THE EFFECT OF GRASSLAND AFFORESTATION ON GROUNDWATER RECHARGE IN THE NEBRASKA SAND HILLS

Acute impacts of land use and land cover changes on groundwater recharge rates have been widely demonstrated across diverse environmental settings. Evaluation of recharge/land use linkages is therefore needed as an important component of proactive water resources management, particularly in semi-arid regions where water scarcity is a critical concern. This study focuses on recharge impacts of tree plantations in a century-old experimental forest surrounded by mixed-grass prairie in the US Northern High Plains (Halsey Nebraska National Forest, located within the Nebraska Sand Hills). Unsaturated zone cores were collected from 10 experimental plantation plots with different vegetation types and planting densities in addition to one grazed grassland site. Cores were collected to a depth of 6 m for recharge estimation with solute mass balances of chloride and sulfate together with moisture contents and matric potentials. Chloride inventories to 6 m depth beneath dense eastern red cedar and ponderosa pine plots forest plots ranged between 225%-240% of the grassland plot. Similarly, sulfate inventories to 6 m depth beneath forest plots ranged between 175%-230% of the grassland plot. The chloride-based recharge estimate for the grazed grassland site corresponds well with independent estimates from water balance (~73 mm/yr). The higher solute accumulation rates beneath the plantation suggest that recharge rates were 4-10 mm/yr, representing an 86-94% reduction relative to the surrounding grassland. Correlations between plot-averaged solute inventories and mean moisture contents in the area's relatively homogenous sandy soils confirm that the solute signals reflect partitioning between drainage and evapotranspiration. While tree plantations are likely to affect the local soil water balance by increasing root water uptake, increased rainfall interception and impacts on near-surface soil water retention properties may also play a role. The relative impacts of these attributes on unsaturated zone processes are under investigation in this ongoing study.