Groundwater Evapotranspiration Research Articles

Revealing mechanisms regulating diurnal groundwater level fluctuation in a temperate fen peatland: A spatiotemporal exploration

Diurnal groundwater level fluctuations (DGLF), closely tied to the metabolic rhythm of wetland vegetation, offer insights into direct groundwater consumption. This study is centered on exploring the spatial and temporal patterns of DGLF within the riparian aquifer of the Upper Biebrza region, motivated by the need to understand those sub-daily ecohydrological dynamics at play in this ecosystem, on a process-oriented scale. Through integrated spatiotemporal multivariate analysis, we aim to identify the key potential factors driving variations in these fluctuations across the landscape gradient, as well as, at different time scales. The study employed a comprehensive approach to gather data on groundwater heads and direct groundwater fluxes, utilizing high-temporal-resolution wells within a monitoring network. Meteorological variables from a dedicated station and high-resolution remote sensing maps were also integrated into the dataset. Analysis revealed distinct seasonal patterns and correlations in diurnal groundwater fluctuations, closely correlated with air temperature, solar radiation and subsequently vegetation phenology. Soil moisture content and summer rainfall events also influenced the intensity of these diurnal fluctuations. Dry periods intensified fluctuations, indicating an elevated reliance on groundwater by phreatophytes, while fluctuations decreased after rainfall events, signaling a shift in vegetation’s water source preference to soil moisture. Based on integrated data interpretation, a couple of potential mechanisms, reasonably forming the spatial variation pattern of DGLF, were formulated. Notably, the influence of local hydraulic gradients at transitional forest landscape edges, where higher amplitude fluctuations occurred, compared to lower fluctuations at midpoints. The proximity to the peat-mineral interface influences diurnal fluctuations, with wells closer to the interface showing sustained high-amplitude fluctuations driven by higher rates of recharge. Additionally, the influence of river proximity was explored, revealing dampened fluctuations in wells closer to the river due to rapidly changing hydraulic gradients. Further systematic experimentation, including numerical and data-driven modeling, is needed to validate these hypotheses. The study findings provide perspectives into (DGLF) patterns and distributions, enabling more accurate groundwater evapotranspiration (ETg) estimation and opening new avenues for considering ecohydrological feedback regarding carbon–water interaction, in relation to the daily-scale water table position.

Wastewater irrigation beneath the water table: analytical model of crop contamination risks

Wastewater irrigation alleviates freshwater scarcity. However, conventional (near)surface irrigation techniques directly expose crops to contaminants. Irrigating wastewater into shallow phreatic zones to raise the water table enhances groundwater evapotranspiration, while using the vadose zone as a bioreactor that attenuates contaminants through dilution, adsorption, and biodegradation. Nevertheless, contaminants may spread across the groundwater, soil, and vegetation. In this study, we focus on the crop contamination risks, and derive a simple analytical model to estimate crop solute uptake. Although crops are not directly exposed to the irrigated wastewater, contaminants (and nutrients) may spread to the root zone. Results show that crop contamination is primarily determined by the root zone water balance, and by solute dispersion and biogeochemical reaction parameters. The model contributes towards identifying hydrogeologically and climatically suitable locations for phreatic zone wastewater irrigation, determining acceptable levels of irrigation water quality, and evaluating crop contamination hazards against the fertigative value of wastewater.

Estimation of Groundwater Evapotranspiration and Extinction Depth Using Diurnal Water Table Fluctuation and Remote Sensing Observations: A Case Study in Agriculture-Dominated Watershed of Eastern India

Estimation of Groundwater Evapotranspiration and Extinction Depth Using Diurnal Water Table Fluctuation and Remote Sensing Observations: A Case Study in Agriculture-Dominated Watershed of Eastern India

Groundwater Recharge from Below under Changing Hydro-Meteorological Conditions in a Forested and Grassland Site of the Great Hungarian Plain

The process of groundwater evapotranspiration and its subsequent recharge are fundamental aspects of the Earth’s natural water cycle and have significant implications for the preservation and functionality of various forested ecosystems. This study presents a case analysis examining the recent fluctuations in groundwater levels and their replenishment in two wells situated at a designated forested experimental area and a control site. The magnitude and temporal fluctuations of groundwater recharge were examined through the utilisation of a novel adaptation of the traditional White method, which was specifically tailored to the local context. We also tested the sensitivity of the White method as an indicator of the system’s behaviour because the signal has changed in relation to the access of the forests to groundwater under the conditions of regionally declining groundwater resources and a warming climate. The novelty of this approach is found in the examination of the temporal fluctuations in groundwater recharge, which are influenced by both a decrease in groundwater levels caused by forest evaporation in response to climate change and a regional reduction in groundwater supplies. As a result, the ongoing decrease in groundwater levels may have significant adverse effects on local forests.

