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Irrigation activities are important for sustaining food production and account for 70 % of total global water withdrawals. In addition, due to increased evapotranspiration (ET) and changes in the leaf area index (LAI), these activities have an impact on hydrology and climate. In this paper, we present a new irrigation scheme within the land surface model ORCHIDEE (ORganising Carbon and Hydrology in Dynamic EcosystEms)). It restrains actual irrigation according to available freshwater by including a simple environmental limit and using allocation rules that depend on local infrastructure. We perform a simple sensitivity analysis and parameter tuning to set the parameter values and match the observed irrigation amounts against reported values, assuming uniform parameter values over land. Our scheme matches irrigation withdrawals amounts at global scale, but we identify some areas in India, China, and the USA (some of the most intensively irrigated regions worldwide), where irrigation is underestimated. In all irrigated areas, the scheme reduces the negative bias of ET. It also exacerbates the positive bias of the leaf area index (LAI), except for the very intensively irrigated areas, where irrigation reduces a negative LAI bias. The increase in the ET decreases river discharge values, in some cases significantly, although this does not necessarily lead to a better representation of discharge dynamics. Irrigation, however, does not have a large impact on the simulated total water storage anomalies (TWSAs) and its trends. This may be partly explained by the absence of nonrenewable groundwater use, and its inclusion could increase irrigation estimates in arid and semiarid regions by increasing the supply. Correlation of irrigation biases with landscape descriptors suggests that the inclusion of irrigated rice and dam management could improve the irrigation estimates as well. Regardless of this complexity, our results show that the new irrigation scheme helps simulate acceptable land surface conditions and fluxes in irrigated areas, which is important to explore the joint evolution of climate, water resources, and irrigation activities. [ABSTRACT FROM AUTHOR]

This work is motivated by the identification of the land‐atmosphere interactions as one of the key sources of uncertainty in climate change simulations. It documents new developments in related processes, namely, boundary layer/convection/clouds parameterizations and land surface parameterization in the Earth System Model of the Institut Pierre Simon Laplace (IPSL). Simulations forced by prescribed oceanic conditions are produced with different combinations of atmospheric and land surface parameterizations. They are used to explore the sensitivity to the atmospheric physics and/or soil physics of major biases in the near surface variables over continents,the energy and moisture coupling established at the soil/atmosphere interface in not too wet (energy limited) and not too dry (moisture limited) soil moisture regions also known as transition or "hot‐spot" regions,the river runoff at the outlet of major rivers. The package implemented in the IPSL‐Climate Model for the Phase 6 of the Coupled Models Intercomparison Project (CMIP6) allows us to reduce several biases in the surface albedo, the snow cover, and the continental surface air temperature in summer as well as in the temperature profile in the surface layer of the polar regions. The interactions between soil moisture and atmosphere in hotspot regions are in better agreement with the observations. Rainfall is also significantly improved in volume and seasonality in several major river basins leading to an overall improvement in river discharge. However, the lack of consideration of floodplains and human influences in the model, for example, dams and irrigation, impacts the realism of simulated discharge. Plain Language Summary: Land surface‐atmosphere interactions play an essential role in the climate system. They strongly modulate the regional climates and have impacts on the global scale for instance through freshwater release into the oceans. Climate hazards (heat waves, droughts) and their impacts on populations also strongly depend on interactions between land and atmosphere and on their evolution with climate change. Climate models are precious tools to investigate how the Earth climate behaves. The sixth phase of the Climate Model Intercomparison Project (CMIP6) provides important tools to measure the progress and address the remaining open questions regarding the continental climate modeling. The representation of the land‐atmosphere coupled system by the IPSL‐Climate Model involved in CMIP6 is thoroughly evaluated against observations and compared with simulations using the CMIP5 version. Several biases concerning the temperature over land and over the ice sheets and with the snow cover are significantly reduced. Numerous improvements were made developping advanced parameterizations and tuning of the radiation and of the turbulent mixing in the atmospheric model. The realism of the seasonal cycle of hydrological variables such as the precipitation or the river discharge is also improved over many regions. The new treatment of hydrology paves the way for future developments on water resource aspects in the climate model. Key Points: The representation of the land‐atmosphere coupled system by the IPSL model is thoroughly evaluatedImprovements with respect to previous versions are documented in the context of the Coupled Model Intercomparison Project, CMIPAdvanced parameterization of land and atmospheric processes, tuning of the radiation, and the turbulent mixing yielded many improvements [ABSTRACT FROM AUTHOR]

The main objective of the present work is to study the impacts of water table depth on the near surface climate and the physical mechanisms responsible for these impacts through the analysis of land-atmosphere coupled numerical simulations. The analysis is performed with the LMDZ (standard physics) and ORCHIDEE models, which are the atmosphere-land components of the Institut Pierre Simon Laplace (IPSL) Climate Model. The results of sensitivity experiments with groundwater tables (WT) prescribed at depths of 1 m (WTD1) and 2 m (WTD2) are compared to the results of a reference simulation with free drainage from an unsaturated 2 m soil (REF). The response of the atmosphere to the prescribed WT is mostly concentrated over land, and the largest differences in precipitation and evaporation are found between REF and WTD1. Saturating the bottom half of the soil in WTD1 induces a systematic increase of soil moisture across the continents. Evapotranspiration (ET) increases over water-limited regimes due to increased soil moisture, but it decreases over energy-limited regimes due to the decrease in downwelling radiation and the increase in cloud cover. The tropical (25°S-25°N) and mid-latitude areas (25°N-60°N and 25°S-60°S) are significantly impacted by the WT, showing a decrease in air temperature (−0.5 K over mid-latitudes and −1 K over tropics) and an increase in precipitation. The latter can be explained by more vigorous updrafts due to an increased meridional temperature gradient between the equator and higher latitudes, which transports more water vapour upward, causing a positive precipitation change in the ascending branch. Over the West African Monsoon and Australian Monsoon regions, the precipitation changes in both intensity (increases) and location (poleward). The more intense convection and the change of the large-scale dynamics are responsible for this change. Transition zones, such as the Mediterranean area and central North America, are also impacted, with strengthened convection resulting from increased ET. [ABSTRACT FROM AUTHOR]

The Durance watershed (14 000 km2), located in the French Alps, generates 10% of French hydropower and provides drinking water to 3 million people. The Catchment land surface model (CLSM), a distributed land surface model (LSM) with a multilayer, physically based snow model, has been applied in the upstream part of this watershed, where snowfall accounts for 50% of the precipitation. The CLSM subdivides the upper Durance watershed, where elevations range from 800 to 4000 m within 3580 km2, into elementary catchments with an average area of 500 km2. The authors first show the difference between the dynamics of the accumulation and ablation of the snow cover using Moderate Resolution Imaging Spectroradiometer (MODIS) images and snow-depth measurements. The extent of snow cover increases faster during accumulation than during ablation because melting occurs at preferential locations. This difference corresponds to the presence of a hysteresis in the snow-cover depletion curve of these catchments, and the CLSM was adapted by implementing such a hysteresis in the snow-cover depletion curve of the model. Different simulations were performed to assess the influence of the parameterizations on the water budget and the evolution of the extent of the snow cover. Using six gauging stations, the authors demonstrate that introducing a hysteresis in the snow-cover depletion curve improves melting dynamics. They conclude that their adaptation of the CLSM contributes to a better representation of snowpack dynamics in an LSM that enables mountainous catchments to be modeled for impact studies such as those of climate change. [ABSTRACT FROM AUTHOR]

