Numerical simulation of the effect of land cover and climate changes on hydrological regimes in an inland Pacific Northwest watershed
The Distributed Hydrology Soil-Vegetation Model (DHSVM) was calibrated at the Mica Creek Experimental Watershed (MCEW) in northern Idaho, and assessed based on measured streamflows at nested watersheds, snowpack dynamics, soil moisture, and transpiration estimated from sap flux. A sensitivity analysis on the hydrologic regime as defined by annual water yield, 5th percentile flows and half-mass date was performed for 30 biophysical parameters by an offset-response method. The model effectively simulated streamflow dynamics along with other key hydrological processes across a range of forest canopy and climate characteristics with reasonable accuracy thus indicating that the model can be used with confidence to assess the effect of land cover and climate changes on hydrologic regimes. Sensitivity test found that overstory LAI, minimum stomatal resistance, and porosity are among the most influential parameters.
Spatial and temporal patterns of canopy alterations suggest that completely clear-cutting the entire watershed would increase runoff by 73% and 5 th percentile streamflows by 66%. The hydrologic effects were less pronounced relative to the degree of canopy removal in a partial-cut scenario where 50% of the canopy was removed across the entire watershed. Streamflow regimes approached baseline conditions after approximately 25 years of forest regeneration, and indicated almost complete recovery after 50 years. Gradually patch-cutting 5% of the watershed area every 6 years caused water yield to increase by 17% and remain very stable after 20 years. Hydrologic responses to various spatial harvest patterns were relatively similar.
Hydrologic simulations using downscaled general circulation model (GCM) data indicate that SWE in the MCEW is projected to decline by 47% to 90% in 2046 and by 55% to 95% in 2081. Annual water yield varied from 55% to 100% of the baseline case in 2046 and from 61% to 128% in 2081. Winter runoff (October–March) was elevated by 47% to 168% in 2046 and 73% to 239% in 2081. Summer streamflows were reduced to 21% to 72% for both future periods. Simulations suggest that the 50th percentile streamflow will advance by 17 to 90 days in the mid- and late-21st century. The study suggests that the hydrologic regime in the MCEW will shift from snow to rain dominated by the middle of the 21st century. Canopy removal emphasizes more on annual water yield changes, whereas climate change influences more on streamflow timing.
0404: Climate Change