Surface-groundwater interactions in hard rock, water-limited environments, simulated at very fine scale using long time-series observations and hydrotope-modeling concept

Hard rock systems are typically characterized by undulated landscapes, short flow path, dense drainage network, and low aquifer storage. If located in water-limited environments (WLE), then typically have variable rainfall characteristics, frequent floods and droughts, and savannah type of vegetation capable to uptake groundwater. In such environments, surface–groundwater interactions are of primary importance. In this study, surface–groundwater interactions are analyzed focusing on spatiotemporal net recharge dependence on hydrotopes, where a hydrotope is conceptualized as spatial unit (or object), which at a given scale of assessment can be assumed as internally homogeneous (e.g. tree canopy or rock outcrop), but hydro(geo)logically different than the adjacent area. The study was carried out in the small (7.6 ha) Trabadillo study area (TSA) in Spain, representative for hard rocks of WLE, using 3D-GSFLOW integrated hydrological model (IHM) at very fine grid (5x5m), extracted from coarser grid (100x100m) of Sardon 3D-GSFLOW model (Hassan et al., 2014, Journal of Hydrology 517, 390–410). The IHM was calibrated using: (i) 20-year head observation; (ii) 8-year soil moisture observations at 4 profiles, each at different hydrotope and each at 4 different depths; and (iii) MODIS satellite earth observation of evapotranspiration. Despite applying spatially uniform rainfall as forcing, surface–groundwater interactions were highly spatially variable, primarily, because of hydrotope-differences in rainfall interception loss and secondarily, because of differences in transpiration, in subsurface evaporation and in hydrological properties of subsurface. The high temporal variability of rainfall, ranging yearly from 310 to 925 mm, resulted in high temporal variability of all water fluxes, including groundwater fluxes; e.g. gross recharge (Rg) varied annually from 2 to 445 mm, net recharge (Rn), from −50 mm to + 89 mm, while the 20-year mean Rn was close to zero, because Rg was counterbalanced by groundwater exfiltration and less by groundwater evapotranspiration. The high temporal variability of groundwater fluxes and many dry years with largely negative Rn, in combination with low aquifer storage, emphasized TSA dependence on rainfall and its vulnerability to droughts. The hydrotope modeling concept applied at the very fine scale, emphasized also large Rn dependence on land cover type, especially on density and species composition of trees, invisible at coarser scale modeling. As such, the proposed hydrotope modelling approach can be applied as a tool to test whether in a given environment planting (or cutting) trees will result in increase or decrease of water resources.

Climatic Control on Mean Annual Groundwater Evapotranspiration in a Three‐Stage Precipitation Partitioning Framework

AbstractA three‐stage precipitation partitioning framework is proposed to study the climate controls on mean annual groundwater evapotranspiration (GWET) for 33 gauged watersheds in west‐central Florida. Daily GWET, total evapotranspiration (ET), groundwater recharge, base flow, and total runoff are simulated by the Integrated Hydrologic Model, which dynamically couples a surface water model (HSPF) and a groundwater flow model (MODFLOW). The roles of GWET on long‐term water balance are quantified by four ratios. The ratios of GWET to total available water, watershed wetting, ET, and recharge decrease exponentially with watershed aridity index (WAI), which is defined as the ratio of potential evapotranspiration to total available water. In the one‐stage precipitation partitioning framework, the contribution of GWET to the ratio between total ET and available water for ET (i.e., the y‐axis of Budyko curve) decreases with WAI. In the two‐stage precipitation partitioning framework, the contribution of GWET to the ratio between total ET and watershed wetting (i.e., Horton index) decreases with WAI. The changes in GWET caused by intra‐monthly (IM) climate variability are the highest among the temporal scales of climate variability investigated to understand controls on GWET. The inter‐annual, intra‐annual, and IM climate variabilities lead to increase of GWET; but the sub‐daily climate variability results in decrease of GWET. For the third stage of partitioning, given the same ratio of potential GWET to available water for GWET, higher percentage of forest and wetland and lower percentage of impervious land contribute to higher ratio of GWET to available water for GWET.

