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Highlights • A Monte Carlo approach is effectively used to sample weights to combine precipitation products. • Modeled soil moisture can be improved by optimally combining different precipitation products. • Combining three precipitation products is more efficient than using one or two to improve soil moisture simulations. Abstract Precipitation is a fundamental forcing variable in land surface modeling, controlling several hydrological and biogeochemical processes (e.g., runoff, carbon cycling, evaporation, transpiration, groundwater recharge, and soil moisture). However, precipitation estimates from rain gauges, ground-based radars, satellite sensors, and numerical models are affected by significant uncertainties, which can be amplified when exposed to highly non-linear land model physics. This work tests the hypothesis that precipitation data from different sources can be optimally merged to minimize the hydrologic response error in surface soil moisture simulations and maximize their correlation with ground observations (multi-objective optimization problem). This hypothesis is tested by merging three precipitation products (one satellite product, a ground-based dataset, and model-base estimates) that force a land surface model trained to minimize soil moisture anomalies. A Monte Carlo-based algorithm is developed to generate weights to linearly combine these precipitation datasets. Optimal combinations of weights are identified by minimizing the errors and maximizing the correlation between the model simulated soil moisture and the satellite-based SMOS soil moisture product. The proposed methodology has been tested over Oklahoma where high-quality, high-resolution (independent) ground-based soil moisture observations are available for validation purposes. Results show that there exist optimal combinations of these precipitation datasets that provide smaller errors and larger correlation coefficients between modeled soil moisture estimates and ground-based data with respect to forcing the land surface model with single precipitation datasets. Specifically, combining three precipitation products from different sources provides the largest correlation coefficient and the lowest root mean square error at several locations across Oklahoma. [ABSTRACT FROM AUTHOR]

Highlights • Evaporation from an array of thin line sources of water between smooth plates. • Bridging 1-D diffusion from bare water surfaces and 3-D diffusion from the leaves. • Understanding evaporation from leaves and soils when water is just below the surface. • High evaporation rate is sustained due to lateral conduction of heat. • Film positioning decides evaporation rate and surface and sub-surface temperatures. Abstract We report evaporation characteristics due to higher heating from above from surfaces having an array of line sources. The line sources are created by vertically stacked rectangular plates in a box with water. Two different types of plates were used. In one case line source or film thickness was 66 µm and open area ratio was 4%, and in the second case film thickness was 28 µm and 20% was the open area ratio. Even at the 4% open area ratio the evaporation rate was ∼85% compared to a bare water surface, at the same heat flux. Lateral conduction of heat from the impervious hotter regions to the line sources and the 2-D nature of diffusion near these tiny line sources enhances the evaporative flux, owing to increase in the concentration gradient of water vapour, explains the high evaporation rate, observed in the present work. This system bridges the gap between the understanding of evaporation from bare water surfaces (1-D vapour diffusion) and leaf surfaces (3-D vapour diffusion). Evaporation rates for the fully saturated conditions are in good agreement with the theoretical predictions of Suzuki and Maeda (1968) and Schlünder (1988). The computed surface temperatures and its width-wise variation match well with the experimental values. We also propose a simple film model for the unsaturated condition of the porous medium and show that the temperature distribution obtained using this model is in reasonably good agreement with the measured values. [ABSTRACT FROM AUTHOR]

Highlights • Soil water evaporation under bare soil was higher than that under plastic mulch in daytime. • At nighttime bare soil evaporation was lower than that under plastic mulch. • Soil water evaporation under plastic mulch was 4.04–7.07% of the total evapotranspiration (374.95–513.84 mm). Abstract Plastic mulching has been widely used in arid regions because it can decrease soil water evaporation and raise soil temperature. Previous studies, however, treated soil water evaporation under plastic mulch to be negligible, assuming that the plastic mulch can prevent water exchange between soil and atmosphere completely. In order to demonstrate validity of this assumption, experiments were conducted from 2014 to 2016 under irrigated maize (Zea mays) field in Northwest China, comparison experiments of soil water evaporation between mulched soil and the bare soil between mulches were carried out in two seed maize fields with different irrigation method, i.e. border irrigation under mulch (Site BM) and drip irrigation under mulch (Site DM), with micro-lysimeters placed under the plastic mulched soil and the bare soil between mulches to evaluate soil water evaporation of each experiment maize field. Our observations indicated that the soil water evaporation under plastic mulch (E ms) was about 4.04–7.07% of the total evapotranspiration, among which E ms in the daytime accounted for 3.58–5.37% of the total evapotranspiration and 0.99–2.10% of the total evapotranspiration in the nighttime. Thus E ms was considered not to be negligible. For two experiment sites, soil water evaporation under bare soil between mulches (E bs) was obviously higher than the soil water evaporation under plastic mulch (E ms) in daytime, but the former was lower than the latter in the nighttime. At night, the mean soil temperature in the mulched soil was higher than that in the bare soil because of the warming effect of the plastic mulch. A soil-mulch-canopy-atmosphere model is used to consider the effects of the mulch, and the modeling results further support this finding. These results provide a new insight for understanding the effect of plastic mulch on water use efficiency. [ABSTRACT FROM AUTHOR]

