LONG-TERM TRENDS AND CORRELATIONS BETWEEN CLIMATE AND HYDROLOGY IN YELLOWSTONE NATIONAL PARK, USA

Loss of snow accumulation due to climate warming causes changes in both the timing and magnitude of spring peak discharge in snowmelt dominated streams. Across western North America, increasing temperatures have caused a hydrograph shift toward earlier peak discharges. We analyzed hydrologic data from 11 USGS stream-flow gaging stations, and weather data from 21 stations of several networks in and near Yellowstone National Park (YNP), to determine the past range of climate variability and periods of distinct climate conditions. The nature and magnitude of the shift in timing of peak spring runoff was determined by examining interannual variation in the date of annual peak flow and by calculating the annual hydrograph centroid (CT) for each gaging station for each water year. Pearson’s correlation was used to test for correlations between peak flow date, CT, and time series, and between CT and site-specific temperature and precipitation indices.

Magnitude of temperature and precipitation variation in YNP is influenced by large-scale climate forcings such as the Pacific Decadal Oscillation and El Nino Southern Oscillation, and is dependent on gradients in elevation, latitude, and rain shadows. Since 1950, mean annual temperatures in YNP increased by 1-4°C. Over the same period, the date of annual peak flow trended earlier on 5 major rivers. For all gaging stations, CT was inversely correlated with temperature index at 92% of climate stations within the drainage area. At 48% of the climate stations, CT was positively correlated with precipitation index. About 40-60% of the interannual variability in CT can be attributed to precipitation and temperature variation. The relative importance of each depends on the geology of the individual watershed, and on the general precipitation regime. Elevation of the climate station and gauging station may also play a role. The remaining variation is attributed to the specific distribution of precipitation and snowpack accumulation within the watershed, the rate of melting, the path taken by snowmelt into surface waters, and amount of storage as soil moisture.