|Holifield Collins, Chandra|
|Van Vactor, Steven|
Submitted to: Interagency Conference on Research in the Watersheds
Publication Type: Proceedings
Publication Acceptance Date: September 6, 2011
Publication Date: N/A
Interpretive Summary: Temperature has increased from roughly 1.5 to 5 degrees F. (1–3'C) across North America in the last 50 years, and this change has had an undetermined affect on natural resources and agriculture. To quantify the effect this climate warming has had on water supply, hydrology and watersheds requires long term, high quality hydrology and climatic data collection at locations representing the diversity of conditions across North America. The USDA-Agricultural Research Service (ARS) has a network of carefully instrumented experimental watersheds - many of which have been in operation since the 1950s and ‘60s. In this study observations from two of the ARS experimental watersheds; Reynolds Creek (RCEW) in southwestern Idaho which is dominated by snow; and, Walnut Gulch (WGEW) in southern Arizona which is dominated by monsoon thunderstorm rainfall, were examined to estimate trends in temperature, precipitation and runoff. Both watershed are experiencing significant trends of increasing temperature where the increase in average yearly daily minimum temperature is greater than daily maximum temperature. In RCEW, this has resulted in significant changes in snowfall and snowmelt. The snow season is at least a month shorter than it was in the mid-1960s with rain constituting a larger proportion of the total annual precipitation. Changes in rainfall and runoff in WGEW were less pronounced. Further work with additional ARS watershed over a wider range of the United States will be undertaken in the future.
Technical Abstract: The U.S. Department of Agriculture–ARS, Northwest and Southwest Watershed Research Centers have operated the Reynolds Creek Experimental Watershed (RCEW) in southwestern Idaho and the Walnut Gulch Experimental Watershed (WGEW) in southern Arizona since the 1950s. Each watershed is densely instrumented with a variety of hydrometeorological instrumentation and has multiple gauged subwatersheds spanning a range of spatial scales. These watersheds have yielded an extensive knowledge base of watershed processes over multiple decades of use as outdoor hydrologic laboratories. Precipitation and runoff generation in RCEW is dominated by snow and snowmelt processes, while WGEW is dominated by thunderstorm-generated rainfall during the summer monsoon. This study compared and contrasted hydroclimatic variables at these experimental watersheds, including temperature, precipitation, and streamflow. Monthly, seasonal, and annual data of temperature, precipitation and runoff were tested for significant trends. RCEW and WGEW both exhibit moderate to strongly significant trends of increasing temperature. The rate of increase in daily minimum temperature (Tmin) at the annual scale (+0.29 to 0.57°C per decade) is greater than daily maximum temperature (Tmax; +0.2 to +0.35°C per decade). In RCEW, this has resulted in the crossing of important thermal thresholds. Consequently, the snow season is at least a month shorter than it was in the mid-1960s with rain constituting a larger proportion of the total annual precipitation. Changes in rainfall and runoff in WGEW were less pronounced. While there were moderately significant trends of decreasing runoff in September, it is unclear if this is related to changes in the seasonal onset of the monsoon or a change in rainfall intensity. These decreases will be investigated in more detail in future work.