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A three‐dimensional variably saturated flow model was developed for assessing future global water resources and parameterized for groundwater pumping. We applied this model to an actual watershed to verify its validity as an Earth System Model. For global applicability, the parameterization method for multi‐layered groundwater pumping was developed and verified through comparison with observations and MODFLOW results. The parameterization proposed in this study is applicable even when multiple groundwater pumping wells are present within one horizontal computational grid and when the well spans multiple vertical grids. This method can be applied at the global scale without parameters such as the well radius, for which data may be difficult to obtain. The parameterization recreated seasonal and annual variations in the observed values. Furthermore, the results were comparable to those of MODFLOW. However, the calculation results were overestimated relative to the observed values. This overestimation was likely to be due to active groundwater pumping in the Central Valley before the start of the unsteady‐state calculation. Therefore, the groundwater level at the beginning of the unsteady‐state calculation was calculated using observed values, improving reproducibility. Furthermore, as observed groundwater levels are unlikely to be available at the global scale, steady‐state calculations were conducted over 15 and 60 years considering groundwater pumping. However, the results were not as reproducible as those obtained using observed groundwater levels. These results suggest that the groundwater level set at the beginning of the calculation is important for global‐scale groundwater flow calculation. Plain Language Summary Water resources are expected to become strained in the future due to climate change and population growth. The main focus of water resource assessments conducted to date has been on river water. Although groundwater is an important component of water resources, it has generally not been considered. This is largely due to the lack of groundwater models that can be applied at a global scale. Therefore, we developed a new groundwater model that was parameterized for groundwater pumping and validated in plains. The results of this study indicate that parameterization of groundwater pumping can produce reasonable results relative to observed values, and that the groundwater level set at the beginning of the calculation is important for the evaluation and prediction of global water resources. In particular, collection of observed groundwater levels or estimation of groundwater pumping are key steps toward more realistic prediction. Key Points A three‐dimensional variably saturated flow model for use as an Earth System Model was developed and parameterized for groundwater pumping The validation results reproduced observations to some extent and were comparable to those of MODFLOW in the Central Valley The results show that setting the groundwater level at the beginning of the unsteady‐state simulation is important for global‐scale

Saturated hydraulic conductivity (Ksat) is a hydrologic flux parameter commonly used to determine water movement through the saturated soil zone. Understanding the influences of land-use-specific Ksat on the model estimation error of water balance components is necessary to advance model predictive certainties and land management practices. An exploratory modeling approach was developed in the physically based Soil and Water Assessment Tool (SWAT) framework to investigate the effects of spatially distributed observed Ksat on local water balance components using three digital elevation model (DEM) resolution scenarios (30 m, 10 m, and 1 m). All three DEM scenarios showed satisfactory model performance during calibration (R2 > 0.74, NSE > 0.72, and PBIAS ≤ ±13%) and validation (R2 > 0.71, NSE > 0.70, and PBIAS ≤ ±6%). Results showed that the 1 m DEM scenario provided more realistic streamflow results (0.315 m3/s) relative to the observed streamflow (0.292 m3/s). Uncertainty analysis indicated that observed Ksat forcings and DEM resolution significantly influence predictions of lateral flow, groundwater flow, and percolation flow. Specifically, the observed Ksat has a more significant impact on model predictive confidence than DEM resolution. Results emphasize the potential uncertainty of using observed Ksat for hydrological modeling and demonstrate the importance of finer-resolution spatial data (i.e., 1 m DEM) applied in smaller watersheds.

A specific 2-year program to monitor and test both the vadose zone and the saturated zone, coupled with a numerical analysis, was performed to evaluate the overall performance of slurry wall systems for containment of contaminated areas. Despite local physical confinement (slurry walls keyed into an average 2-m-thick aquitard), for at least two decades, high concentrations of chlorinated solvents (up to 110 mg l  − 1 ) have been observed in aquifers that supply drinking water close to the city of Milan (Italy). Results of monitoring and in situ tests have been used to perform an unsaturated-saturated numerical model. These results yielded the necessary quantitative information to be used both for the determination of the hydraulic properties of the different media in the area and for the calibration and validation of the numerical model. Backfill material in the shallower part of the investigated aquifer dramatically affects the natural recharge of the encapsulated area. A transient simulation from wet to drought periods highlights a change in the ratio between leakages from lateral barriers that support a specific scenario of water loss through the containment system. The combination of monitoring and modelling allows a reliable estimate of the overall performance of the physical confinement to be made without using any invasive techniques on slurry wall.

Transport of Cryptosporidium parvum through macroporous soils is poorly understood yet critical for assessing the risk of groundwater contamination. We developed a conceptual model of the physics of flow and transport in packed, tilted, and vegetated soilboxes during and immediately after a simulated rainfall event and applied it to 54 experiments implemented with different soils, slopes, and rainfall rates. Using a parsimonious inverse modeling procedure, we show that a significant amount of subsurface outflow from the soilboxes is due to macropore flow. The effective hydraulic properties of the macropore space were obtained by calibration of a simple two-domain flow and transport model that accounts for coupled flow in the matrix and in the macropores of the soils. Using linear mixed-effects analysis, macropore hydraulic properties and oocyst attenuation were shown to be associated with soil bulk density and rainfall rate. Macropore flow was shown to be responsible for bromide and C. parvum transport through the soil into the underlying pore space observed during the 4-h experiments. We confirmed this finding by conducting a pair of saturated soil column studies under homogeneously repacked conditions with no macropores in which no C. parvum transport was observed in the effluent. The linear mixed-effects and logistic regression models developed from the soilbox experiments provide a basis for estimating macropore hydraulic properties and the risk of C. parvum transport through shallow soils from bulk density, precipitation, and total shallow subsurface flow rate. The risk assessment is consistent with the reported occurrence of oocysts in springs or groundwater from fractured or karstic rocks protected only by shallow overlying soils.