Summary: The groundwater flow in land surface models (LSMs) is receiving increasing attention, and different groups have recognised the need for an improved representation of the saturated zone. For example, the hydrological model TOPMODEL is now included in several LSMs, which allows the simulation of a shallow water table. In this article, we present an adaptation of the catchment land surface model (CLSM), which is an LSM using the concepts of TOPMODEL to generate runoff and soil moisture patterns, to the Somme River basin located in northern France. This catchment is heavily influenced by the deep groundwater flow in the Chalk aquifer, and groundwater storage exerts a strong buffering effect on the streamflow. However, the TOPMODEL shallow water table is not adapted to store water over long timescales. To account for this process, we propose the implementation of an additional linear storage reservoir (LR). Using 18 years of meteorological and streamflow data, we demonstrate that this parameterisation considerably improves the discharge simulation performance. In particular, it allows the maintanance of low flows and the reduction of overestimated peak flows that were generated by CLSM without this reservoir. Many simulations with different parameter combinations are analysed to investigate the parameter sensitivities. The impact of the LR on the energy budget is assessed using soil temperature data. We conclude that the new LR parameterisation contributes to a better representation of water transfers in an LSM that enables a groundwater-fed catchment to be modelled for impact studies such as those of climate change. [Copyright &y& Elsevier]

Large-scale runoff routing models (RRMs) are important as a validation tool for GCMs, and to close the hydrological cycle in fully-coupled climate models. The model RiTHM was developed to simulate the discharge of large rivers from the total runoff simulated by the LMD GCM. It uses a 1024×800 grid, nested in the 64×50 grid of the LMD GCM. The runoff simulated in a GCM grid cell is uniformly distributed over the underlying cells, where a series of two reservoirs accounts for the delay related to infiltration through the unsaturated zone and aquifers. The resulting riverflow is routed assuming pure translation along the drainage network, extracted with a GIS from a 5 min DEM. The transfer time from a cell to the outlet depends on topography, and on a basin-wide parameter, the time of concentration. RiTHM was calibrated in 11 river basins, using a realistic runoff forcing (computed by the land surface model SECHIBA from reanalyzed meteorological forcing). This led to a very satisfactory reproduction of observed hydrographs. The main problems were related to hydraulic processes neglected in RiTHM (reservoirs, diversion of riverflow because of flooding or irrigation). These results helped to validate SECHIBA, except for its snow processes, shown to be too simple. With the same parameters, RiTHM was also forced with runoff from the LMD GCM. This induced an important degradation of the simulated hydrographs, regarding both volume and timing. It was largely explained by errors in precipitation, and more generally climate, in the GCM. The direct calibration of RiTHM under the GCM-runoff forcing markedly improved the timing of simulated discharge, which could be interesting for land–atmosphere–ocean coupling. This work demonstrated that the usefulness of RRMs for GCMs strongly depends on their adequate calibration. [Copyright &y& Elsevier]

The three-layer snow model of Lynch-Stieglitz is coupled to the global catchment-based land surface model of the National Aeronautics and Space Administration’s Seasonal to Interannual Prediction Project, and the combined models are used to simulate the growth and ablation of snow cover over the North American continent for the period of 1987–88. The various snow processes included in the three-layer model, such as snow melting and refreezing, dynamic changes in snow density, and snow insulating properties, are shown (through a comparison with the corresponding simulation using a much simpler snow model) to lead to an improved simulation of ground thermodynamics on the continental scale. This comparison indicates that the three-layer model, originally developed and validated at small experimental catchments, does indeed capture the important snow processes that control the growth and the ablation of continental-scale snowpack and its snow insulation capabilities. [ABSTRACT FROM AUTHOR]

Examines the sensitivity of the hydrological cycle to soil water-holding capacity. Use of a general circulation model coupled to a land surface model; Increase in annual mean evaporation; Split between increased annual precipitation and decreased annual mean moisture convergence.

Soil physical properties play an important role for estimating soil water and energy fluxes. Since field measurement of these properties is difficult and costly, many hydrology and land surface models (LSMs) use soil texture maps to infer soil hydraulic parameters. The impact of soil texture on soil water processes has been extensively assessed, however, to our knowledge, it has never been studied at global or subcontinental scales. In the present study, we investigated the impact of soil texture on global soil water fluxes and storage using the ORCHIDEE LSM, which offers a process-based description of soil hydrology, combining a 1D-vertical Richards equation and the Van Genuchten-Mualem model for unsaturated hydraulic parameters. This model was run off-line at the 0.5° resolution, and we prescribed multiple input soil texture maps from the literature to this model to examine its response to each texture, and to each map. Among all water fluxes, surface runoff and infiltration exhibited the highest sensitivity to soil texture change, and medium textured soils showed the highest evapotranspiration rates and the lowest total runoff. Yet, the partitioning of precipitation between evapotranspiration and total runoff is shown to be primarily dependent on climatic conditions and less dependent on soil texture. In particular, the different soil texture maps resulted in globally similar water budgets, although high local differences were observed. Compared to several observation-based products, we also found that ORCHIDEE satisfactorily simulated the global patterns and mean of evapotranspiration whichever the soil texture map, including with a uniform loamy soil. In line with few other results, this study suggests that soil texture, although used as the only soil descriptor in many hydrology and land surface models, may not be a relevant control of soil hydrology, at least at the 0.5° resolution explored here. [ABSTRACT FROM AUTHOR]

Groundwater (GW) constitutes by far the largest volume of liquid freshwater on Earth. The most active part is soil moisture (SM), recognized as a key variable of land/atmosphere interactions, especially in so-called transition zones, where/when SM varies between wet and dry values. But GW can also be stored in deeper reservoirs than soils, in particular unconfined aquifer systems, in which the saturated part is called the water table (WT). The latter is characterized by slow and mostly horizontal water flows towards the river network, with well-known buffering effects on streamflow variability. Where/when the WT is shallow enough, it can also sustain SM by means of capillary rise, thus increase evapotranspiration (ET), with potential impact on the climate system (including temperatures and precipitation). The large residence time of GW may also increase the Earth system's memory, with consequences on the persistence of extreme events, hydro-climatic predictability, and anthropogenic climate change, particularly the magnitude of regional warming. Here, our main goal is to explore the impacts of GW on historical and future climate, by comparing integrations from three different climate models (CESM, CNRM-CM, IPSL-CM). Their land surface component explicitly describe the spatio-temporal dynamics of GW with GW-SM interactions, but these processes are based on different physical assumptions representative of the state of the art (e.g. scale of GW flow, active depth, input parameters). For each climate model, two transient land-atmosphere simulations are performed, one with GW and the other one without GW, by deactivating the related processes. Each transient simulation is made of the concatenation of (i) one AMIP simulation, with historical sea forcing over 1979-2014, (ii) one "FutureAMIP" simulation from 2015 to 2100, corresponding to the SSP5-8.5 radiative forcing, with sea forcing deduced from a corresponding fully coupled CNRM-CM simulation (from ScenarioMIP). Within this framework, we want to assess the sensitivity of the simulated climate to GW in a systematic way, by trying to identify robust features among the three models. Our main objectives are twofold: (1) Compare GW and NO-GW AMIP simulations to observations to assess if accounting for GW improves some simulated land or climate parameters; (2) Compare FutureAMIP to AMIP simulations to assess if GW is able to alter the manifestations of climate change. For instance, can we get weaker regional warming in areas with significant GW-SM interactions? [ABSTRACT FROM AUTHOR]

A comprehensive estimation of water budget components, particularly groundwater storage (GWS) and fluxes, is crucial. In this study, we evaluate the terrestrial water budget of the Donga basin (Benin, West Africa), as simulated by three land surface models (LSMs) used in the African Monsoon Multidisciplinary Analysis Land Surface Model Intercomparison Project, phase 2 (ALMIP2): CLM4, Catchment LSM (CLSM), and Minimal Advanced Treatments of Surface Interaction and Runoff (MATSIRO). All three models include an unconfined groundwater component and are driven by the same ALMIP2 atmospheric forcing from 2005 to 2008. Results show that all three models simulate substantially shallower water table depth (WTD) with smaller seasonal variations, approximately 1–1.5 m compared to the observed values that range between 4 and 9.6 m, while the seasonal variations of GWS are overestimated by all the models. These seemingly contradictory simulation results can be explained by the overly high specific yield prescribed in all models. All models achieve similar GWS simulations but with different fractions of precipitation partitioning into surface runoff, base flow, and evapotranspiration (ET), suggesting high uncertainty and errors in the terrestrial and groundwater budgets among models. The poor performances of models can be attributed to bias in the hydrological partitioning (base flow vs surface runoff) and sparse subsurface data. This analysis confirms the importance of subsurface hydrological processes in the current generation of LSMs and calls for substantial improvement in both surface water budget (which controls groundwater recharge) and the groundwater system (hydrodynamic parameters, vertical geometry). [ABSTRACT FROM AUTHOR]