Highlights • The FAO Irrigation and Drainage Paper 56 can be adapted for bare soil evaporation. • Lysimeters were simulated by a water balance model based on FAO 56. • The reduction in bare soil evaporation compared to cropped areas is quantified. Abstract Unlike evaporation from open water, the magnitude of evaporation from bare soil decreases as the water table falls. Bare soil evaporation studies have included field and laboratory experiments, mathematical formulations and semi-empirical models. However, there is only limited field information, especially concerning evaporation from bare sand. The semi-empirical approach of the FAO 1 ETo – Evapotranspiation; FAO – Food and Agriculture Organisation; REW – Readily Evaporable Water; TEW – Total Evaporable Water. 1 Irrigation and Drainage Paper 56, which contains guidelines for computing crop water requirements, can be adapted for bare soil evaporation with a three stage process. The suitability of the FAO 56 approach for bare sand evaporation is investigated by installing lysimeters in sand dams. Sand dams are shallow groundwater storage systems, which are designed on the assumption of reduced evaporation as the water table falls. The field results from the lysimeters are simulated adequately by a water balance model based on FAO 56 with an additional component to represent both the difference between the variable saturation with depth, which occurs in practice, and the assumption in standard water balance models of a sudden change from dry to fully-saturated conditions at the water table. This study demonstrates and quantifies the reduction in bare soil evaporation compared to open water or cropped areas and confirms the validity of the three stage FAO semi-empirical approach. [ABSTRACT FROM AUTHOR]

HYPROP (Hydraulic Property Analyzer) system works based on the simplified evaporation method, which determines soil hydraulic properties in the laboratory. This system simultaneously monitors the changes in weight of soil samples as well as the soil matric potential at two depths throughout a drying process governed by evaporation from the soil surface. In this study, we examined the performance of the pseudo continuous pedotranfer function (PC-PTF) to estimate the soil water retention curve (SWRC) using high resolution data measured by HYPROP system. The dataset consisted of 7963 measured water retention data points obtained from 81 Turkish soil samples from which 60%, 20% and 20% were randomly selected for training, cross-validation and test subsets, respectively. The best PC-PTF developed in this study with a mean absolute error of 0.023 m 3 m −3 (a root mean square error of 0.033 m 3 m −3 ) and a correlation coefficient of 0.96 showed promising performance considering the typical performance range (RMSE = 0.034–0.085 m 3 m −3 ) for parametric PTFs estimating SWRC. The best PC-PTF used soil textural information, soil bulk density, the percentage of stable aggregates, soil organic matter content and the initial water content as the input attributes. PTFs developed in this study also ranked high among previously developed PC-PTFs (with RMSE ranging from 0.027 to 0.056 m 3 m −3 ) using sparse datasets collected via the traditional equilibrium approach (i.e. sandbox apparatus /pressure plates extractor). We, therefore, recommend further application of the PC-PTF approach for development of PTFs using high resolution data obtained by HYPROP system. [ABSTRACT FROM AUTHOR]

The evaporation method (EM) is an effective technique for measuring soil hydraulic properties. However, the EM generally fails to measure hydraulic conductivity curves ( K–h ; K and h are the hydraulic conductivity and the pressure head, respectively) in the wet (low suction) range when the hydraulic gradient within soil samples exceeds the minimum accuracy of the pressure transducer. In this study, we developed a new experimental device that uses the EM to perform conductivity measurements in the wet range. By making two major improvements, we successfully expanded the measurable range of K–h to cover the wet range, enabling accurate and effective measurements. Firstly, the accuracy of the hydraulic gradient measurement was remarkably improved by the use of a differential pressure transducer. Assuming the minimum evaporation speed (0.18 cm/day at the center of soil sample), the estimated measurable range of K–h is up to 27 cm/day for any type of soil. That is, our device has expanded the range of K–h measurable by the EM by more than two orders of magnitude on the wet end. Secondly, our device can execute the saturated/unsaturated steady-state infiltration method (SSIM/USIM) and the EM on the same sample. This diminishes not only time and cost but also errors between different methods, ensuring good accuracy and representativeness. The range of K–h measurable by the USIM was determined to be more than 21 cm/day, which overlaps with the range measurable by the EM. As a result, our device can obtain consecutive K–h curves from saturation to the medium pressure range (−500 <  h   ⩽  0 cm) for any type of soil. [ABSTRACT FROM AUTHOR]

The process of evaporation interacts with the soil, which has various comprehensive mechanisms. Multiphase flow models solve air, vapour, water, and heat transport equations to simulate non-isothermal soil moisture transport of both liquid water and vapor flow, but are only applied in non-vegetated soils. For (sparsely) vegetated soils often energy balance models are used, however these lack the detailed information on non-isothermal soil moisture transport. In this study we coupled a multiphase flow model with a two-layer energy balance model to study the impact of non-isothermal soil moisture transport on evaporation fluxes (i.e., interception, transpiration, and soil evaporation) for vegetated soils. The proposed model was implemented at an experimental agricultural site in Florida, US, covering an entire maize-growing season (67 days). As the crops grew, transpiration and interception became gradually dominated, while the fraction of soil evaporation dropped from 100% to less than 20%. The mechanisms of soil evaporation vary depending on the soil moisture content. After precipitation the soil moisture content increased, exfiltration of the liquid water flow could transport sufficient water to sustain evaporation from soil, and the soil vapor transport was not significant. However, after a sufficient dry-down period, the soil moisture content significantly reduced, and the soil vapour flow significantly contributed to the upward moisture transport in topmost soil. A sensitivity analysis found that the simulations of moisture content and temperature at the soil surface varied substantially when including the advective (i.e., advection and mechanical dispersion) vapour transport in simulation, including the mechanism of advective vapour transport decreased soil evaporation rate under wet condition, while vice versa under dry condition. The results showed that the formulation of advective soil vapor transport in a soil-vegetation-atmosphere transfer continuum can affect the simulated evaporation fluxes, especially under dry condition. [ABSTRACT FROM AUTHOR]

