Improving Estimates of Recharge in the Mississippi Alluvial Plain Using the Soil-Water-Balance (SWB) Model

Author(s): Westenbroek, S.; Ladd, D.

Independently derived estimates of recharge calculated from spatially explicit inputs can significantly improve evaluations of groundwater movement. The U.S. Geological Survey’s Soil-Water-Balance (SWB) code is a tool to estimate distribution and timing of net infiltration (recharge) out of the root zone using readily available data, such as gridded climate, land-use, and soils. Calculations are completed on a daily time step and gridded results are summarized on monthly and annual time scales. In addition, irrigation water requirements (crop water demand) are optionally estimated to evaluate the effects of agricultural water use on the water budget. Version 2.0 of the SWB code was recently released and is being applied to the Mississippi Alluvial Plain (MAP) study area. Current work combines historical and recent land use, agricultural, and daily weather data to produce estimates of crop water demand and net infiltration at a 1-km grid resolution. Previous modeling in the study area assumed some fraction of precipitation (on the order of 1.0E-4 to 7.0E-2) becomes recharge, supplied to the model through 19 zones of similar soil type and geomorphology. The SWB approach allows recharge to be estimated with greater spatial resolution than the original zone method, but the limited knowledge of crop patterns from the early 20th century introduces uncertainty to SWB estimates from that time period. SWB recharge estimates will be constrained by other complementary datasets generated for this project, such as the Empirical Water Budget model. Our initial work generates recharge values up to an order of magnitude greater than the previous model estimates in some locations, particularly relative to the initial estimates based on data from early in the 20th century.

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