Effects of climate variability and change on infiltration and recharge beneath natural grasslands in semiarid regions of the High Plains, USA

Global circulation models indicate that future climate of some semiarid regions of North America will have increased air temperatures and changes to the spatiotemporal patterns of precipitation, which will likely affect infiltration, recharge, and ultimately groundwater resources. To best predict, manage, and adapt to the effects of climate change on groundwater resources in semiarid regions, the relations between current climate variability and physical processes in the vadose zone that control recharge must be quantified. Therefore, the objectives of this research are to characterize the response of infiltration rates and water fluxes in thick vadose zones to current climate variability, and ultimately, to calibrate a 1-dimensional unsaturated zone numerical model to forecast the effects of climate change on recharge to groundwater resources in semiarid regions. During the period of 2006-2010, total water potentials were continuously monitored from multiple depths within relatively thick (28-50 m) vadose zones beneath two study sites in natural grasslands of the semiarid High Plains region of the United States. These study sites are part of a larger vadose zone monitoring network to quantify recharge and chemical transport to the High Plains aquifer. Following some precipitation events, total water potentials increased sharply and were lagged in time at subsequently deeper depths in the vadose zone at the study sites near the Cimarron National Grasslands, Kansas and Imperial, Nebraska. The data indicate that wetting fronts rapidly moved below the root zone and reached depths of 5 and 23 m each within approximately 1 year at the respective sites. At both sites, total water potentials decrease monotonically with depth indicating the potential for deep percolation through the vadose zone. Therefore, episodic recharge is likely to have occurred in regions near the study sites with depths to water less than 5 and 23 m. Estimates of the maximum water flux of the wetting fronts range from 185-5,366 mm/yr and are orders of magnitude greater than previously reported site-specific water fluxes, which were estimated using environmental tracers that likely reflect a combination of episodic and long-term water flux rates. Initial analysis of precipitation records at the sites indicate that the observed changes in total potential at both study sites are indeed event-based (episodic) and seasonal responses to spatiotemporal patterns of precipitation. These unexpected and previously unobserved rapid and deep wetting events within the vadose zone are important indicators that episodic recharge of the High Plains aquifer is possible under current climate conditions. The rapid water flux likely indicates preferential flow through the vadose zone and increases the vulnerability of chemical transport to the water table. Also, under current water practices, pumping rates exceed recharge rates in many locations resulting in a declining water table. To continue using the High Plains aquifer as a viable water resource, water managers need additional information on aquifer infiltration and recharge in order to adapt water policy to current and future climate conditions.