Quantifying Extreme Rainfall Events and their Hydrologic Response in the Walnut Gulch Experimental Watershed in Southeastern Arizona

Extended Abstract. Hydrologists are concerned with high intensity rainfall and peak runoff rates for stormwater infrastructure designs, post-event assessments and mitigation of environmental impacts. In the Southwestern US the need for accurate information about these rates is increasingly important as population growth and associated development is projected to exceed national averages. Design storm totals for various durations and return period frequencies are routinely derived from NOAA Atlas 14 and are commonly used as input to hydrologic models to estimate peak runoff rates and runoff volumes. For the Southwestern US during the North American Monsoon, NOAA relies on sparse rain gauge networks to measure rainfall from limited area convective storms primarily at daily time steps and estimates of sub-daily event intensities are derived by temporal down-scaling from a few point locations. The US Department of Agriculture, Agricultural Research Service, Southwest Watershed Research Center (SWRC) operates the Walnut Gulch Experimental Watershed (WGEW) in the vicinity of Tombstone, Arizona. SWRC maintains a database of 60 years of sub-daily, high temporal-precision rainfall intensities and runoff rates for WGEW (Goodrich et al., 2008). Updated, temporally-extended, rainfall intensity-duration-frequency relations for WGEW are presented. The current analysis includes intensity-duration-frequency relations for July, August and September for 53 years, 1961-2013, for 2-, 5-, 10-, 15-, 30- and 60-minute durations and 2-, 5-, 10-, 25-, 50-, 100- and 1000-year return periods. The 149 km 2 WGEW is large enough to select groups of four rain gauges whose event totals are independent. This allows combining of the four independent gauges’ 53-year time series into a longer time series of 212 years. A comparison of WGEW-generated intensity-duration-frequency curves to those of NOAA Atlas 14 indicated good agreement. However, across the range of durations, many observed events on WGEW from gauges not used in the frequency analysis are much greater than the estimated 100-year event.