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Earth's ecosystems are increasingly threatened by "hot drought," which occurs when hot air temperatures coincide with precipitation deficits, intensifying the hydrological, physiological, and ecological effects of drought by enhancing evaporative losses of soil moisture (SM) and increasing plant stress due to higher vapor pressure deficit (VPD). Drought-induced reductions in gross primary production (GPP) exert a major influence on the terrestrial carbon sink, but the extent to which hotter and atmospherically drier conditions will amplify the effects of precipitation deficits on Earth's carbon cycle remains largely unknown. During summer and autumn 2020, the U.S. Southwest experienced one of the most intense hot droughts on record, with record-low precipitation and record-high air temperature and VPD across the region. Here, we use this natural experiment to evaluate the effects of hot drought on GPP and further decompose those negative GPP anomalies into their constituent meteorological and hydrological drivers. We found a 122 Tg C (>25%) reduction in GPP below the 2015-2019 mean, by far the lowest regional GPP over the Soil Moisture Active Passive satellite record. Roughly half of the estimated GPP loss was attributable to low SM (likely a combination of record-low precipitation and warming-enhanced evaporative depletion), but record-breaking VPD amplified the reduction of GPP, contributing roughly 40% of the GPP anomaly. Both air temperature and VPD are very likely to continue increasing over the next century, likely leading to more frequent and intense hot droughts and substantially enhancing drought-induced GPP reductions.

Large datasets of greenhouse gas and energy surface-atmosphere fluxes measured with the eddy-covariance technique (e.g., FLUXNET2015, AmeriFlux BASE) are widely used to benchmark models and remote-sensing products. This study addresses one of the major challenges facing model-data integration: To what spatial extent do flux measurements taken at individual eddy-covariance sites reflect model- or satellite-based grid cells? We evaluate flux footprints—the temporally dynamic source areas that contribute to measured fluxes—and the representativeness of these footprints for target areas (e.g., within 250–3000 m radii around flux towers) that are often used in flux-data synthesis and modeling studies. We examine the land-cover composition and vegetation characteristics, represented here by the Enhanced Vegetation Index (EVI), in the flux footprints and target areas across 214 AmeriFlux sites, and evaluate potential biases as a consequence of the footprint-to-target-area mismatch. Monthly 80% footprint climatologies vary across sites and through time ranging four orders of magnitude from 103 to 107 m2 due to the measurement heights, underlying vegetation- and ground-surface characteristics, wind directions, and turbulent state of the atmosphere. Few eddy-covariance sites are located in a truly homogeneous landscape. Thus, the common model-data integration approaches that use a fixed-extent target area across sites introduce biases on the order of 4%–20% for EVI and 6%–20% for the dominant land cover percentage. These biases are site-specific functions of measurement heights, target area extents, and land-surface characteristics. We advocate that flux datasets need to be used with footprint awareness, especially in research and applications that benchmark against models and data products with explicit spatial information. We propose a simple representativeness index based on our evaluations that can be used as a guide to identify site-periods suitable for specific applications and to provide general guidance for data use.

Growing season length (GSL) is a key unifying concept in ecology that can be estimated from eddy covariance-derived estimates of net ecosystem production (NEP). Previous studies disagree on how increasing GSLs may affect NEP in evergreen coniferous forests, potentially due to the variety of methods used to quantify GSL from NEP. We calculated GSL and GSL-NEP regressions at eleven evergreen conifer sites across a broad climatic gradient in western North America using three common approaches: (1) variable length (3-7 days) regressions of day of year versus NEP, (2) a smoothed threshold approach, and (3) the carbon uptake period, followed by a new approach of a method-averaged ensemble. The GSL and the GSL-NEP relationship differed among methods, resulting in linear relationships with variable sign, slope, and statistical significance. For all combinations of sites and methods, the GSL explained between 6% and 82% of NEP with p-values ranging from 0.45 to

Changes in the volume, rate, and timing of the snowmelt water pulse have profound implications for seasonal soil moisture, evapotranspiration (ET), groundwater recharge, and downstream water availability, especially in the context of climate change. Here, we present an empirical analysis of water available for runoff using five eddy covariance towers located in continental montane forests across a regional gradient of snow depth, precipitation seasonality, and aridity. We specifically investigated how energy‐water asynchrony (i.e., snowmelt timing relative to atmospheric demand), surface water input intensity (rain and snowmelt), and observed winter ET (winter AET) impact multiple water balance metrics that determine water available for runoff (WAfR). Overall, we found that WAfR had the strongest relationship with energy‐water asynchrony (adjusted r2 = 0.52) and that winter AET was correlated to total water year evapotranspiration but not to other water balance metrics. Stepwise regression analysis demonstrated that none of the tested mechanisms were strongly related to the Budyko‐type runoff anomaly (highest adjusted r2 = 0.21). We, therefore, conclude that WAfR from continental montane forests is most sensitive to the degree of energy‐water asynchrony that occurs. The results of this empirical study identify the physical mechanisms driving variability of WAfR in continental montane forests and are thus broadly relevant to the hydrologic management and modelling communities. [ABSTRACT FROM AUTHOR]