Many maps of open water and wetlands have been developed based on three main methods: (i) compiling national and regional wetland surveys, (ii) identifying inundated areas via satellite imagery and (iii) delineating wetlands as shallow water table areas based on groundwater modeling. However, the resulting global wetland extents vary from 3 % to 21 % of the land surface area because of inconsistencies in wetland definitions and limitations in observation or modeling systems. To reconcile these differences, we propose composite wetland (CW) maps, combining two classes of wetlands: (1) regularly flooded wetlands (RFWs) obtained by overlapping selected open-water and inundation datasets; and (2) groundwater-driven wetlands (GDWs) derived from groundwater modeling (either direct or simplified using several variants of the topographic index). Wetlands are statically defined as areas with persistent near-saturated soil surfaces because of regular flooding or shallow groundwater, disregarding most human alterations (potential wetlands). Seven CW maps were generated at 15 arcsec resolution (ca. 500 m at the Equator) using geographic information system (GIS) tools and by combining one RFW and different GDW maps. To validate this approach, these CW maps were compared with existing wetland datasets at the global and regional scales. The spatial patterns were decently captured, but the wetland extents were difficult to assess compared to the dispersion of the validation datasets. Compared with the only regional dataset encompassing both GDWs and RFWs, over France, the CW maps performed well and better than all other considered global wetland datasets. Two CW maps, showing the best overall match with the available evaluation datasets, were eventually selected. These maps provided global wetland extents of 27.5 and 29 million km 2 , i.e., 21.1 % and 21.6 % of the global land area, which are among the highest values in the literature and are in line with recent estimates also recognizing the contribution of GDWs. This wetland class covers 15 % of the global land area compared with 9.7 % for RFW (with an overlap of ca. 3.4 %), including wetlands under canopy and/or cloud cover, leading to high wetland densities in the tropics and small scattered wetlands that cover less than 5 % of land but are highly important for hydrological and ecological functioning in temperate to arid areas. By distinguishing the RFWs and GDWs based globally on uniform principles, the proposed dataset might be useful for large-scale land surface modeling (hydrological, ecological and biogeochemical modeling) and environmental planning. The dataset consisting of the two selected CW maps and the contributing GDW and RFW maps is available from PANGAEA at 10.1594/PANGAEA.892657 (Tootchi et al., 2018). [ABSTRACT FROM AUTHOR]

The river routing scheme (RRS) in the Organising Carbon and Hydrology in Dynamic Ecosystems (ORCHIDEE) land surface model is a valuable tool for closing the water cycle in a coupled environment and for validating the model performance. This study presents a revision of the RRS of the ORCHIDEE model that aims to benefit from the high-resolution topography provided by the Hydrological data and maps based on SHuttle Elevation Derivatives at multiple Scales (HydroSHEDS), which is processed to a resolution of approximately 1 km. Adapting a new algorithm to construct river networks, the new RRS in ORCHIDEE allows for the preservation of as much of the hydrological information from HydroSHEDS as the user requires. The evaluation focuses on 12 rivers of contrasting size and climate which contribute freshwater to the Mediterranean Sea. First, the numerical aspect of the new RRS is investigated, in order to identify the practical configuration offering the best trade-off between computational cost and simulation quality for ensuing validations. Second, the performance of the new scheme is evaluated against observations at both monthly and daily timescales. The new RRS satisfactorily captures the seasonal variability of river discharge, although important biases stem from the water budget simulated by the ORCHIDEE model. The results highlight that realistic streamflow simulations require accurate precipitation forcing data and a precise river catchment description over a wide range of scales, as permitted by the new RRS. Detailed analyses at the daily timescale show the promising performance of this high-resolution RRS with respect to replicating river flow variation at various frequencies. Furthermore, this RRS may also eventually be well adapted for further developments in the ORCHIDEE land surface model to assess anthropogenic impacts on river processes (e.g. damming for irrigation operation). [ABSTRACT FROM AUTHOR]

Many maps of open water and wetlands have been developed based on three main methods: (i) compiling national/regional wetland surveys; (ii) identifying inundated areas via satellite imagery; and (iii) delineating wetlands as shallow water table areas based on groundwater modelling. However, the resulting global wetland extents vary from 3 % to 21 % of the land surface area because of inconsistencies in wetland definitions and limitations in observation or modelling systems. To reconcile these differences, we propose composite wetland (CW) maps, combining two classes of wetlands: (1) regularly flooded wetlands (RFW) obtained by overlapping selected open-water and inundation datasets; and (2) groundwater-driven wetlands (GDW) derived from groundwater modelling (either direct or simplified using several variants of the topographic index). Wetlands are statically defined as areas with persistent near-saturated soil surfaces because of regular flooding or shallow groundwater. Seven CW maps were generated at the 15 arc-sec resolution (ca 500 m at the Equator) using geographic information system (GIS) tools and by combining one RFW and different GDW maps. To validate this approach, these CW maps were compared with existing wetland datasets at the global and regional scales. The spatial patterns were decently captured, but the wetland extents were difficult to assess against the dispersion of the validation datasets. Compared with the only regional dataset encompassing both GDWs and RFWs, over France, the CW maps performed well and better than all other considered global wetland datasets. Two CW maps, showing the best overall match with the available evaluation datasets, were eventually selected. These maps provided global wetland extents of 27.5 and 29 million km², i.e., 21.1 % and 21.6 % of global land area, which are among the highest values in the literature and in line with recent estimates also recognizing the contribution of GDWs. This wetland class covers 15 % of the global land area compared with 9.7 % for RFW (with an overlap of ca. 3.4 %), including wetlands under canopy/cloud cover, leading to high wetland densities in the tropics and small scattered wetlands that cover less than 5 % of land but are highly important for hydrological and ecological functioning in temperate to arid areas. By distinguishing the RFWs and GDWs based globally on uniform principles, the proposed dataset might be useful for large-scale land surface modelling (hydrological, ecological and biogeochemical modelling) and environmental planning. The dataset consisting of the two selected CW maps and the contributing GDW and RFW maps is available from PANGAEA at https://doi.pangaea.de/10.1594/PANGAEA.892657 [ABSTRACT FROM AUTHOR]

This study presents a revised river routing scheme (RRS) for the Organising Carbon and Hydrology in Dynamic Ecosystems (ORCHIDEE) land surface model. The revision is carried out to benefit from the high resolution topography provided the Hydrological data and maps based on SHuttle Elevation Derivatives at multiple Scales (HydroSHEDS), processed to a resolution of approximately 1 kilometer. The RRS scheme of the ORCHIDEE uses a unit-to-unit routing concept which allows to preserve as much of the hydrological information of the HydroSHEDS as the user requires. The evaluation focuses on 12 rivers of contrasted size and climate which contribute freshwater to the Mediterranean Sea. First, the numerical aspect of the new RRS is investigated, to identify the practical configuration offering the best trade-off between computational cost and simulation quality for ensuing validations. Second, the performance of the revised scheme is evaluated against observations at both monthly and daily timescales. The new RRS captures satisfactorily the seasonal variability of river discharges, although important biases come from the water budget simulated by the ORCHIDEE model. The results highlight that realistic streamflow simulations require accurate precipitation forcing data and a precise river catchment description over a wide range of scales, as permitted by the new RRS. Detailed analyses at the daily timescale show promising performances of this high resolution RRS for replicating river flow variation at various frequencies. Eventually, this RRS is well adapted for further developments in the ORCHIDEE land surface model to assess anthropogenic impacts on river processes (e.g. damming for irrigation operation). [ABSTRACT FROM AUTHOR]