Snížení základního odtoku kvůli evapotranspiraci povrchové a podzemní vody z mokřadu v povodí Liběchovky: porovnání měření a výpočtu Oudinovou metodou

This study is focused on the base flow decrease due to surface water and groundwater evapotranspiration (ET) in wetlands of the Liběchovka catchment. Evapotranspiration in wetlands can significantly affect stream discharge, and its influence will probably still increase in the future due to the global rise of temperatures caused by climate change. The study site is located in the upper part of Liběchovka catchment (Fig. 1). This wetland hosts a small stream supplied by rather steady groundwater inflow, representing the favourable site for detection of the effect of ET. V notch weir and two piezometers were used to monitor stream discharge and wetland water table in 30 minute periods (Fig. 2). Diurnal periodical oscillations of water flow and water table caused by ET were observed (Fig. 3). They occurred in the summer part of the year, especially on sunny days without rain. The amplitude of water table level oscillation was increasing with increasing temperature (Fig. 4). Calculation showed that evapotranspiration reduced the water flow, on average, by 15 % on the warm sunny days. The maximum daily reduction was up to 32 %. Direct measurements were compared with the potential ET calculated using the Oudin’s method (Oudin 2005). The ET from the measurement was 45 % of the potential ET calculated by Oudin’s method. In the next step, the Oudin’s method was used to calculate the potential ET of all wetlands in Liběchovka catchment upstream from the Želízy village, where gauging station of ČHMÚ is situated. The original discharge not reduced by the wetland ET was defined as the sum of the wetland potential ET and the mean annual measured discharge. Due to the ET from wetlands (years 2015–2020), the discharge was reduced on average by 13 %. If only the summer months of July and August were considered, it was reduced, on average, by 26 % and by 39 % in the driest day recorded in this period. To estimate the role of groundwater abstraction on the Liběchovka stream during the period of 2015–2020, the sum of measured stream discharge, groundwater abstraction and wetland ET was calculated. From this sum, 71.5 % represented the directly measured water flow, 18.2 % was the groundwater abstraction, and 10.3 % was the wetland ET (Fig. 8).

The effect of saturation and salts on the rigidity of fine-grained material

Repeated multi-channel analysis of surface waves (MASW) and horizontal to vertical spectral ratio (HVSR) experiments were conducted on a hummock within a large landslide in east-central Utah adjacent to a small wetland. Stakes were placed into the ground to ensure identical array geometries and geophone locations. The landslide is an admixture of fine-grained material (clay, quartz and Na-plagioclase) and intermediate-composition volcanic clasts. A prominent crust of Na-salts covered the hummock surface, produced by evapotranspiration of shallow groundwater associated with the adjacent fen.The experiments produced three primary observations. First, on any given day, the Rayleigh wave frequency spectra were highly reproducible between individual experiments. Second, from May to October the spectra exhibited high variability between experiments conducted weeks apart, especially at frequencies <20 Hz. Third, a persistent subsurface boundary exists across which Vs increases from ~175 to ~300 m/s. However, that boundary migrated over time from a depth of ~5 to ~4 m.The low velocity of the upper ~5 m subsurface layer represents poorly-compacted landslide material, perhaps weakened from high Na+ concentrations. However, the variability in frequency spectra and upward migration of the boundary must have arisen from changing moisture and salt content over time. The fen was instrumented with a piezometer that showed an overall ~1 m decline in water levels from May to October with small, superimposed recharge events resulting from summer monsoon rains.This study leads to two important conclusions. Specific to this site and perhaps others, variable moisture content can affect capillary forces and the proportion of evaporite cements in the shallow subsurface that are manifested in strikingly different frequency spectra and migrating stiffness (Vs) boundaries. More generally, moisture and salt content can affect estimates of subsurface stiffness, including the position of apparent boundaries within unconsolidated materials.