Studies on evapotranspiration partitioning under eddy covariance (EC) towers rarely address the separate effects of transpiration and evaporation on groundwater resources. Such partitioning is important to accurately assess groundwater resources, especially in arid and semi-arid areas. The main objective of this study was to partition (evaluate separately) the evaporation and transpiration components of evapotranspiration, originated either from saturated or unsaturated zone, and estimate their contributions in a semi-arid area characterized by relatively shallow groundwater Table (0–10 m deep). Evapotranspiration, tree transpiration and subsurface evaporation were estimated with EC tower, using sap flow methods and HYDRUS1D model, respectively. To set up the HYDRUS1D model, soil material properties, soil moisture, soil temperature, soil matric potential and water table depth were measured in the area. The tree transpiration was sourced into groundwater and unsaturated zone components (∼0.017 mm d −1 for both) and accounted for only ∼6% of the evapotranspiration measured by the EC tower (∼0.565 mm d −1 ), due to the low canopy coverage in the study area (7%). The subsurface evaporation fluxes were also sourced into groundwater and unsaturated zone components using the SOURCE package, and their relative relevance in total evapotranspiration was assessed. Subsurface evaporation was the main flux year-round (∼0.526 mm d −1 ). During late autumn, winter and early spring time, the unsaturated zone evaporation was dominant, while in dry summer the relevance of groundwater evaporation increased, reaching one third of evapotranspiration, although errors in the water balance closure point still at its possible underestimation. The results show that, in arid and semi-arid areas with sparse vegetation, the often neglected groundwater evaporation is a relevant contribution to evapotranspiration, and that water vapor flow should be taken into account in the calculation of extinction depth. [ABSTRACT FROM AUTHOR]

Soil evaporation is an important component in the water and energy cycles on land, especially for areas that are moderately or densely covered by bare soil. Soil evaporation parameterizations that scale down potential evaporation with the soil surface temperature ( T s ) and/or the air humidity are regionally applicable because of the advantage of omitting pixel-scale near-surface soil moisture. In this paper, we provide an intercomparison study among these parameterizations. Potential evaporation indices are estimated from the Priestley-Taylor method, the Penman method, and the mass transfer method (with or without T s ). The surface dryness indices that indicate the water availability of the soil surface are based on T s and/or the air humidity. We establish and evaluate ten such soil evaporation parameterizations through combinations of different types of potential evaporation indices and surface dryness indices at near-instantaneous scales (30 min). The results show that incorporating the soil temperature in the surface dryness index instead of the potential evaporation index can improve soil evaporation estimations. Poorer but still reasonable estimations are achieved when only the air humidity-based surface dryness index is used. In addition, the energy balance factor is crucial in the surface dryness indices. Our study indicates that the potential evaporation indices that are based on the Penman equation are generally more useful and robust than those that are based on the Priestley-Taylor approach or the mass transfer method. However, when the surface dryness index is only based on air humidity data, the Priestley-Taylor potential evaporation index performs as well as the index that is estimated from the Penman equation. In contrast, a soil evaporation parameterization that estimates the potential evaporation through the mass transfer method (with T s ) and the surface dryness index from the soil moisture content did not perform as well as the above ten parameterizations. [ABSTRACT FROM AUTHOR]

Summary Discriminating moisture sources with precision is an important requirement to better understand the processes involved in extreme rainfall episodes. In a previous contribution by Gangoiti et al. (2011b), an innovative technique was presented to assess surface moisture sources contributing to a target precipitation within a Lagrangian framework. The technique was based in transporting parcels of vapor, representing the target precipitation, across a set of nested grids covering a large area at different resolutions. A mesoscale model estimated the meteorological variables to transport and redistribute the vapor back into its original sources, all of them assumed to be at the surface. The sequence of extreme rainfall events, which occurred over central Europe on August 11–13, 2002, was chosen to put the methodology to test. An important innovation has now been introduced. This new advance allows discriminating not only the terrestrial and oceanic sources but also the evaporation from precipitation occurring below the clouds and falling either on land or on the open sea. It is also able to detect with greater precision the relative importance of remote versus local sources, together with the sequence of evaporation of a rainfall event. After its application to the same episode and targets, our results confirm a similar distribution and strength of surface terrestrial and marine sources. Furthermore, the estimated direct evaporation from precipitation columns contributes to the precipitation episode with a significant amount of moisture which averages around 18% of the total sources, with a main fraction evaporated over land and close to the target regions in central Europe. This contribution adds to the surface sources, and it is consistent with the existence of an important mechanism of positive feedback for the inland transport efficiency of moisture and precipitation, operating at the regional level for this type of episodes. Significant regional differences are found in the contribution to different rainfall targets, with a lower fraction of 14% for the southern target (Upper Austria), and 22% for the northern one (Bohemia). [ABSTRACT FROM AUTHOR]