While it is well accepted that climate change and growing water needs affect long-term sustainable water resourcesmanagement, performing accurate simulations of water cycle and energy balance dynamics at regional scale remains a challenging task. Traditional Soil-Vegetation-Atmosphere-Transfer (SVAT) models are used for numerical surface water and energy simulations. These models, by conception, do not account for the groundwater lower boundary that permits a full hydrosystem representation. Conversely, while addressing important features such as subsurface heterogeneity and river-aquifer exchanges, groundwater models often integrate overly simplified upper boundary conditions ignoring soil heating and the impacts of vegetation processes on radiation fluxes and root-zone uptakes. In this paper, one of the first attempts to jointly model water and energy fluxes with a special focus on both surface and groundwater at the regional scale is proposed on the Seine hydrosystem (78,650 km²), which overlays one of the main multi-aquifer systems of Europe. This study couples the SVAT model ORCHIDEE and the process-based hydrological-hydrogeological model CaWaQS, which describes water fluxes, via a one-way coupling approach from ORCHIDEE toward CaWaQS based on the blueprint published by de Marsily et al. [1978]. An original transport library based on the resolution of the diffusion/advection transport equation was developed in order to simulate heat transfer in both 1D-river networks and pseudo-3D aquifer systems. In addition, an analytical solution is used to simulate heat transport through aquitards and streambeds. Simulated ORCHIDEE surface water and energy fluxes feed fast surface runoff and slow recharge respectively and then is used as CaWaQS forcings to compute river discharges, hydraulic heads and temperature dynamics through space and time, within each of the hydrosystem compartments. The tool makes it possible to establish a fully consistent water and energy budget over a period of 17 years. It also simulates temperature evolution in each aquifer and evaluates that river thermal regulation mostly relies by order of importance on short wave radiations (109.3W·m-2), groundwater fluxes (48.1W·m-2) and surface runoff (22.7W·m-2). [ABSTRACT FROM AUTHOR]

The representation of groundwater dynamics in land surface models and their roles in global precipitation variations has received attention in recent years. Studies have revealed the overall higher soil moisture but rather diverse precipitation changes after incorporating the groundwater component in climate models. However, groundwater effects on large-scale atmospheric energy transport, the fundamental atmospheric variable regulating Earth's climate, have not been explored thoroughly. In this study, a pair of idealized experiments corresponding to contrast globally fixed water table depths by AMIP-type simulations in the Community Earth System Model was conducted. In the wet (shallow water table) experiments, an increased meridional surface temperature gradient makes the mean meridional energy transports and Hadley circulation stronger than dry (deep water table) experiments over the tropics. Such energy transport changes are primarily attributed to the dynamic contribution (intensified Hadley circulation). The wet experiments make the simulated world be like an aquaplanet simulation with less land–sea temperature contrast and the enhancement (reduction) of mean meridional circulation (stationary eddies) energy transports. Furthermore, the South Asian monsoon circulation in the wet experiment shows a southward shift in the premonsoon season (April–June) and slight weakening in the mature phase (July and August). This study explores the impacts of the soil conditions caused by various water table depths on global energy transport and has further implications for climate model developments and experiment designs. [ABSTRACT FROM AUTHOR]

Continental- to global-scale hydrologic and land surface models increasingly include representations of the groundwater system. Such large-scale models are essential for examining, communicating, and understanding the dynamic interactions between the Earth system above and below the land surface as well as the opportunities and limits of groundwater resources. We argue that both large-scale and regional-scale groundwater models have utility, strengths, and limitations, so continued modeling at both scales is essential and mutually beneficial. A crucial quest is how to evaluate the realism, capabilities, and performance of large-scale groundwater models given their modeling purpose of addressing large-scale science or sustainability questions as well as limitations in data availability and commensurability. Evaluation should identify if, when, or where large-scale models achieve their purpose or where opportunities for improvements exist so that such models better achieve their purpose. We suggest that reproducing the spatiotemporal details of regional-scale models and matching local data are not relevant goals. Instead, it is important to decide on reasonable model expectations regarding when a large-scale model is performing "well enough" in the context of its specific purpose. The decision of reasonable expectations is necessarily subjective even if the evaluation criteria are quantitative. Our objective is to provide recommendations for improving the evaluation of groundwater representation in continental- to global-scale models. We describe current modeling strategies and evaluation practices, and we subsequently discuss the value of three evaluation strategies: (1) comparing model outputs with available observations of groundwater levels or other state or flux variables (observation-based evaluation), (2) comparing several models with each other with or without reference to actual observations (model-based evaluation), and (3) comparing model behavior with expert expectations of hydrologic behaviors in particular regions or at particular times (expert-based evaluation). Based on evolving practices in model evaluation as well as innovations in observations, machine learning, and expert elicitation, we argue that combining observation-, model-, and expert-based model evaluation approaches, while accounting for commensurability issues, may significantly improve the realism of groundwater representation in large-scale models, thus advancing our ability for quantification, understanding, and prediction of crucial Earth science and sustainability problems. We encourage greater community-level communication and cooperation on this quest, including among global hydrology and land surface modelers, local to regional hydrogeologists, and hydrologists focused on model development and evaluation. [ABSTRACT FROM AUTHOR]

The Sahara was significantly wetter and greener than today during the mid‐Holocene (∼6,000 years before present), and those conditions were likely maintained by feedbacks from evaporating wetlands and riparian zones. A lack of spatially continuous wetland reconstruction is the major obstacle to investigating their impacts on climate and vegetation during that epoch. Here, we estimate high‐resolution gridded wetland distribution up to 15″ in the mid‐Holocene North Africa obtained with three statistical and hydrological modeling approaches forced by enhanced and calibrated precipitation from climate models. These wetland models have good performance for present‐day conditions and reproduce mid‐Holocene hydrological elements evaluated by 297 paleo‐records. Simulation results show that 18.9 ± 4.0% of land surface in North Africa was covered by wetlands during the mid‐Holocene. Our results highlight the impact of natural climate change on wetland areas and provide a data set for modeling studies to include wetland feedbacks. Plain Language Summary: Several lines of evidence show that northern Africa was considerably wetter and greener than today at ∼6,000 years ago, which is known as the mid‐Holocene Green Sahara (GS). However, most current models could not reproduce climate in the GS. The importance of wetland feedbacks on sustaining a wetter climate has partially been recognized while large uncertainties in wetland coverage make it difficult to examine wetland feedbacks in climate models. We trained several wetland models under present climate and applied them to wetland reconstructions of GS. The produced wetland maps could capture dense wetlands indicated by 297 paleo‐records. The total wetland fraction in the mid‐Holocene North Africa is 18.9 ± 4.0%, which is more than five times the area today (∼2.8%). The relationship between wetland fraction and precipitation is examined in our models. This work improves our understanding of the GS enigma, and has implications for potential greening and wetting of Sahel and Sahara in the future. Key Points: We present high‐resolution reconstructions of wetland distribution up to 15″ for the mid‐Holocene North AfricaDuring the mid‐Holocene, 18.9 ± 4.0% of land surface in North Africa was covered by wetlandsThe relationship between precipitation and wetland fraction in the mid‐Holocene Sahara is explored [ABSTRACT FROM AUTHOR]