Current understanding of the dynamic flow paths and subsurface water storages that support streamflow in mountain catchments is inhibited by the lack of long-term hydrologic data and the frequent use of single age tracers that are not applicable to older groundwater reservoirs. To address this, the current study used both multiple metrics and tracers to characterize the transient nature of flow paths with respect to change in catchment storage at Marshall Gulch, a sub-humid headwater catchment in the Santa Catalina Mountains, Arizona, USA. The fraction of streamflow that was untraceable using stable water isotope tracers was also estimated. A Gamma-type transit time distribution (TTD) was appropriate for deep groundwater analysis, but there were errors in the TTD shape parameters arising from the short record length of ³H in deep groundwater and stream water, and inconsistent seasonal cyclicity of the precipitation ³H time series data. Overall, the mean transit time calculated from ³H data was more than two decades greater than the mean transit time based on δ18O at the same site. The fraction of young water (Fyw) in shallow groundwater was estimated from δ18O time series data using weighted wavelet transform (WWT), iteratively re-weighted least squares (IRLS), and TTD-based methods. Estimates of Fyw depended on sampling frequency, the method of estimation, bedrock geology, hydroclimate, and factors affecting streamflow generation processes. The coupled use of Fyw and discharge sensitivity indicated highly dynamic flow paths that reorganized with changes in shallow catchment storage. The utility of ³H to determining Fyw in deeper groundwater was limited by data quality. Given that Fyw, discharge sensitivity, and mean transit time all yield unique information, this work demonstrates how co-application of multiple methods can yield a more complete understanding of the transient flow paths and observable storage volumes that contribute to streamflow in mountain headwater catchments. [ABSTRACT FROM AUTHOR]

Hydrologic models have been applied to predict land surface water and energy budgets in mountainous watersheds that are characterized by complex geological features and climatic variability. A common practice is to calibrate the models and achieve the best performing parameter set according to historical observations, and then the calibrated model was used to do future projections. One drawback is that the influence of parameter uncertainty on model projections is not well discussed. In this study, we applied multiple objective functions to choose a group of best performing parameter sets to the Boulder Creek Watershed, USA to investigate how parameter uncertainties can propagate to future projections. We used 16 parameter sets that have similar performance in simulating streamflow amount and regime historically, and applied the same parameter sets to predict hydrologic variables including streamflow, evapotranspiration, and soil moisture in two future phases (Phase 1 is 2040-2069 and Phase 2 is 2070-2099). The results show that variability due to parameter uncertainty was up to 10 % annually and 26 % monthly under future climate change scenarios, and the uncertainties are especially prominent during May to September. The different parameter sets can result to annual streamflow changes in opposite directions. The results indicate that a single parameter set may yield biased representation of hydrologic variability. It is necessary to consider multiple optimal parameter sets in applying hydrologic models for hydrological projections and water resources decision making. [ABSTRACT FROM AUTHOR]

Background:The eddy covariance (EC) technique provides a direct measure of water vapour and carbon dioxide fluxes between ecosystems and the atmosphere. Aims:This review article highlights the findings of various studies that have integrated EC observations into basic meteorological, hydrological and ecological research questions in two ecosystems near Niwot Ridge, Colorado, and synthesises these studies into a catchment-scale model of water and carbon cycling, within the context of regional disturbance and environmental change. Methods:EC was implemented continuously over subalpine forest and alpine tundra vegetation at Niwot Ridge, and resulting data were compared with discrete measurements and modelling studies. Results:Sensible heat fluxes were generally in excess of latent heat fluxes, indicating that the forest and tundra ecosystems were moisture limited. Snow cover regulated the annual sum of primary productivity in the forest, and beneath-snow respiration represented a significant portion of ecosystem respiration at both locations. Conclusions:Changes in the magnitude, timing, or spatial distribution of snow are likely to have the greatest impact on ecological processes in these semi-arid mountain catchments, but possibly in compensatory ways. Ultimately, the degree to which net carbon losses from alpine tundra offset forest carbon sequestration will determine the future magnitude of the Western United States carbon sink. [ABSTRACT FROM PUBLISHER]

This study coupled long‐term hydrometric and stable water isotope data to identify links between subsurface water storage and vegetation in a subhumid mountain catchment in Arizona, USA. Specific observations included catchment‐scale hydrologic fluxes and soil water storage and stable water isotopes from stream water, soil water, groundwater, and sap water from Arizona pine (Pinus arizonica) and Douglas fir (Pseudotsuga menziesii) individuals. Here, we find that tightly bound soil water was sufficient to meet dry period vegetation water demand when the former was defined in terms of field capacity as opposed to a matric tension threshold. This water was a mixture of summer and winter precipitation that predominates in both shallow and deep soil waters, and contributed significantly to streamflow. We also identified a less common mobile water type that did not contribute significantly to streamflow and was related to infiltration during isotopically depleted precipitation events. Although each water type was used by both Arizona pine and Douglas fir vegetation, the second water type was dominant in Douglas fir sap water. Therefore, we conclude that Arizona pine and Douglas fir can occupy different ecohydrological niches at this subhumid mountain location. Further, a lack of isotopic distinction between tightly bound and inferred mobile soil water signals that the ecohydrological water source separation hypothesis is not entirely applicable at this site. The results of this study broadly highlight how alternative definitions of tightly bound water can influence interpretation of data, and contribute to a more thorough understanding of interactions between subsurface storage and plant water dynamics. [ABSTRACT FROM AUTHOR]