Paper No. 332-6
Presentation Time: 9:00 AM-6:30 PM
IMPACT OF GRASSLAND PLANTATIONS ON GROUNDWATER RECHARGE UNDER HISTORICAL AND PROJECTED CLIMATE CONDITIONS
This study focuses on impacts of tree plantations on groundwater recharge in a century-old experimental forest surrounded by native grasslands in the Nebraska Sand Hills. Historical and projected (1950 to 2099) recharge rates were estimated using numerical modeling for 2 plots representing grassland and dense pine plantation conditions at Halsey National Forest. We estimated the effective van Genuchten soil hydraulic parameters for each plot by inverse modeling. For this purpose, the DiffeRential Evolution Adaptive Metropolis (DREAM (ZS)) global optimizer algorithm was coupled with Markov-chain Monte-Carlo sampling scheme. The method was applied to the measured monthly soil moisture contents at 10 different depths for a uniform soil profile ranging from 10 cm to 220 cm over a period of 4 years and validated over 3 years. Afterwards, long-term HYDRUS-1D numerical forward simulations with the estimated parameters were performed for both the grassland and pine plots with 0.5 cm and 200 cm maximum rooting depths, respectively. The impact of land use change on recharge and other water budget components was assessed under the same climate forcings. Despite considerable variation in soil moisture content and the coarse monthly resolution, the calibrated model characterized the soil moisture dynamics over the period of simulation reasonably well. The estimates suggest that while recharge is projected to increase slightly in the next century, grassland conversions to pine plantations reduce historical and projected mean annual groundwater recharge rates by ~60% which is likely attributable to a combined effect of rooting depth, leaf area index, and surface soil water repellency. The calibrated model estimates for the native grassland plot are also generally consistent with the findings of other recharge studies in the area. The results highlight the significance of grassland cover for recharge in the Nebraska Sand Hills.