Abstract
Previous land surface modeling efforts to predict and understand water budgets in the U.S. Southeast for soil water management have struggled to characterize parts of the region due to an extensive presence of fragipan soils for which current calibration approaches are not adept at handling. This study presents a physically based approach for calibrating fragipan-dominated regions based on the “effective” soil moisture capacity concept, which accounts for the dynamic perched saturation zone effects created by the low hydraulic capacities of the fragipan layers. The approach is applied to the Variable Infiltration Capacity model to develop a hydrologic model of the Obion River Watershed (ORW), TN, which has extensive fragipan coverage. Model calibration was performed using observed streamflow data, as well as evapotranspiration and soil moisture data, to ensure correct partitioning of surface and subsurface fluxes. Estimated Nash-Sutcliffe coefficients for the various sub-drainage areas within ORW were all greater than 0.65, indicating good model performance. The model results suggest that ORW has a high responsivity and high resilience. Despite forecasted temperature increases, the simulation results suggest that water budget trends in the ORW are unlikely to change significantly in the near future up to 2050 due to sufficient precipitation amounts.
Highlights
The soil water balance (Figure 1) is a prime control of the structure and function of intensively managed agroecosystems, driving the dynamics of both soil microbial communities, and crops [1].Stress resulting from soil moisture deficits nearly halts nutrient cycling [2], while anoxic conditions that develop if the soil is too wet can alter microbial decomposition processes [3]
The combination of poor drainage and relatively shallow depth to the fragipan layer creates a very flashy hydrologic system [37], especially during more intensive rainfall events, in areas where the fragipan soils are close to the surface
As explained in the introduction, a shortcoming of these studies is the assumption of a static soil moisture capacity distribution, which is not directly applicable to watersheds with distributions of fragipan soils that vary in space, both laterally and with depth
Summary
The soil water balance (Figure 1) is a prime control of the structure and function of intensively managed agroecosystems, driving the dynamics of both soil microbial communities, and crops [1]. Estimating the b-parameter in this case requires an approach based on the effective moisture capacity that considers the hydraulic conductivity in addition to the depth to the restrictive layer Such an approach has not been examined to date, despite the extensive coverage of fragipan soils in Southeastern U.S This could potentially explain past difficulties/biases in simulating streamflows in the U.S Southeast with land surface models. Unless the ranges provided are physically based and restricted to capture for example the effective moisture capacity needed to represent fragipan soils, uncertainties in model predictions related to the calibration parameters will persist even if observed data are perfectly matched by model simulations [27,28,29].
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