The simplified evaporation method (SEM) is a technique to determine soil hydraulic properties, i.e. the water retention curve and the hydraulic conductivity curve, from evaporation experiments in the laboratory. The method relies on simplifying assumptions like spatial linearity in pressure head and water fluxes, which are violated in the evaporation process. Therefore, its validity and accuracy has been checked in the past by numerical simulation using the Richards equation and subsequent evaluation of the computer-generated data. These numerical tests were conducted using models of the soil hydraulic properties like the van Genuchten–Mualem model, which account only for water retention and conductivity in completely filled capillaries and thus give biased descriptions of the hydraulic properties at low moisture contents. We reinvestigate the accuracy of the SEM by a numerical simulation study for a broad spectrum of soils with textures ranging from clay to pure sand, using more realistic functions of the soil hydraulic properties, which account for water adsorption, flow in incompletely filled capillaries and isothermal vapor flow. Our results show that the previous numerical studies underestimated the non-linearities of suction and water content profiles which occur during bare-soil evaporation, in particular for sandy soils. Nevertheless, the SEM approximates the true hydraulic functions of most soils very well, with mean deviations below 0.1% volumetric water content and below 0.1 for log 10 of hydraulic conductivity. Larger columns or higher evaporation rates increase this error only slightly. Only for the extremes in the texture spectrum, we found bias in the estimated soil hydraulic properties. We furthermore investigate different strategies for calculating a mean suction and show that a weighted combination of the arithmetic and geometric mean leads to the best results. [ABSTRACT FROM AUTHOR]

Highlights: [•] The condensed water in the greenhouse comes from underground water. [•] Daily evaporation is only a small part of the water fluxes in the upper soil layers. [•] The daily evaporation speed can be expressed as a sinusoidal curve. [ABSTRACT FROM AUTHOR]

Highlights: [•] A non-equilibrium model of water transport in the vadose zone is developed. [•] A non-equilibrium isothermal phase change relation is proposed and characterized. [•] Soil column experiments are carried out and compared to numerical simulations. [•] In our cases, the local equilibrium assumption is not valid and must be rejected. [•] The evaporation front is explicitly defined and localized. [ABSTRACT FROM AUTHOR]

Summary: Evaporation from a salty soil generates salt accumulation near the surface with the subsequent deterioration of the soil quality. Salinization mechanisms are poorly understood despite their global impact. Vapor flux and solute transport were studied under evaporation conditions. Laboratory experiments consisted of open sand and silt columns initially saturated with epsomite (MgSO4·7H2O) or halite (NaCl) solutions. Salt precipitation occurred only above the evaporation front, which occupied a very narrow region. Vapor flowed both upwards and downwards from this front. The downward vapor flow condensed further down the column, diluting the solution. This gave rise to two areas: a high salinity area above the evaporation front, and a diluted solution area below it. The effects of thermal, suction and osmotic gradients on water fluxes were studied in order to better understand the underlying mechanisms of this phenomenon. [ABSTRACT FROM AUTHOR]

Summary: Surface soil moisture plays a key role in regulating a variety of processes associated with water and energy, especially evaporation. Most prevailing methods measure soil water content only for soil blocks or layers of a certain depth, rather than directly on the soil surface. Diffuse reflectance spectroscopy (DRS) provides a way to approximate surface soil water content by utilizing surface reflectance to quantify soil water. The direct measurement of surface water content could potentially improve the understanding and modeling accuracy of the processes associated with surface water dynamics. The present study investigated the reflectance variations in one artificially constructed sample set, one set of natural soil cores, and one set of natural surface soils, with various moisture levels. Results showed that the reflectance decreased non-linearly with an increase in soil moisture at single wavelengths for all three sample sets. The models derived from partial least square (PLS) regression with segmented cross-validation using natural logarithm transformed wave bands or full spectra provided accurate and stable prediction for all moisture ranges studied. Although it is still problematic to construct a single, universal model for surface soil moisture, segmented models based on soil property classes (i.e. texture, color) are able to accurately predict surface soil water content from measured reflectance. Specific calibrations for soils with similar soil texture or color are expected to provide promising prediction of surface soil moisture. [ABSTRACT FROM AUTHOR]

Summary: The main goal of this study was to evaluate different methods for the estimate of the evaporation from a bare soil. The starting point was the experimental data set of a field survey carried out for a period of 16months in the area of Parco Lambro, in Milano (Italy), where meteorological and soil measurements were collected to estimate the local recharge to the phreatic aquifer. The evaporation from the nearly bare soil was assessed with two different approaches. A first, indirect approach was based on the evaluation of the actual evaporation as a fraction of the potential evaporation, which was computed with both Penman’s and Hamon’s equations. As a second approach, the Bowen ratio method, that directly estimates the actual evaporation from atmospheric data, was applied. Attention was mainly focused to the effects of the configuration of the measurement instruments (particularly the measurement heights of air temperature and humidity) on the estimate of the potential and actual evaporation and to the numerical stability of each method, in relation to the measurement uncertainty. In particular, numerical problems related to the intrinsic instability of the Bowen ratio equation were deeply investigated and an original method, that identifies the data leading to unphysical results on the basis of the error propagation, was proposed. The obtained results were discussed in order to evaluate the suitability of each approach, taking into account several aspects. Penman’s and Hamon’s methods resulted to be quite stable with respect to the measurement uncertainty and the potential evaporation estimates were not affected by the configuration of the field instruments. Conversely, the Bowen ratio estimates were affected by large uncertainties due to both the intrinsic numerical instability of the method and to the sensitivity to the heights of measurement; nevertheless, the application of a suitable criterion for the data rejection allowed for a significant reduction of uncertainties on the estimates of the actual evaporation. [Copyright &y& Elsevier]