Intermittent rivers and ephemeral streams (IRES) are now recognized to support specific freshwater biodiversity and ecosystem services and represent approximately half of the global river network, a fraction that is likely to increase in the context of global changes. Despite large research efforts on IRES during the past few decades, there is a need for developing a systemic approach to IRES that considers their hydrological, hydrogeological, hydraulic, ecological, and biogeochemical properties and processes, as well as their interactions with human societies. Thus, we assert that the interdisciplinary approach to ecosystem research promoted by critical zone sciences and socio‐ecology is relevant. These approaches rely on infrastructure—Critical Zone Observatories (CZO) and Long‐Term Socio‐Ecological Research (LTSER) platforms—that are representative of the diversity of IRES (e.g., among climates or types of geology. We illustrate this within the French CZO and LTSER, including their diversity as socio‐ecosystems, and detail human interactions with IRES. These networks are also specialized in the long‐term observations required to detect and measure ecosystem responses of IRES to climate and human forcings despite the delay and buffering effects within ecosystems. The CZO and LTSER platforms also support development of innovative techniques and data analysis methods that can improve characterization of IRES, in particular for monitoring flow regimes, groundwater‐surface water flow, or water biogeochemistry during rewetting. We provide scientific and methodological perspectives for which this interdisciplinary approach and its associated infrastructure would provide relevant and original insights that would help fill knowledge gaps about IRES. This article is categorized under:Water and Life > Stresses and Pressures on EcosystemsScience of Water > Hydrological ProcessesWater and Life > Conservation, Management, and Awareness [ABSTRACT FROM AUTHOR]

Continental- to global-scale hydrologic and land surface models increasingly include representations of the groundwater system. Such large-scale models are essential for examining, communicating, and understanding the dynamic interactions between the Earth System above and below the land surface as well as the opportunities and limits of groundwater resources. We argue that both large-scale and regional-scale groundwater models have utility, strengths and limitations so continued modeling at both scales is essential and mutually beneficial. A crucial quest is how to evaluate the realism, capabilities and performance of large-scale groundwater models given their modeling purpose of addressing large-scale science or sustainability questions as well as limitations in data availability and commensurability. Evaluation should identify if, when or where large-scale models achieve their purpose or where opportunities for improvements exists so that such models better achieve their purpose. We suggest that reproducing the spatio-temporal details of regional-scale models and matching local data is not a relevant goal. Instead, it is important to decide on reasonable model expectations regarding when a large scale model is performing "well enough" in the context of its specific purpose. The decision of reasonable expectations is necessarily subjective even if the evaluation criteria is quantitative. Our objective is to provide recommendations for improving the evaluation of groundwater representation in continental- to global-scale models. We describe current modeling strategies and evaluation practices, and subsequently discuss the value of three evaluation strategies: 1) comparing model outputs with available observations of groundwater levels or other state or flux variables (observation-based evaluation); 2) comparing several models with each other with or without reference to actual observations (model-based evaluation); and 3) comparing model behavior with expert expectations of hydrologic behaviors in particular regions or at particular times (expert-based evaluation). Based on evolving practices in model evaluation as well as innovations in observations, machine learning and expert elicitation, we argue that combining observation-, model-, and expert-based model evaluation approaches, while accounting for commensurability issues, may significantly improve the realism of groundwater representation in large-scale models. Thus advancing our ability for quantification, understanding, and prediction of crucial Earth science and sustainability problems. We encourage greater community-level communication and cooperation on this quest, including among global hydrology and land surface modelers, local to regional hydrogeologists, and hydrologists focused on model development and evaluation. [ABSTRACT FROM AUTHOR]

The CONterminous United States (CONUS) presents a large range of climate conditions and biomes where terrestrial primary productivity and its inter‐annual variability are controlled regionally by rainfall and/or temperature. Here, the response of ecosystem productivity to those climate variables was investigated across different biomes from 2010 to 2018 using three climate datasets of precipitation, air temperature or drought severity, combined with several proxies of ecosystem productivity: a remote sensing product of aboveground biomass, an net primary productivity (NPP) remote sensing product, an NPP model‐based product and four gross primary productivity products. We used an asymmetry index (AI) where positive AI indicates a greater increase of ecosystem productivity in wet years compared to the decline in dry years, and negative AI indicates a greater decline of ecosystem productivity in dry years compared to the increase in wet years. We found consistent spatial patterns of AI across the CONUS for the different products, with negative asymmetries over the Great Plains and positive asymmetries over the southwestern CONUS. Shrubs and, to a lesser extent, evergreen forests show a persistent positive asymmetry, whilst (natural) grasslands appear to have transitioned from positive to negative anomalies during the last decade. The general tendency of dominant negative asymmetry response for ecosystem productivity across the CONUS appears to be influenced by the negative asymmetry of precipitation anomalies. AI was found to be a function of mean rainfall: more positive AIs were found in dry areas where plants are adapted to drought and take advantage of rainfall pulses, and more negative AIs were found in wet areas, with a threshold delineating the two regimes corresponding to a mean annual rainfall of 200–400 mm/year. [ABSTRACT FROM AUTHOR]

In this second part of a two-part study, we performed a simulation of the carbon and water budget of the Lena catchment with the land surface model ORCHIDEE MICT-LEAK, enabled to simulate dissolved organic carbon (DOC) production in soils and its transport and fate in high-latitude inland waters. The model results are evaluated for their ability to reproduce the fluxes of DOC and carbon dioxide (CO2) along the soil–inland-water continuum and the exchange of CO2 with the atmosphere, including the evasion outgassing of CO2 from inland waters. We present simulation results over the years 1901–2007 and show that the model is able to broadly reproduce observed state variables and their emergent properties across a range of interacting physical and biogeochemical processes. These include (1) net primary production (NPP), respiration and riverine hydrologic amplitude, seasonality, and inter-annual variation; (2) DOC concentrations, bulk annual flow, and their volumetric attribution at the sub-catchment level; (3) high headwater versus downstream CO2 evasion, an emergent phenomenon consistent with observations over a spectrum of high-latitude observational studies. These quantities obey emergent relationships with environmental variables like air temperature and topographic slope that have been described in the literature. This gives us confidence in reporting the following additional findings: of the ∼34 Tg C yr -1 left over as input to soil matter after NPP is diminished by heterotrophic respiration, 7 Tg C yr -1 is leached and transported into the aquatic system. Of this, over half (3.6 Tg C yr -1) is evaded from the inland water surface back into the atmosphere and the remainder (3.4 Tg C yr -1) flushed out into the Arctic Ocean, mirroring empirically derived studies. These riverine DOC exports represent ∼1.5 % of NPP. DOC exported from the floodplains is dominantly sourced from recent more "labile" terrestrial production in contrast to DOC leached from the rest of the watershed with runoff and drainage, which is mostly sourced from recalcitrant soil and litter. All else equal, both historical climate change (a spring–summer warming of 1.8 ∘ C over the catchment) and rising atmospheric CO2 (+85.6 ppm) are diagnosed from factorial simulations to contribute similar significant increases in DOC transport via primary production, although this similarity may not hold in the future. [ABSTRACT FROM AUTHOR]

Few Earth system models adequately represent the unique permafrost soil biogeochemistry and its respective processes; this significantly contributes to uncertainty in estimating their responses, and that of the planet at large, to warming. Likewise, the riverine component of what is known as the "boundless carbon cycle" is seldom recognised in Earth system modelling. The hydrological mobilisation of organic material from a ∼1330 –1580 PgC carbon stock to the river network results in either sedimentary settling or atmospheric "evasion", processes widely expected to increase with amplified Arctic climate warming. Here, the production, transport, and atmospheric release of dissolved organic carbon (DOC) from high-latitude permafrost soils into inland waters and the ocean are explicitly represented for the first time in the land surface component (ORCHIDEE) of a CMIP6 global climate model (Institut Pierre Simon Laplace – IPSL). The model, ORCHIDEE MICT-LEAK, which represents the merger of previously described ORCHIDEE versions MICT and LEAK, mechanistically represents (a) vegetation and soil physical processes for high-latitude snow, ice, and soil phenomena and (b) the cycling of DOC and CO2 , including atmospheric evasion, along the terrestrial–aquatic continuum from soils through the river network to the coast at 0.5 to 2 ∘ resolution. This paper, the first in a two-part study, presents the rationale for including these processes in a high-latitude-specific land surface model, then describes the model with a focus on novel process implementations, followed by a summary of the model configuration and simulation protocol. The results of these simulation runs, conducted for the Lena River basin, are evaluated against observational data in the second part of this study. [ABSTRACT FROM AUTHOR]

