Propagation of meteorological variability from atmosphere to groundwater

Abstract:

Climate change impacts groundwater supplies through a variety of mechanisms. The most straightforward mechanism is variability in air temperature and precipitation changing groundwater recharge rates. Precipitation and air temperature are key input variables to distributed surface water-groundwater models, and their spatiotemporal variability is expected to propagate into groundwater via changes in recharge and subsurface flow dynamics. In climate impact assessments, future climate change projections are often downscaled, and irrespective of the downscaling approach, gridded datasets built from gauge-based observations are utilized for method development and/or validation. Gridded forcing datasets, however, are plagued by uncertainty, especially in mountainous terrain where gauge networks are sparse. To examine the mechanisms via which spatial variability in meteorological forcing propagates to groundwater, we perform a series of numerical experiments using an integrated surface water-groundwater hydrologic model, ParFlow.CLM. The Kaweah River watershed in California, USA is used as our virtual catchment laboratory. By applying the three cornered hat method, we quantify uncertainty in four publically available meteorological forcing datasets and their simulated hydrology in a spatially distributed manner. Simulations demonstrate that snowmelt is the primary source of groundwater recharge in the mountainous study domain. But uncertainty in simulated groundwater storage is primarily a result of topographic redistribution of uncertainty in precipitation forcing. The choice of air temperature dataset does not alter the importance of topographic redistribution. Variability in air temperature forcing impacts groundwater recharge to a similar degree as variability in precipitation forcing, but the impacts of both are not additive. Finally, we utilize simulations of the 2012-2014 California hot drought to demonstrate how extreme meteorological variability impacts groundwater storage in locations with complex topography. These simulations demonstrate the importance of topography and lateral water flow within the critical zone in controlling the groundwater response to climate variability.