Summary: This paper is a review of physically-based rainfall interception modelling. Fifteen models were selected, representing distinct concepts of the interception process. Applications of these models to field data sets published before March 2008 are also analysed. We review the theoretical basis of the different models, and give an overview of the models’ characteristics. The review is designed to help with the decision on which model to apply to a specific data set. The most commonly applied models were found to be the original and sparse Gash models (69 cases) and the original and sparse Rutter models (42 cases). The remaining 11 models have received much less attention, but the contribution of the Mulder model should also be acknowledged. The review reveals the need for more modelling of deciduous forest, for progressively more sparse forest and for forest in regions with intensive storms and the consequent high rainfall rates. The present review also highlights drawbacks of previous model applications. Failure to validate models, the few comparative studies, and lack of consideration given to uncertainties in measurements and parameters are the most outstanding drawbacks. Finally, the uncertainties in model input data are rarely taken into account in rainfall interception modelling. [Copyright &y& Elsevier]

Summary: Stable water isotopes such as oxygen 18, are tracers of water movement within the soil–vegetation–atmosphere system. They provide useful information for a better understanding of evaporation and water vapor transport within soils. In part I of this paper, we presented a novel control experimental set-up under non steady conditions, dedicated to the measurement of the evaporation flux and corresponding isotopic composition from six bare soil columns. Data analysis raised several questions about the soil depth controlling the isotopic composition of the evaporated water vapor, suggesting different behavior before and after the appearance of back diffusion. Experimental data also suggested a time variable value of the kinetic fractionation factor. The present paper presents the modeling of the experimental results using the coupled heat, water and stable isotope transfer model SiSPAT_Isotope. Model results were used for investigating the above questions more in details. For this purpose, model parameters were calibrated for each soil column in order to reproduce the data. Then model results were inverted to estimate the kinetic fractionation factor. The results show that the hypothesis that the kinetic fractionation factor varies in time is plausible but the uncertainty is too large to derive firm conclusions. The largest uncertainty is found when the soil relative humidity is lower than one but water vapor is still negligible. When back diffusion has occurred, model results are the most robust and confirm that the isotopic composition of the evaporated water vapor is controlled by the soil isotopic composition of the liquid water at the peak. In this case, the retrieved kinetic fractionation factor is close to 18.9‰, corresponding to laminar flow. [Copyright &y& Elsevier]

Summary: Stable water isotopes such as oxygen 18, are natural tracers of water movement within the soil–vegetation–atmosphere continuum. They provide useful information for a better understanding of evaporation and water vapor transport within soils. This paper presents a novel controlled experimental setup. It is dedicated to detailed measurements of the evaporation fluxes from bare soil columns, as well as to the corresponding isotopic composition of the water vapor, under non-steady state conditions. The experiment allowed an accurate determination of these quantities. The formulae encountered in the literature were used to estimate the isotopic composition of the evaporated water vapor. None of them was able to correctly reproduce the measured isotopic composition of water. The data were then used to estimate the value of the isotopic composition of the soil liquid water, which should be used to get the right results for the isotopic composition of the evaporated water vapor. Results suggest that, when liquid transfer is dominant within the soil, the isotopic composition of evaporation was controlled by the isotopic composition of the liquid water within very thin soil surface layers. When there is a peak in the isotopic profile, i.e. when water vapor is dominant close to the surface, the isotopic composition of the evaporated water seems to be governed by the isotopic composition of the soil liquid water at the peak. The data were also used to estimate the kinetic fractionation factor. The results suggest that the latter is not constant in time. The values seem to depend on the shape of the isotopic profile. In both cases, the uncertainty on the results is very large. The estimation of the kinetic fractionation factor is studied more in details using the modeling results presented in Part II of a companion paper where the data set is modeled using the SiSPAT_Isotope model. [Copyright &y& Elsevier]

Summary: Groundwater from unconfined chalk aquifers constitutes a major water resource in the UK. The unsaturated zone in such systems plays a crucial role in the hydrological cycle, determining the timing and magnitude of recharge, and the transport and fate of nutrients. However, despite more than three decades of study, our physical understanding of this system is incomplete. In this research, state of the art instrumentation provided high temporal resolution readings of soil moisture status, rainfall and actual evaporation from two sites in the Pang and Lambourn catchments (Berkshire, UK), for a continuous two year period (2004/5). A parsimonious, physically based model for the flow of water through the chalk unsaturated zone, including a novel representation of the soil and weathered chalk layers, was developed. The parameters were identified by inverse modelling using field measurements of water content and matric potential. The model was driven by rainfall and evaporation data, and simulated fluxes throughout the profile (including the discrete matrix and fracture components), down to the water table (but not the water table response). Results showed that the model was able to reproduce closely the observed changes in soil moisture status. Recharge was predominantly through the matrix, and the recharge response was strongly attenuated with depth. However, the activation of fast recharge pathways through fractures in the chalk unsaturated zone was highly sensitive to rainfall intensity. Relatively modest increases in rainfall led to dramatically different recharge patterns, with potentially important implications for groundwater flooding. The development and migration of zero flux planes with time and depth were simulated. The simulations also provided strong evidence that, for water table depths greater than 5m, recharge fluxes persist throughout the entire year, even during drought conditions, with important implications for the calculation of specific yield from baseflow estimates and the representation of recharge in groundwater models. [Copyright &y& Elsevier]

