Present and future effects of agricultural practices and climate change on coupled hydrologic and biologic regimes in a western U.S. river
Amy M. Marcarelli1, Robert W. Van Kirk2, and Colden V. Baxter1. (1) Stream Ecology Center, Department of Biological Sciences, Idaho State University, Pocatello, ID 83202-8007, (2) Department of Mathematics, Humboldt State University, Arcata, CA 95521
Many rivers worldwide are degraded by both eutrophication and altered flow regimes. However, biologic and hydrologic alterations have largely been addressed independently by scientists and watershed managers. We hypothesized that changes in hydrology (due to climate change, irrigation withdrawals, etc.) may change the timing of peak flow relative to peaks in biological activity (daily metabolism). We tested this hypothesis in the Portneuf River, southeast Idaho, which is typical of many western U.S. rivers that have been strongly affected by agricultural practices. Reconstruction of historical hydrographs demonstrated that spring snowmelt runoff has decreased 50% and summer baseflow 75% due to water use by irrigated agriculture, while winter baseflow remains similar. A regional-scale analysis predicts that changes in hydrologic regime over the next several decades are as likely to occur from agricultural water use as from climate change. Patterns of daily metabolism under the current hydrologic regime demonstrate that primary production is principally related to flow (R2= 0.49), while ecosystem respiration is principally related to temperature and secondarily to flow (R2 = 0.38). Using these relationships, we will predict metabolism under current and future hydrologic regimes and discuss the implications of these patterns for nutrient and water management in western U.S. watersheds.