A Review of Advances in Groundwater Evapotranspiration Research

Groundwater evapotranspiration (ETg) is an important component of the hydrological cycle in water-scarce regions and is important for local ecosystems and agricultural irrigation management. However, accurate estimation of ETg is not easy due to uncertainties in climatic conditions, vegetation parameters, and the hydrological parameters of the unsaturated zone and aquifers. The current methods for calculating ETg mainly include the WTF method and the numerical groundwater model. The WTF method often requires data supplementation from the numerical unsaturated model to reduce uncertainty; in addition, it relies on point-monitoring data and cannot solve the spatial heterogeneity of ETg. The ETg calculation module of the numerical groundwater model is set up too simply and ignores the influence from the unsaturated zone and surface cover. Subsequent research breakthroughs should focus on the improvement of WTF calculation theory and the setting up of an aquifer water-table fluctuation monitoring network. The numerical groundwater model should couple the surface remote sensing data with the unsaturated zone model to improve the accuracy of ETg calculation.

Increasing contribution of alluvial groundwater to riparian cottonwood forest water use through warm and dry summers

In semi-arid regions, riparian forests access alluvial groundwater to supplement the limited local precipitation. Understanding the sensitivity of riparian forests to variations in weather and climate, and water resources management is essential for guiding riparian conservation measures, especially on rivers impacted by dams and water withdrawal. We anticipated a seasonal trade-off in ecosystem water sources, with increasing relative groundwater use through the summer to compensate for the reduced availability of the shallow soil moisture. To test this hypothesis, we assessed the temporal patterns of alluvial groundwater uptake by a riparian cottonwood (Populus species) forest along a regulated river in southern Alberta, Canada over a three-year interval with relatively warm and dry weather, and low river flows. In support of our hypothesis, groundwater evapotranspiration, as calculated from diurnal fluctuations in the alluvial water table, displayed relative seasonal consistency until the autumnal leaf senescence period. In contrast, the ecosystem-scale evapotranspiration, measured by eddy covariance, gradually receded through the summer as precipitation inputs that fed the shallow soil layers became limited. The consistent uptake rate of alluvial groundwater by cottonwoods enables riparian forests to maintain high rates of water use and productivity in this semi-arid prairie region. We conclude the consistent groundwater use through the warm and dry summer reflected: (1) perennial river flow that replenished the floodplain aquifer; (2) adaptations of cottonwoods to the riparian hydrology and the semi-arid climate; and (3) environmental flow operations that maintained the minimum river and alluvial groundwater levels. Our study provides information on the riparian forest water sources and water use patterns that could help water managers effectively regulate river flows to support healthy downstream riparian ecosystems.

Hydrogeological processes and hydrochemical effects in the Manas river catchment, Northwest China, over the past 60 years

The arid inland regions around the world are suffering from extreme water shortages and ecological crises due to the impacts of climate change and enhanced human activities. This study used multiple methods including trend and abrupt analysis, correlation analysis, water-balance analysis, and hydrochemical analysis to evaluate the hydrogeological processes and hydrochemical effects in 5 periods in the Manas River catchment, a typical arid inland catchment in China. The results show that groundwater storage in the catchment decreased by 5.45 × 108 m3 over the past 60 years, while glacial melt, snowmelt, and precipitation showed an increasing trend since the 1980 s. A drop in the groundwater level in the alluvial fan and a 28 % reduction in spring discharge in the lower alluvial fan since 1959 were caused by reduced river infiltration owing to the water diversion projects. Meanwhile, the concentrations of SO42- and Cl- in the alluvial fan groundwater have more than doubled over the period 1957–2018 as a result of the decrease of both groundwater level and flow rate. Long-term over-exploitation of groundwater associated with irrigation after 1978 strongly depleted the groundwater storage and altered the flow path of groundwater in the oasis zone. In addition, the expansion of arable land and increased temperatures enhanced evapotranspiration and salinization of shallow groundwater in the oasis and desert zones. Based on the results, we propose making full use of natural underground reservoirs to increase infiltration of river water and flood water in the alluvial fan zone, and reducing the arable land area to ensure the sustainability of water resources and ecology in the middle and lower reaches of the catchment. The results will provide reference for the sustainable development of water resources and environmental protection in arid inland regions and similar catchments worldwide.