Summary: The quantification of soil evaporation and of soil water content dynamics near the soil surface are critical in the physics of land-surface processes on regional and global scales, in particular in relation to mass and energy fluxes between the ground and the atmosphere. Although it is widely recognized that both liquid and gaseous water movement are fundamental factors in the quantification of soil heat flux and surface evaporation, their computation is still rarely considered in most models or practical applications. Moreover, questions remain about the correct computation of key factors such as the soil surface resistance or the soil surface temperature. This study was conducted to: (a) implement a fully coupled numerical model to solve the governing equations for liquid water, water vapor, and heat transport in bare soils, (b) test the numerical model with detailed measurements of soil temperature, heat flux, water content, and evaporation from the surface, and (c) test different formulations for the soil surface resistance parameter and test their effect on soil evaporation. The code implements a non-isothermal solution of the vapor flux equation that accounts for the thermally driven water vapor transport and phase changes. Simulated soil temperature, heat flux, and water content were in good agreement with measured values. The model showed that vapor transport plays a key role in soil mass and energy transfer and that vapor flow may induce sinusoidal variations in soil water content near the surface. Different results were obtained for evaporation calculations, depending on the choice of the soil surface resistance equation, which was shown to be a fundamental term in the soil–atmosphere interactions. The results also demonstrated that soil water dynamics are strongly linked to temperature variations and that it is important to consider coupled transport of heat, vapor and liquid water when assessing energy dynamics in soils. [Copyright &y& Elsevier]

Summary: Regional estimates of aquifer recharge are needed in data-scarce regions such as the Basin of Mexico, where nearly 20 million people are located and where the Basin’s aquifer system represents the main water source. In order to develop the spatio-temporal estimates of aquifer recharge and to analyze to what extent urban growth has affected aquifer recharge, this work presents a daily soil water balance which uses different vegetation and soil types as well as the effect of topography on climatological variables and evapotranspiration. The soil water balance was applied on a daily time step in the Basin of Mexico for the period 1975–1986, obtaining an annually-lumped potential recharge flow of 10.9–23.8m3/s (35.9–78.1mm) in the entire Basin, while the monthly values for the year with the largest lumped recharge value (1981=78.1mm) range from 1m3/s (0.3mm) in December to 87.9m3/s (23.7mm) in June. As aquifer recharge in the Basin mainly occurs by subsurface flow from its enclosing mountains as Mountain Block Recharge, urban growth has had a minimal impact on aquifer recharge, although it has diminished recharge in the alluvial plain. [Copyright &y& Elsevier]

Summary: The extent to which climate change will affect evapotranspiration and water deficits is still uncertain. Temperature increase was recently shown to lead to enhanced drought in the Netherlands. In contrast, experimental evidence shows that elevated atmospheric CO2 concentrations tend to reduce stomatal opening in plants. This leads to lower transpiration rates, although models of atmospheric and soil water feedback show that reductions may be smaller than expected from stomatal closure. We combined the various effects and feedbacks. First, we inferred partial corrections on ‘crop factors’ used in simple evaporation equations such as Makkink’s, for a range of crops and vegetation types in the Netherlands. Second, we applied these corrected factors to infer the likely effects on water deficits in the Netherlands, using a coupled set of hydrological models and national climate scenarios. The combined effects of CO2 on evapotranspiration are generally modest, between a reduction of a few percent for short crops to about 15% for tall, rough vegetation. These reductions are, however, of comparable but opposite magnitude to predicted temperature-induced increases in evapotranspiration. We show that, if combined within the coupled hydrological model, the CO2-effect would lead to a much-reduced desiccating effect of climate change. In general, it is argued that, especially for sub-regional spatial scales and seasonal time-scales, CO2 is likely to be a significant factor in the water balance even of relatively wet regions. [Copyright &y& Elsevier]

Summary: The Atacama Desert is hyper-arid, and areas where adequate moisture exists for evaporation are spatially highly restricted. Nevertheless, water resources exist and their evaluation requires knowledge of this elusive but important component of the hydrological cycle. Evaporation may occur in four typical areas: rivers and associated riparian zones, localized springs, large playas and extensive areas of bare soil after infrequent precipitation events. Transpiration is locally possible where moisture is sufficiently close to the surface to allow phreatophytes or scarce grass cover to grow, but virtually no information is available for quantification. Pan evaporation data from 11 stations for the period 1977–1991 is analyzed and complemented by analysis of an evaporation study conducted in the Salar de Atacama during 1987/1988. The results show that pan evaporation, and hence maximum potential evaporation may be considered largely a function of maximum temperature and elevation as well as density of the evaporating fluid. Actual evaporation is limited by available moisture and diminishes rapidly as the level of soil moisture saturation drops below the soil surface, extinguishing at ca. 2m depth. Evaporation is greatest during the summer, but at higher elevations convective cloudiness develops during January and February reducing evaporating rates at a time when significant precipitation may occur. Inter-annual variations in pan evaporation are considerable and weakly correlated with ENSO, but variations in actual evaporation are damped by comparison. Regression equations are developed which have widespread applicability and may be used to estimate evaporation in areas where no site-specific data exists. [Copyright &y& Elsevier]

