MODEL ANALYSIS OF HYDROLOGIC RESPONSE TO CLIMATE CHANGE IN THE UPPER DESCHUTES BASIN, OREGON

The hydrologic response in western watersheds to climate change is of considerable importance to water managers and users. In Oregon’s upper Deschutes Basin, changes in the flow of groundwater-fed streams are of particular interest. Much of the precipitation occurring in the basin’s groundwater recharge zone falls as snow, so the timing of runoff and recharge depend on both accumulation and melting of the snowpack. Here, a recharge model coupled to a groundwater model, driven by a suite of climate projections, is used to investigate how global-warming induced changes in snowpack affect the hydrology of the basin.

A daily mass balance, Deep Percolation Model (DPM), was developed for the basin in the 1990s. The DPM uses spatially distributed daily climate data to calculate both daily and monthly mass balance for major components of the hydrologic budget across the basin. We use the results of high and low emissions scenarios from three general circulation models (GCMs) downscaled to 1/16th degree spatial resolution and disaggregated to daily time steps to drive the DPM. The three GCMs were selected because they have been shown to perform well in the Pacific Northwest. Each simulation is run to the end of the 21st century.

All of our model simulations show changes in the timing of runoff and recharge as well as significant reductions in snowpack throughout the Deschutes Basin. Although the total recharge and runoff calculated by the DPM vary according to GCM and emission scenario, all simulations show loss of the spring snowmelt runoff/recharge peak as well as an increase in evapotranspiration as time progresses.

The response of the groundwater system to changes in the timing and amount of recharge is expected to vary spatially. Short groundwater flow paths in the upper part of the basin are likely more sensitive to the change in seasonality of recharge than are longer flow paths lower in the basin. This is because geologic controls on the system cause variations in the recharge signal to attenuate as it propagates through the groundwater system into the lower portions of the basin. This scale-dependent variation in the response of the groundwater system to changes in seasonality and magnitude of recharge is explored by applying DPM-generated recharge to an existing regional groundwater flow model.