Assessing farmers' adaptation responses to water conservation policies through modular recursive hydro-micro-macro-economic modeling

Farmers' adaptation responses to water conservation policies involve a complex decision-making process that depends on a range of criteria, including water availability, profits, and risks, which are in turn dependent on (and might have consequences at) broader scale processes including water systems and the macroeconomy. The non-consideration of the complex interactions between and within natural and human systems often leads to unforeseen consequences and sub-optimal water policy design. There exists a fundamental need to improve our understanding of complex human-water (e.g. hydro-economic) and human-human (e.g. micro-macroeconomic) systems' interactions so to better inform policy-makers. This paper develops an innovative modeling framework for capturing the richness of interactions in complex human-water systems by: i) considering the rationale behind farmers' behavior and responses through microeconomic models; ii) assessing the complex interactions among economic sectors and regions within an economy through macroeconomic models; iii) simulating responses on water cycle dynamics within a river basin by means of hydrologic modeling; and iv) representing the interconnected dynamics and two-way feedback responses between human-water and human-human (through micro-macro-economic) systems. The proposed modeling framework operates through a recursive modular approach built from independent modules which are, in turn, connected through a set of protocols that control the exchange of information. Methods are illustrated considering an incremental agricultural water charging policy in the Spanish part of the Douro River Basin (DRB). Results show that local land use reallocations have an impact on the supply of irrigated (rainfed) agricultural commodities at the macroeconomic level, which further leads to higher (or lower) commodity prices that partially offset changes in crop profitability due to changing water charges and readjustments in the crop portfolio. These, in turn, result in non-linear responses in land and water use with non-trivial impacts on water system's dynamics, where evapotranspiration, surface runoff, and groundwater evapotranspiration are the main hydrologic components affected. We conclude that the integration of hydro-micro-macro-economic modules through a set of protocols can provide crucial information for promoting the efficient design of agricultural water policies.

A New Method of Estimating Groundwater Evapotranspiration at Sub-Daily Scale Using Water Table Fluctuations

Riparian ecosystems fundamentally depend on groundwater, and accurate estimations of groundwater evapotranspiration (ETG) are important for understanding ecosystem functionality and managing regional water resources. Over the past several decades, various methods have been proposed to estimate groundwater evapotranspiration based on water table fluctuations. However, the majority of methods cannot resolve sub-daily variations in ETG. In this study, we proposed a new hydraulic theory-based ETG estimation method at a sub-daily time scale. To evaluate its performance, we employed a variety of measurements (i.e., water table levels, latent heat flux and soil water contents) at a riparian forest (T. ramosissima) in Northwest China from 25 July to 10 October in 2017. The results indicated that the proposed method can successfully estimate ETG at both sub-daily (R2 = 0.75) and daily (R2 = 0.88) time scales, but the variations in the specific yield under different water table conditions should be carefully taken into account. In addition, we investigated the seasonal variations in water uptake source of the riparian plant, and found that it had strong plasticity in water usage during the study period. That is, it consumed approximately equal amounts of soil water and groundwater when soil moisture was available, and tended to consume more groundwater for survival as the soil moisture was depleted. To verify the seasonal patterns of the water uptake of the riparian forest, systematic isotope-based studies are needed in future study.

Evaluating the different methods for estimating groundwater evapotranspiration using diurnal water table fluctuations

Riparian ecosystems fundamentally depend on groundwater, and thus accurate estimates of daily groundwater evapotranspiration rate (ETg) is curial for understanding the interactions between groundwater and vegetation. Up to now, various methods (i.e., White, 1932; Hays, 2003; Soylu et al., 2012) have been proposed to estimate daily ETg using the water table fluctuations. The objective of this paper was to evaluate and compare their performances under various water table conditions in field environments. For this purpose, we employed a variety of measurements (i.e., water table levels, latent heat flux and soil water contents) at a riparian forest (T. ramosissima) in Northwest China in 2017. Our results showed that the White method is suitable for use during the water table declining periods. Also, the selected time intervals could have impacts on the estimations of ETg. The Soylu method and the Hays method performed well under various water table conditions. Thus, it seems that the Hays and Soylu methods are more suitable for long-term ETg estimates in field environments. In addition, the percentage of plant transpired water from groundwater varied during the growing seasons. The riparian plants mainly use soil water during the early growing stages, and it tended to use groundwater during the late growing stages. In the future, more isotope-based studies were needed to verify the seasonal patterns of water uptake of the riparian forest.