Summary: This study was conducted in the Owens Valley, California to determine the relative contribution of groundwater and soil water to evapotranspiration (ET) from phreatophytic meadow and scrub plant communities. Groundwater uptake during the growing season was estimated from the difference between ET measured using eddy covariance, and the sum of soil water depletion, precipitation, and evaporation from the water table. Total ET during the growing season (March 26 to October 15) ranged from 53 to 646mm among all sites and years. ET during winter was small, averaging approximately 40mm. Estimates of evaporation from the water table based on soil properties and ET measurements at night both suggested this flux was a small component of the water balance. For alkali meadows with water table depths of 1–3m, groundwater uptake accounted for 60–81% of ET. Shrub-dominated sites had lower cover and transpiration, and relied less on groundwater than meadows. Groundwater uptake was correlated with water table depth and leaf area index (r 2 =0.62 and 0.70, respectively) even though water table depth and vegetation cover were less correlated (r 2 =0.44). A slightly higher correlation was observed between groundwater uptake per unit leaf area and water table depth (r 2 =0.73). Annual ET results from this study could assist the management of groundwater pumping in areas of phreatophytic vegetation by improved accounting of the sources of ET as vegetation leaf area and water table depths vary. [Copyright &y& Elsevier]

Summary: Oxygen isotopes in precipitation and soil water were monitored at two sites along a climate gradient on the east side of the Cascade Mountains (Washington, USA). Precipitation at both sites is dominated by snowfall in the winter months. δ 18O values of precipitation show a typical seasonal variation from around −17‰ in the winter to around −8‰ in the summer at the western site (Cle Elum), and slightly lower values at the eastern site (Ellensburg). Soil water isotope composition and moisture contents relate to hydrological season: (1) late summer to fall – cooling, increased soil moisture; (2) winter – snow cover; (3) late winter to early spring – snow melts, recharge of soils and groundwater; (4) late spring – initial drying of soils. A simple mass balance model that conserves isotopes and mass is used to constrain the rates of water loss due to evaporation and non-fractionating losses (transpiration and downward percolation) during these hydrological seasons. Non-fractionating losses at both sites are similar during the spring at approximately 0.8–1.0mm/day. In contrast, during the fall, these non-fractionating losses are near zero at the drier site (Ellensburg) and remain at ≈0.8mm/day at the wetter site (Cle Elum). The model yields evaporation rates between 0.01 and 0.2mm/day for Ellensburg and between 0.2 and 0.9mm/day for Cle Elum. [Copyright &y& Elsevier]

Abstract: Soil moisture content, θ, of a bare and vegetated UK gravelly sandy loam soil (in situ and repacked in small lysimeters) was measured using various dielectric instruments (single-sensor ThetaProbes, multi-sensor Profile Probes, and Aquaflex Sensors), at depths ranging between 0.03 and 1m, during the summers of 2001 (in situ soil) and 2002 (mini-lysimeters). Half-hourly values of evaporation, E, were calculated from diurnal changes in total soil profile water content, using the soil water balance equation. For the bare soil field, Profile Probes and ML2x ThetaProbes indicated a diurnal course of θ that did not concur with typical soil physical observations: surface layer soil moisture content increased from early morning until about midday, after which θ declined, generally until the early evening. The unexpected course of θ was positively correlated to soil temperature, T s, also at deeper depths. Aquaflex and ML1 ThetaProbe (older models) outputs, however, reflected common observations: θ increased slightly during the night (capillary rise) and decreased from the morning until late afternoon (as a result of evaporation). For the vegetated plot, the spurious diurnal θ fluctuations were less obvious, because canopy shading resulted in lower amplitudes of T s. The unrealistic θ profiles measured for the bare and vegetated field sites caused diurnal estimates of E to attain downward daytime and upward night-time values. In the mini-lysimeters, at medium to high moisture contents, θ values measured by (ML2x) ThetaProbes followed a relatively realistic course, and predictions of E from diurnal changes in vertically integrated θ generally compared well with lysimeter estimates of E. However, time courses of θ and E became comparable to those observed for the field plots when the soil in the lysimeters reached relatively low values of θ. Attempts to correct measured θ for fluctuations in T s revealed that no generally applicable formula could be derived. [Copyright &y& Elsevier]

