Impact of grassland conversion to forest on groundwater recharge in the Nebraska Sand Hills

Study region: Nebraska National Forest in the High Plains Aquifer, Nebraska Sand Hills, U.S.A. Study focus: This research aimed to investigate the effects of grassland conversions to forest on recharge rates in a century-old experimental forest. The DiffeRential Evolution Adaptive Metropolis (DREAM(ZS)) global optimization algorithm was used to calibrate the effective soil hydraulic parameters from observed soil moisture contents for 220 cm deep uniform soil profiles. The historical recharge rates were then estimated by applying the numerical model HYDRUS 1-D for simulation of two plots representing grasslands and dense pine forest conditions. New hydrological insights: The results indicate that conversion from grasslands to dense pine forests led to vegetation induced changes in soil hydraulic properties, increased rooting depth, and greater leaf area index, which together altered the water budget considerably. The impacts of land use change, expressed in percent of gross precipitation, include a 7% increase in interception associated with an increase in leaf area index, a nearly 10% increase in actual evapotranspiration, and an overall reduction of groundwater recharge by nearly 17%. Simulated average annual recharge rates decreased from 9.65 cm yr(-1) in the grassland to 0.07 cm yr(-1) in the pine plot. These outcomes highlight the significance of the grassland ecology for water resources, particularly groundwater recharge, in the Nebraska Sand Hills and the overall sustainability and vitality of the High Plains Aquifer.