A hydrographic method to identify the groundwater net recharge, barometric effect and evapotranspiration from a complicated semidiurnal water table fluctuation

AbstractA hydrographic method was proposed to separate out the hourly scaled groundwater level changes caused by net recharge, barometric effects and evapotranspiration from a semidiurnal water table fluctuation. A characteristic midnight time, with a turning point of the barometric pressure change and high relative air humidity, which meant that neither the barometric effect nor groundwater evapotranspiration occurred, was proposed for determining the net recharge rate rnet. Then, the barometric efficiency fbar was estimated using the other time period without evapotranspiration, and the evapotranspiration rate rETG was finally obtained using the remainder of the water level changes. A case example illustrated that estimation of fbar using the proposed method was more accurate than that using the traditional error analysis method, which may result in a significant underestimation under the condition of the present water level changes. Additionally, the abnormal semidiurnal fluctuations, more specifically, two step‐down fluctuations, which may be a common pattern when the groundwater level is controlled by net recharge, barometric effects and evapotranspiration, can be well understood using the three components separated out. The results also showed that nighttime groundwater evapotranspiration, accounting for an average of 23% of that during the daytime, cannot be ignored.

An approximate analytical solution of depth to water table driven by periodical precipitation and evapotranspiration in shallow groundwater zones

Depth to water table (DWT) for an unconfined aquifer affects a wide range of hydrological, ecological, and agricultural problems such as soil salinization, plant type, and crop output. In a world without human impacts, DWT in shallow groundwater zones is mainly driven by evapotranspiration and precipitation. In the real-world however, human impacts on groundwater hydrological processes are ubiquitous, which hinders one to fully understand the natural groundwater hydrological rule. The purpose of this study is to derive an analytical solution of precipitation-evapotranspiration driven DWT irrespective of the human impacts. Through pretreatment of practical data to eliminate human impacts, we show that the proposed method is efficient in dealing with the real-world problems. Specifically, we establish a mathematical model describing impacts of periodic evapotranspiration and precipitation on DWT. An innovative quasi-analytical solution using the Fourier series is developed and analyzed in detail based on the model. This study indicates that the amplitude of DWT is correlated linearly with the amplitude of precipitation and quadratically with the amplitude of potential evapotranspiration. The phase position of DWT is subject to the phase lag between the precipitation and potential evapotranspiration. In arid regions, the evapotranspiration has a greater influence on the phase of DWT than the precipitation. This quasi-analytical solution is limited for a shallow groundwater situation in which DWT is less than the extinction depth of groundwater evapotranspiration.

Assessing environmental water requirement for groundwater-dependent vegetation in arid inland basins by combining the copula joint distribution function and the dual objective optimization: An application to the Turpan Basin, China

Preserving groundwater-dependent terrestrial ecosystems through environmental water allocation is critical for sustainable development in arid inland basins. Assessing the environmental water requirement is challenging due to the complex relationship between vegetation growth and groundwater depth. This study proposed a new assessment method by combining the copula joint distribution function and the dual objective optimization. The copula joint distribution function was used to describe the relationship between vegetation and groundwater depth instead of the traditional regression analysis. Given an ecological protection target, the conditional probability of achieving the target was estimated using the copula joint distribution. The groundwater depth interval with relatively high probability was suitable for vegetation growth and correspondingly conducive for ecological protection. In addition to ecological protection, the socio-economic water requirement was incorporated into the environmental water assessment, resulting in a dual optimization problem that could be resolved by the ideal point method. The optimization analysis revealed a groundwater depth with a high probability of successful ecological protection and low groundwater evapotranspiration to balance vegetation and human demands for groundwater. The proposed method of environmental water assessment by combing copula joint distribution function and dual objective optimization was applied in the Turpan Basin, an arid inland basin in Northwest China. The environmental groundwater depth ranged between 6 and 20 m, and the optimized interval was 7–8 m. The optimal environmental groundwater depth resulted in a probability of 0.46 to achieve the ecological protection target and annual evapotranspiration of 983 mm. The proposed method was practical and reliable and could be an effective tool for assessing the environmental water requirement of groundwater-dependent vegetation in arid inland basins.