The importance of northern peatlands in the global carbon cycle has been recognized, especially for long-term changes. Yet, the complex interactions between climate and peatland hydrology, carbon storage, and area dynamics make it challenging to represent these systems in land surface models. This study describes how peatlands are included as an independent sub-grid hydrological soil unit (HSU) in the ORCHIDEE-MICT land surface model. The peatland soil column in this tile is characterized by multilayered vertical water and carbon transport and peat-specific hydrological properties. The cost-efficient version of TOPMODEL and the scheme of peatland initiation and development from the DYPTOP model are implemented and adjusted to simulate spatial and temporal dynamics of peatland. The model is tested across a range of northern peatland sites and for gridded simulations over the Northern Hemisphere (>30 ∘ N). Simulated northern peatland area (3.9 million km 2), peat carbon stock (463 Pg C), and peat depth are generally consistent with observed estimates of peatland area (3.4–4.0 million km 2), peat carbon (270–540 Pg C), and data compilations of peat core depths. Our results show that both net primary production (NPP) and heterotrophic respiration (HR) of northern peatlands increased over the past century in response to CO2 and climate change. NPP increased more rapidly than HR, and thus net ecosystem production (NEP) exhibited a positive trend, contributing a cumulative carbon storage of 11.13 Pg C since 1901, most of it being realized after the 1950s. [ABSTRACT FROM AUTHOR]

In this second part of a two-part study, we perform a simulation of the carbon and water budget of the Lena catchment with the land surface model ORCHIDEE MICT-LEAK, enabled to simulate dissolved organic carbon (DOC) production in soils and its transport and fate in high latitudes inland waters. The model results are evaluated in their ability to reproduce the fluxes of DOC and carbon dioxide (CO2) along the soil-inland water continuum, and the exchange of CO2 with the atmosphere, including the evasion outgassing of CO2 from inland waters. We present simulation results over years 1901-2007, and show that the model is able to broadly reproduce observed state variables and their emergent properties across a range of interacting physical and biogeochemical processes, including: 1) Net primary production (NPP), respiration and riverine hydrologic amplitude, seasonality and inter-annual variation; 2) DOC concentrations, bulk annual flow and their volumetric attribution at the sub-catchment level; 3) High headwater versus downstream CO2 evasion, an emergent phenomenon consistent with observations over a spectrum of high latitude observational studies. (4) These quantities obey emergent relationships with environmental variables like air temperature and topographic slope that have been described in the literature. This gives us confidence in reporting the following additional findings: (5) Of the ~34TgCyr-1 left over as input to terrestrial and aquatic systems after NPP is diminished by heterotrophic respiration, 7TgCyr-1 is leached and transported into the aquatic system. Of this, over half (3.6TgCyr-1) is evaded from the inland water surface back into the atmosphere and the remainder (3.4TgCyr-1) flushed out into the Arctic Ocean, proportions in keeping with other, empirically derived studies. (6) DOC exported from the floodplains is dominantly sourced from recent, more "labile" terrestrial production, in contrast to DOC leached from the rest of the watershed with runoff and drainage, which is mostly sourced from recalcitrant soil and litter. (7) All else equal, both historical climate change (a spring/summer warming of 1.8°C over the catchment) and rising atmospheric CO2 (+85.6ppm) are diagnosed from factorial simulations to contribute similar, significant increases in DOC transport via primary production, although this similarity may not hold in the future. [ABSTRACT FROM AUTHOR]

Few Earth System models adequately represent the unique permafrost soil biogeochemistry and its respective processes; this significantly contributes to uncertainty in estimating their responses, and that of the planet at large, to warming. Likewise, the riverine component of what is known as the "boundless carbon cycle" is seldom recognized in Earth System modeling. Hydrological mobilization of organic material from a ~1330-1580PgC carbon stock to the river network results either in sedimentary settling or atmospheric "evasion", processes widely expected to increase with amplified Arctic climate warming. Here, the production, transport and atmospheric release of dissolved organic carbon (DOC) from high-latitude permafrost soils into inland waters and the ocean is explicitly represented for the first time in the land surface component (ORCHIDEE) of a CMIP6 global climate model (IPSL). The model, ORCHIDEE MICT-LEAK, mechanistically represents (a) vegetation and soil physical processes for high latitude snow, ice and soil phenomena, and (b) the cycling of DOC and CO2, including atmospheric evasion, along the terrestrial-aquatic continuum from soils through the river network to the coast, at 0.5° to 2° resolution. This paper, the first in a two-part study, presents the rationale for including these processes in a high latitude specific land surface model, then describes the model with a focus on novel process implementations, followed by a summary of the model configuration and simulation protocol. The results of these simulation runs, conducted for the Lena River basin, are evaluated against observational data in the second part of this study. [ABSTRACT FROM AUTHOR]

The importance of northern peatlands in the global carbon cycle has recently been recognized, especially for long-term changes. Yet, the complex interactions between climate and peatland hydrology, carbon storage and area dynamics make it challenging to represent these systems in land surface models. This study describes how peatland are included as an independent sub-grid hydrological soil unit (HSU) into the ORCHIDEE-MICT land surface model. The peatland soil column in this tile is characterized by multi-layered vertical water and carbon transport, and peat-specific hydrological properties. A cost-efficient TOPMODEL approach is implemented to simulate the dynamics of peatland area, calibrated by present-day wetland areas that are regularly inundated or subject to shallow water tables. The model is tested across a range of northern peatland sites and for gridded simulations over the Northern Hemisphere (> 30° N). Simulated northern peatland area (3.9 million km²), peat carbon stock (463 PgC) and peat depth are generally consistent with observed estimates of peatland area (3.4-4.0 million km²), peat carbon (270-540 PgC) and data compilations of peat core depths. Our results show that both net primary production (NPP) and heterotrophic respiration (HR) of northern peatlands increased over the past century in response to CO2 and climate change. NPP increased more rapidly than HR, and thus net ecosystem production (NEP) exhibited a positive trend, contributing a cumulative carbon storage of 11.13 Pg C since 1901, most of it being realized after the 1950s. [ABSTRACT FROM AUTHOR]

The high-latitude regions of the Northern Hemisphere are a nexus for the interaction between land surface physical properties and their exchange of carbon and energy with the atmosphere. At these latitudes, two carbon pools of planetary significance -- those of the permanently frozen soils (permafrost), and of the great expanse of boreal forest -- are vulnerable to destabilization in the face of currently observed climatic warming, the speed and intensity of which are expected to increase with time. Improved projections of future Arctic and boreal ecosystem transformation require improved land surface models that integrate processes specific to these cold biomes. To this end, this study lays out relevant new parameterizations in the ORCHIDEE-MICT land surface model. These describe the interactions between soil carbon, soil temperature and hydrology, and their resulting feedbacks on water and CO2 fluxes, in addition to a recently developed fire module. Outputs from ORCHIDEEMICT, when forced by two climate input datasets, are extensively evaluated against (i) temperature gradients between the atmosphere and deep soils, (ii) the hydrological components comprising the water balance of the largest highlatitude basins, and (iii) CO2 flux and carbon stock observations. The model performance is good with respect to empirical data, despite a simulated excessive plant water stress and a positive land surface temperature bias. In addition, acute model sensitivity to the choice of input forcing data suggests that the calibration of model parameters is strongly forcingdependent. Overall, we suggest that this new model design is at the forefront of current efforts to reliably estimate future perturbations to the high-latitude terrestrial environment. [ABSTRACT FROM AUTHOR]

