Water balance analysis in Iowa using satellite-based evapotranspiration, precipitation, and streamflow data

Understanding the water balance in intensively managed agricultural landscapes is critical for sustainable water resource management. This study utilizes publicly available satellite-based evapotranspiration, gridded precipitation, and in-situ streamflow data to conduct water balance analysis for Iowa from 2000 to 2023. The objectives of the study were to: 1) identify areas of groundwater-dependent evapotranspiration, 2) evaluate the Budyko framework for estimating annual runoff, and 3) quantify the impact of historical land cover change on watershed hydrology. Our analysis revealed that during dry years, annual evapotranspiration exceeded precipitation across nearly 36,260 km2 of cropland. Approximately, 20 % of this area is sustained by irrigation, and the remaining 30,000 km2 rely on natural subsidies like stored soil moisture, or shallow groundwater. For a selected watershed, we showed that the Budyko framework is a good proxy for observed annual runoff (R²=0.83). Finally, we quantified that the historical conversion from native prairie to corn and soybean cropland has increased average annual runoff by approximately 57 mm (23 %), a direct result of the lower evapotranspiration rates of annual row crops.