The spatial distribution of soil physical properties is essential for modeling and understanding hydrological processes. In this study, the different spatial information (the conventional soil types map-based spatial information (STMB) versus refined spatial information map (RSIM)) of soil physical properties, including field capacity, soil porosity and saturated hydraulic conductivity are used respectively as input data for Water Flow Model for Lake Catchment (WATLAC) to determine their effectiveness in simulating hydrological processes and to expound the effects on model performance in terms of estimating groundwater recharge, soil evaporation, runoff generation as well as partitioning of surface and subsurface water flow. The results show that: 1) the simulated stream flow hydrographs based on the STMB and RSIM soil data reproduce the observed hydrographs well. There is no significant increase in model accuracy as more precise soil physical properties information being used, but WATLAC model using the RSIM soil data could predict more runoff volume and reduce the relative runoff depth errors; 2) the groundwater recharges have a consistent trend for both cases, while the STMB soil data tend to produce higher groundwater recharges than the RSIM soil data. In addition, the spatial distribution of annual groundwater recharge is significantly affected by the spatial distribution of soil physical properties; 3) the soil evaporation simulated using the STMB and RSIM soil data are similar to each other, and the spatial distribution patterns are also insensitive to the spatial information of soil physical properties; and 4) although the different spatial information of soil physical properties does not cause apparent difference in overall stream flow, the partitioning of surface and subsurface water flow is distinct. The implications of this study are that the refined spatial information of soil physical properties does not necessarily contribute to a more accurate prediction of stream flow, and the selection of appropriate soil physical property data needs to consider the scale of watersheds and the level of accuracy required.

Accurate simulation and prediction of flow and transport of solutes in a heterogeneous vadose zone requires the appropriate hydraulic properties corresponding to the spatial scale of interest. Upscaling techniques are needed to provide effective properties for describing the vadose zone system's behavior with information collected at a much smaller scale. Numerical experiments were performed to investigate the effective unsaturated hydraulic conductivity of soils with different degrees and dimensionalities of heterogeneity. Researchers have extended Matheron's method for determining the hydraulic conductivity of soils with one-dimensional heterogeneity under a saturated condition to unsaturated conditions. In this work, Matheron's method was further extended to the unsaturated soils with two- and three-dimensional heterogeneity. It was found that the first-order approximation of the extended formula is similar to those based on the small-perturbation approach. The extended Matheron's method was verified using multistep numerical experiments of gravity-induced flow into synthetic soils with different degrees of heterogeneity. Results showed that the dimensionality of soil heterogeneity has a significant impact on the effective unsaturated hydraulic conductivity, and the extended Matheron's method can well estimate the effective conductivity of the soils with multidimensional heterogeneity.

Hydrologic modelling is pre-requisite to water resources management. Unfortunately, hydrologic modelling in data scare basin has always been difficult. The current study, explored the use of “data limited” model Soil Water Assessment Tool (SWAT) in modelling lower Aswa basin located in northern Uganda. The study adopted different techniques in generating and estimating various missing model parameters and input especially solar radiation, saturated soil hydraulic conductivity, available soil water content, Universal Soil Lost Equation erodibility factor and moist soil albedo. Soil Water Assessment Tool model was then manually calibrated using monthly historical streamflow records. The calibration was successful with coefficient of determination (R 2 ) value of 0.618 and the Nash and Sutcliffe efficiency value of 0.47. Validation of the calibrated model using independent dataset shows even better model performance with Nash and Sutcliffe efficiency value of 0.64 and coefficient of determination (R 2 ) value of 0.56. Successful calibration of hydrologic model Soil Water Assessment Tool under the data scarcity still proves the potential of the application of the model even in data limited basin, but more especially by water resources managers who needs understanding of existing condition and modelling possible future.

Multi-compartment samplers (MCSs) measure unsaturated solute transport in space and time at a given depth. Sorting the breakthrough curves (BTCs) for individual compartments in descending order of total solute amount and plotting in 3D produces the leaching surface. The leaching surface is a useful tool to organize, present, and analyze MCS data. We present a novel method to quantitatively characterize leaching surfaces. We fitted a mean pore-water velocity and a dispersion coefficient to each BTC, and then approximated their values by functions of the rank order of the BTCs. By combining the parameters of these functions with those of the Beta distribution fitted to the spatial distribution of solutes, we described an entire leaching surface by four to eight parameters. This direct characterization method allows trends to be subtracted from the observations, and incorporates the effects of local heterogeneity. The parametric fit creates the possibility to quantify concisely the leaching behavior of a soil in a given climate under given land use, and eases the quantitative comparison of spatio-temporal leaching behavior in different soils and climates.