Land surface processes play an important role in the West African monsoon variability. In addition, the evolution of hydrological systems in this region, and particularly the increase of surface water and runoff coefficients observed since the 1950s, has had a strong impact on water resources and on the occurrence of floods events. This study addresses results from phase 2 of the African Monsoon Multidisciplinary Analysis (AMMA) Land Surface Model Intercomparison Project (ALMIP2), carried out to evaluate the capability of different state-of-the-art land surface models to reproduce surface processes at the mesoscale. Evaluation of runoff and water fluxes over the Mali site is carried out through comparison with runoff estimations over endorheic watersheds as well as evapotranspiration (ET) measurements. Three remote-sensing-based ET products [ALEXI, MODIS, and Global Land Evaporation Amsterdam Model (GLEAM)] are also analyzed. It is found that, over deep sandy soils, surface runoff is generally overestimated, but the ALMIP2 multimodel mean reproduces in situ measurements of ET and water stress events rather well. However, ALMIP2 models are generally unable to distinguish among the two contrasted hydrological systems typical of the study area. Employing as input a soil map that explicitly represents shallow soils improves the representation of water fluxes for the models that can account for their representation. Shallow soils are shown to be also quite challenging for remote-sensing-based ET products, even if their effect on evaporative loss was captured by the diagnostic thermal-based ALEXI. A better representation of these soils, in soil databases, model parameterizations, and remote sensing algorithms, is fundamental to improve the estimation of water fluxes in this part of the Sahel. [ABSTRACT FROM AUTHOR]

Lateral transfer of carbon (C) from terrestrial ecosystems into the inland water network is an important component of the global C cycle, which sustains a large aquatic CO2 evasion flux fueled by the decomposition of allochthonous C inputs. Globally, estimates of the total C exports through the terrestrial-aquatic interface range from 1.5 to 2.7 Pg C yr-1 (Cole et al., 2007; Battin et al., 2009; Tranvik et al., 2009), i.e. in the order of 2-5% of the terrestrial NPP. Earth System Models (ESM) of the climate system ignore these lateral transfers of C, and thus likely overestimate the terrestrial C sink. In this study, we present the implementation of fluvial transport of dissolved organic carbon (DOC) and CO2 into ORCHIDEE, the land surface scheme of the Institut Pierre-Simon Laplace ESM. This new model branch, called ORCHILEAK, represents DOC production from canopy and soils, DOC and CO2 leaching from soils to streams, DOC decomposition and CO2 evasion to the atmosphere during its lateral transport in rivers, as well as exchange with the soil carbon and litter stocks on floodplains and in swamps. We parameterized and validated ORCHILEAK for the Amazon basin, the world's largest river system with regard to discharge and one of the most productive ecosystems of the world. With ORCHILEAK, we are able to reproduce observed terrestrial and aquatic fluxes of DOC and CO2 in the Amazon basin, both in terms of mean values and seasonality. In addition, we are able to resolve the spatio-temporal variability in C fluxes along the canopy-soil-aquatic continuum at high resolution (1°, daily) and to quantify the different terrestrial contributions to the aquatic C fluxes. We simulate that more than 2/3 of the Amazon's fluvial DOC export is contributed by the decomposition of submerged litter. Throughfall DOC fluxes from canopy to ground are about as high as the total DOC inputs to inland waters. The latter, however, are mainly sustained by litter decomposition. Decomposition of DOC and submerged plant litter contributes slightly more than half of the CO2 evasion from the water surface, while the remainder is contributed by soil respiration. Total CO2 evasion from the water surface equals about 5 % of the terrestrial NPP. Our results highlight that ORCHILEAK is well suited to simulate carbon transfers along the terrestrial-aquatic continuum of tropical forests. It also opens the perspective that provided parameterization, calibration and validation is performed for other biomes, the new model branch could improve the quantification of the global terrestrial C sink and help better constrain carbon cycle-climate feedbacks in future projections. [ABSTRACT FROM AUTHOR]

The high-latitude regions of the northern hemisphere are a nexus for the interaction between land surface physical properties and their exchange of carbon and energy with the atmosphere. At these latitudes, two carbon pools of planetary significance - those of the permanently frozen soils (permafrost), and of the great expanse of boreal forest - are vulnerable to destabilization in the face of currently observed climatic warming, the speed and intensity of which are expected to increase with time. Improved projections of future Arctic and boreal ecosystem transformation require improved land surface models that integrate processes specific to these cold biomes. To this end, this study lays out relevant new parameterizations in the ORCHIDEE-MICT land surface model. These describe the interactions between soil carbon, soil temperature and hydrology, and their resulting feedbacks on water and CO2 fluxes, in addition to a recently-developed fire module. Outputs from ORCHIDEE-MICT, when forced by two climate input data sets, are extensively evaluated against: (i) temperature gradients between the atmosphere and deep soils; (ii) the hydrological components comprising the water balance of the largest high-latitude basins, and (iii) CO2 flux and carbon stock observations. The model performance is good with respect to empirical data, despite a simulated excessive plant water stress and a positive land surface temperature bias. In addition, acute model sensitivity to the choice of input forcing data suggests that the calibration of model parameters is strongly forcing-dependent. Overall, we suggest that this new model design is at the forefront of current efforts to reliably estimate future perturbations to the high-latitude terrestrial environment. [ABSTRACT FROM AUTHOR]

The Land Surface, Snow and Soil Moisture Model Intercomparison Project (LS3MIP) is designed to provide a comprehensive assessment of land surface, snow and soil moisture feedbacks on climate variability and climate change, and to diagnose systematic biases in the land modules of current Earth system models (ESMs). The solid and liquid water stored at the land surface has a large influence on the regional climate, its variability and predictability, including effects on the energy, water and carbon cycles. Notably, snow and soil moisture affect surface radiation and flux partitioning properties, moisture storage and land surface memory. They both strongly affect atmospheric conditions, in particular surface air temperature and precipitation, but also large-scale circulation patterns. However, models show divergent responses and representations of these feedbacks as well as systematic biases in the underlying processes. LS3MIP will provide the means to quantify the associated uncertainties and better constrain climate change projections, which is of particular interest for highly vulnerable regions (densely populated areas, agricultural regions, the Arctic, semi-arid and other sensitive terrestrial ecosystems). The experiments are subdivided in two components, the first addressing systematic land biases in offline mode ("LMIP", building upon the 3rd phase of Global Soil Wetness Project; GSWP3) and the second addressing land feedbacks attributed to soil moisture and snow in an integrated framework ("LFMIP", building upon the GLACE-CMIP blueprint). [ABSTRACT FROM AUTHOR]

The Land Surface, Snow and Soil Moisture Model Intercomparison Project (LS3MIP) is designed to provide a comprehensive assessment of land surface, snow, and soil moisture feedbacks on climate variability and climate change, and to diagnose systematic biases in the land modules of current Earth System Models (ESMs). The solid and liquid water stored at the land surface has a large influence on the regional climate, its variability and predictability, including effects on the energy, water and carbon cycles. Notably, snow and soil moisture affect surface radiation and flux partitioning properties, moisture storage and land surface memory. They both strongly affect atmospheric conditions, in particular surface air temperature and precipitation, but also large-scale circulation patterns. However, models show divergent responses and representations of these feedbacks as well as systematic biases in the underlying processes. LS3MIP will provide the means to quantify the associated uncertainties and better constrain climate change projections, which is of particular interest for highly vulnerable regions (densely populated areas, agricultural regions, the Arctic, semi-arid and other sensitive terrestrial ecosystems). The experiments are subdivided in two components, the first addressing systematic land biases in offline mode ("LMIP", building upon the 3rd phase of Global Soil Wetness Project; GSWP3) and the second addressing land feedbacks attributed to soil moisture and snow in an integrated framework ("LFMIP", building upon the GLACE-CMIP blueprint). [ABSTRACT FROM AUTHOR]

