Why has streamflow in a northern Idaho creek increased while flows from many other watersheds in the US Pacific Northwest have decreased over the past sixty years?
Wednesday, 17 December 2014
As global warming proceeds, evapotranspiration demand will increase, the precipitation regime may change, and water cycling in many ecosystems may be affected. Streamflow in the Pacific Northwest (PNW) region of the USA decreased in the last ~60 year possibly due to decreasing precipitation at high elevations and/or increasing evapotranspiration. However, an increasing trend of streamflow was observed at a 4km2 watershed in the Priest River Experimental Forest (PREF) in northern Idaho. We used the process-based soil-vegetation-atmosphere Simultaneous Heat and Water (SHAW) model, to simulate the changes in the water cycle at PREF. Independent measurements were used to parameterize the model, including forest transpiration, stomatal responses to vapor pressure, forest properties (height, leaf area index, and biomass), soil properties, soil moisture, snow depth, and snow water equivalent. The model reasonably simulated the streamflow dynamics during the evaluation period from 2003 to 2010, which verified the ability of SHAW to simulate the water cycle at PREF. We then ran the model using historical vegetation cover and climate data to reveal the drivers of the changes in water budget of PREF over the past 60 years. Historical vegetation cover was obtained from a 1939 digitized historical vegetation map. The biggest change was the decline of western white pine (Pinus monticola Dougl. ex D. Don), a fast growing and deep rooted species with high transpiration rates, which was once a predominant species in PREF in the early 20th century. This was followed by a subsequent increase and decrease in fir species, followed by the emergence of western red cedar (Thuja plicata) as the current dominant tree species. The tree species shifts under this successional trajectory would have produced continually decreasing transpiration rates, which may explain the steady increase in observed runoff over the last ~60 years, which was likewise simulated with the SHAW model.