Abstract: Numerical models simulating changes in soil water content with time rely on accurate estimation of root water uptake. This paper considers two root water uptake modules that have a compensation mechanism allowing for increased root uptake under conditions of water stress. These modules, proposed by Lai and Katul and Li et al. [Adv. Water Resour. 23 (2000) 427 and J. Hydrol. 252 (2001) 189] use potential transpiration weighted, for each soil layer, by a water stress and a compensation function in order to estimate actual transpiration. The first objective of the paper was to assess the accuracy of the proposed root extraction modules against two existing data sets, acquired under dry conditions for a winter wheat and a soybean crop. In order to perform a fair comparison, both modules were included as possible root water extraction modules within the Simple Soil Plant Atmosphere Transfer (SiSPAT) model. In this first set of simulations, actual transpiration was calculated using the solution of the surface energy budget as implemented in the SiSPAT model. Under such conditions, both root extraction modules were able to reproduce accurately the time evolution of soil moisture at various depths, soil water storage and daily evaporation. Results were generally improved when we activated the compensation mechanisms. However, we showed that Lai and Katul [Adv. Water Resour. 23 (2000) 427] module was sensitive to soil hydraulic properties through its water stress function, whereas the Li et al. [J. Hydrol. 252 (2001) 189] module was not very sensitive to the specification of its parameter. The latter module is therefore recommended for inclusion into a larger scale hydrological model, due to its robustness. When water balance models are run at larger scales or on areas with scarce data, actual transpiration is often calculated using models based on potential transpiration without solving the surface energy balance. The second objective of the paper was to assess the loss of accuracy in such conditions for the Lai and Katul and Li et al. [Adv. Water Resour. 23 (2000) 427 and J. Hydrol. 252 (2001) 189] modules. For this purpose we compared results from the SiSPAT model solving the surface energy balance with those of a degraded version where only potential evapotranspiration was imposed as input data. We found that actual transpiration and evapotranspiration were in general underestimated, especially for the Lai and Katul [Adv. Water Resour. 23 (2000) 427] module, when we used the potential evapotranspiration as calculated from FAO standards. The use of crop coefficients improved the simulation although standard values proposed by the FAO were too small. The definition of the potential evapotranspiration was the major source of error in simulating soil moisture and daily evaporation rather than the choice of the root extraction modules or the inclusion of a compensation mechanism. When used for water management studies, a sensitivity to the definition of potential evapotranspiration used to run the models is therefore advisable. [Copyright &y& Elsevier]

The evapotranspiration and groundwater recharge from two natural areas with high (oak) and low (heath) vegetation were estimated by calibrating a semi-physical numerical soil water and heat model to fit 8 and 7 years of TDR-measurements of water content, respectively. The measurements were made between the surface and 7 m depth. For the oak stand, the estimated annual recharge for the years 1992–1999 is 390 mm, the evaporation from soil and interception is 205 mm, and the transpiration is 285 mm. For the heath area estimation was carried out for the years 1993–1999. However, the heath was struck by a heavy beetle attack in 1994, which strongly affected the vegetation and thus the water balance for the following 3 years. For years not affected, the estimated recharge is 733 mm (about 50% larger than for the oak stand for the same years), the evaporation is 316 mm, and the transpiration is 128 mm. The estimated recharge values compare fairly well to estimates obtained from bromide tracer experiments. However, the recharge estimates obtained from the tracer experiments are very uncertain. The uncertainty is mainly due to spatial heterogeneity making the three replicate samples taken here for each time and depth insufficient.The analyses of TDR-measurements and tracer data showed that water front movement depends on the antecedent soil water content. Some layers are bypassed, especially at low water contents, and at high soil water contents preferential flow was observed at the heath site. [Copyright &y& Elsevier]

Soil hydrological processes play an important role in land-atmosphere system. In most climate models, these processes are described by soil moisture variations in the first 2 m of soil resulting from precipitation, evaporation, and transpiration. Groundwater effects on soil moisture variations and surface evaporation are either neglected or not explicitly treated. Although groundwater may have a small effect on soil moisture in areas with a deep groundwater table, groundwater can act as a soil water source and have substantial effects in areas where the water table is near or within a model''s soil column. How groundwater affects soil moisture, its vertical distribution, as well as the surface water flux are the issues addressed in this study.A soil hydrological model was developed to include groundwater effects by allowing water exchange between the unsaturated zone and groundwater. The model uses a vertically varying saturation hydraulic conductivity, and is evaluated using observations at one station in the Nebraska Sand Hills. Model results show its ability to describe the roles of groundwater in maintaining the observed soil moisture, especially in deep layers. In addition, comparisons show that the soil moisture content in the first meter of the soil column from the model with groundwater is 21% greater than that from a model without groundwater. High soil moisture content in the root zone results in increased evapotranspiration (ET). The average ET in three periods from 1998 to 2000 is 7–21% higher when groundwater is considered in the model. Because of the groundwater effects, spatial variations in the groundwater table can create an additional spatial variability of soil moisture and surface water flux. This additional variability could be important in development of storms in regions whose domain has a large portion with high groundwater table. [Copyright &y& Elsevier]

In this paper, we analyse the spatio-temporal variability of instantaneous and cumulated evapotranspiration (ET) fluxes measured during two long drying periods with a soil water balance approach on 28 point measurements located within a small maize cropped field. Then we test three sampling procedures to estimate field averaged ET and their associated uncertainty. Results of this study show that the ‘within-field’ spatial variability of the local scale instantaneous measured ET is large with coefficients of variation ranging between 15.5 and 46.3%. The corresponding cumulated ET for the first and the second period ranges between 39.4–107.5 and 30.3–55.7 mm, respectively. We show that this variability of ET fluxes can be related in some way to the measured spatially variable crop growth (2.2