Widespread negative correlations between summertime-mean temperatures and precipitation over land regions are a well-known feature of terrestrial climate. This behavior has generally been interpreted in the context of soil moisture-atmosphere coupling, with soil moisture deficits associated with reduced rainfall leading to enhanced surface sensible heating and higher surface temperature. The present study revisits the genesis of these negative temperature-precipitation correlations using simulations from the Global Land-Atmosphere Coupling Experiment-phase 5 of the Coupled Model Intercomparison Project (GLACE-CMIP5) multimodel experiment. The analyses are based on simulations with five climate models, which were integrated with prescribed (noninteractive) and with interactive soil moisture over the period 1950-2100. While the results presented here generally confirm the interpretation that negative correlations between seasonal temperature and precipitation arise through the direct control of soil moisture on surface heat flux partitioning, the presence of widespread negative correlations when soil moisture-atmosphere interactions are artificially removed in at least two out of five models suggests that atmospheric processes, in addition to land surface processes, contribute to the observed negative temperature-precipitation correlation. On longer time scales, the negative correlation between precipitation and temperature is shown to have implications for the projection of climate change impacts on near-surface climate: in all models, in the regions of strongest temperature-precipitation anticorrelation on interannual time scales, long-term regional warming is modulated to a large extent by the regional response of precipitation to climate change, with precipitation increases (decreases) being associated with minimum (maximum) warming. This correspondence appears to arise largely as the result of soil moisture-atmosphere interactions. [ABSTRACT FROM AUTHOR]

Summary: The main objective of this study is to provide a realistic simulation of river stage in regional river networks in order to improve stream-aquifer interactions and better assess stream discharge and hydraulic head in aquifer units. The study focuses on the Oise basin (a 17,000km2 sub-basin of the Seine River basin, in Northern France). An upscaling method is proposed to benefit from high resolution hydraulic modeling at local scale to improve the simulation of river stages at regional scale. The methodology is based on simulation of the main rivers with a 1D Saint–Venant approach, from which functional stage-discharge relationships, are derived and projected onto each grid-cell of the regional model. At regional scale, the rating curves are used as boundary conditions and allow to calculate river stage, which is then used to calculate the exchanges between aquifer units and river. The approach is efficient to better simulate water pathways and stream-aquifer interactions at regional scale with low computing cost. The model was used to quantify stream to aquifer exchanges, that are in average 39mmyr−1 for aquifer to stream fluxes and 2mmyr−1 for stream to aquifer fluxes, mainly due to storage in aquifer units during storm events. [ABSTRACT FROM AUTHOR]

Multi-model ensembles have become a standard tool for assessing global climate change impacts. Interpretation of such ensembles is complicated because each model group has a different modeling concept and framework. For example, global scale land surface, water and vegetation models have been widely applied to understand the complex hydrological cycle of the Earth and to assess associated past and future changes. Additionally to this purpose, land surface models assess energy and biogeochemical cycles while vegetation models assess vegetation and carbon cycles. Therefore, all these models differ with respect to the specific processes of the hydrological cycle included in their structure. In this study, we demonstrate how the similarities and differences between models can be better understood and illustrated by using a standard representation of the main model features. We analyze twelve models from the global water sector of the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP) phase 2b: six land surface models (LSMs), five global hydrological models (GHMs) and one dynamic global vegetation model (DGVM). The majority of the models are run with a daily temporal resolution and with a spatial resolution of 0.5°. Part of these models include a reservoir scheme and water use sectors. The heuristic mappings of the models are designed to ensure the opportunity to choose a model at the initial stage of the analysis, based on the most important qualities, relationships and characteristics, which provide users with significant time saving. Therefore, the review study will provide the basis for: (i) achieving further model (inter)comparison; (ii) selecting the right model(s) output(s) for specific applications; and (iii) assessing the similarities and differences among the models. The models characteristics will be presented in three levels of complexity allowing to reach a large audience. The target audience includes the modeling community, the stakeholder community, and the general public interested in understanding large-scale models, simulating climate change and its impacts. Additionally, stakeholder insights, gathered mostly in Eastern Europe and West Africa, have been considered in the study design. Stakeholders were identified according to their need for climate-impact information provided within the ISIMIP framework and included academics, government officials, employees working in international organizations, NGOs, consultancies, and private companies. In conclusion, the presentation describes the study approach and preliminary results, with particular emphasis on the standard model diagram, differences between the models, and the stakeholder engagement. [ABSTRACT FROM AUTHOR]

During the early to middle Holocene, the Sahara received enhanced precipitation and was covered by steppe-like vegetation with a large-scale hydrographic network of lakes, wetlands and fans, which is known as the Green Sahara (GS). However, most coupled land-atmosphere models underestimate the precipitation and vegetation cover, suggesting that critical atmospheric or land surface processes are lacking in those models. Climate-induced vegetation cover change can modify soil texture and physical properties over the long term, which in turn have feedbacks on vegetation. In this study, we examine five plausible soil-vegetation processes in a land surface model, which are expected to increase soil moisture for plants and possibly sustain equilibrium vegetation for a lower rainfall level. The annual precipitation required during the GS epoch to match the modelled vegetation distribution with paleorecords is inferred. Results demonstrate that these soil-vegetation processes have strong positive impacts on vegetation and soil moisture, especially the increase of soil evaporative resistance. After including all soil feedbacks on vegetation, the model requires only a mean precipitation of ∼400 mm/yr to reproduce the pollen-inferred GS vegetation, instead of ∼600 mm/yr when no soil feedback is included. From the mid-Holocene to pre-industrial period, we infer that terrestrial carbon stocks decrease by ∼58 PgC due to the removal of carbon in vegetation, soil and litter pools of the GS. This work highlights the importance of soil-vegetation interactions for simulating dry-region vegetation coverage in models, and the impacts of natural land cover change on carbon budgets in the geological past. • A dynamic vegetation model is used to reproduce the Green Sahara vegetation. • Strong soil-vegetation interactions determine modelled vegetation distribution. • The relative importance of each soil feedback is separated by factorial simulations. • The carbon stocks in the mid-Holocene Sahara are ∼58 PgC higher than today. [ABSTRACT FROM AUTHOR]

Groundwater (GW) constitutes by far the largest volume of liquid freshwater on Earth. The most active part is soil moisture (SM), recognized as a key variable of land/atmosphere interactions. But GW can also be stored in deeper reservoirs than soils, characterized by slow and mostly horizontal water flows towards the river network. These flows end up forming baseflow, with well-known buffering effects on streamflow variability. But they also contribute to sustain high SM in many areas, like in lowland areas surrounding streams, which are among the most frequent wetlands (floodplains and smaller riparian wetlands).In the standard version of the ORCHIDEE land surface model, the sole effect of GW is on river discharge, using a linear reservoir model to represent GW storage and baseflow in each grid-cell, with no feedback on local SM. To describe the interactions between GW flow and soil moisture, we introduced a new sub-grid fraction, corresponding to the lowland parts of the grid-cell, and acting as a buffer between the upland areas, supplying most GW recharge, and the river system eventually draining GW flow. For simplicity, this fraction is constant in each grid-cell, and prescribed from a global-scale wetland map recently designed to this purpose (Tootchi et al., 2019). The lowland fraction is described as a separate hydrological element, with physically-based water flows relying on a fine vertical discretization. It is effectively wet when/where GW flow is sufficient, in which case a water table can build up, and feeds baseflow to the river, as well as enhanced evapotranspiration compared to the upland fraction.This new version is called ORCHIDEE-GWF (GW-fed Fraction) and we present here its first evaluation at global scale, both off-line (with prescribed atmospheric conditions from a meteorological forcing dataset) and coupled to the IPSL climate model. Compared to the reference version of ORCHIDEE, the main impacts of this new version relate to SM, evapotranspiration, river discharge, and soil thermal properties. They are quantified spatially, compared to several observation-based products to assess if ORCHIDEE-GWF helps obtaining more realistic simulations, and the sensitivity to key assumptions of this new parametrization ORCHIDEE-GWF is explored (soil depth, baseflow parameters).Tootchi, A., Jost, A., and Ducharne, A. (2019). Multi-source global wetland maps combining surface water imagery and groundwater constraints, Earth Syst. Sci. Data., accepted, https://doi.org/10.5194/essd-2018-87 [ABSTRACT FROM AUTHOR]