Assessing Long‐Term Changes in Regional Groundwater Recharge Using a Water Balance Model for New Mexico

AbstractIn New Mexico in the arid southwestern United States, groundwater recharge is crucial to sustain groundwater (GW), which is vitally important to life, agriculture, industry, and ecosystems. To better understand the changes in recharge statewide, we explored the changes in groundwater recharge (RE), precipitation (P), surface water inflow (SWin), outflow (SWout), diversions (SWdiv), returns (SWret), and surface water and GW evapotranspiration (SWE and GWET) in five New Mexico counties: Taos, Torrance, Doña Ana, Eddy, and Lea during 1975–2015. The results show that the change‐point of RE was in the 1990s, leading to contrasting tendencies in RE before and after the change‐point with a decreasing rate during the latter one. There was a significant positive relationship between RE and P for Taos. The highest contribution rate (CR) to RE was SWout for Taos, whereas GWET ranked as the top CR for the other counties. Furthermore, the annual CR of GWET to RE increased significantly in all counties except Lea. The above results reveal that P and surface water flows played the dominant role in impacting RE in northern New Mexico where surface water is the primary local water resources, whereas GWET had the highest and increasingly continuous influence in central and southern New Mexico where the surface water is much less than that in northern New Mexico. This study shows that water budgets are important to identify differences in regional hydrological regimes that affect planning to maintain RE to groundwater.

Modified method for the estimation of groundwater evapotranspiration under very shallow water table conditions based on diurnal water table fluctuations

Groundwater evapotranspiration (ETG) is often estimated using the White method from diurnal water table fluctuations. However, under very shallow water table conditions (<1 m in this study), the groundwater replenishment to ET-induced depleted soils during nighttime is significant, but is ignored in the White method. A variably saturated flow model, HYDRUS-1D, was used to elucidate the impact of this omission on the accuracy of ETG under various soils, depths to the water table, climatic conditions, vegetation coverage, and root distributions. We found that the mean ETG was underestimated by 33–69% by the original White method because of the inaccurate estimation of the water table recovery rate during nighttime. The daily hydrostatic equilibrium recovery (Re) term was introduced to modify the original White method by considering the ET-induced disequilibrium in soil moisture. Using the modified method, the estimated ETG approaches its real value, and the mean difference between them is only approximately 2%. The modified method was applied to estimate ETG in a wetland in NW China, resulting in better agreement with potential evapotranspiration measurements than the original White method.

Modeling the behavior of shallow groundwater system in sustaining arid agroecosystems with fragmented land use

Shallow groundwater often presents multiple behaviors in supporting the sustainability of arid agroecosystems. The behaviors are especially important and complicated for the arid irrigated areas with fragmented land-use types. In the view of understanding the role of shallow groundwater in such areas, the MODFLOW and MT3DMS (with the assistance of HYDRUS-1D) were used to simulate groundwater flow and salt transport in a typical observation site of the arid upper Yellow River basin (YRB). The original MODFLOW was modified with introducing a variable expression of specific yield, aiming to improve the modeling accuracy in shallow groundwater environments. The model was calibrated and validated using the observation data of groundwater depth and drainage in 2013 and 2012, respectively. Groundwater evapotranspiration and lateral water-salt exchange among different land-use types were simulated and analyzed; meanwhile, the dry drainage effect of non-cultivated natural land was quantitatively interpreted. Results indicated that the fragmented land-use/planting pattern (with different irrigation scheduling) resulted in significant lateral groundwater flow and water/salt redistribution among different land-use fields during irrigation. The groundwater evapotranspiration ranged at about 160–220 mm for different land-use types, reaching to 0.30–0.45 of plant evapotranspiration (ETpt) during the growing seasons. This effectively supplemented the plant water use in arid climate, especially for the unirrigated cultivated land (during some stages) and the non-irrigated natural land. Meanwhile, the lateral groundwater flow drove salt migrating from the cultivated land to the natural land. The natural land accumulated 27–40% of salts introduced by irrigation, playing a significant role in accommodating salts (i.e. the dry drainage effect). Moreover, our modeling work further implied that the reliable groundwater modeling in arid and shallow groundwater areas required a more detailed model conceptualization and more accurate information of land-use distribution, irrigation scheduling, and local microrelief than the general understanding in the past.