Quantifying Extreme Rainfall Events and Their Hydrologic Response in Southeastern Arizona

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 United States the need for accurate information about these rates is increasingly important as population growth and associated development are projected to exceed national averages. Design storm totals for various durations and return period frequencies are routinely derived from the National Oceanic and Atmospheric Administration (NOAA) Atlas 14 and are commonly used as input to hydrologic models to estimate peak runoff rates and runoff volumes. For the southwestern United States 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 subdaily event intensities are derived by temporal downscaling from a few point locations. The USDA, 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 subdaily, high temporal-precision rainfall intensities and runoff rates for WGEW. 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 durations of 2, 5, 10, 15, 30, and 60 min and return periods of 2, 5, 10, 25, 50, 100, and 1,000 years. The $149{\mathrm{km}}^2$ 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. The dense gauge network appears to capture a substantially greater number of low-frequency, extreme rainfall events not typically observed in sparse networks. To assess the hydrologic consequences of these extreme events they were used as input to a well-tested watershed model for a small gauged watershed that did not experience events of similar magnitude. Simulated runoff volumes and peak discharge rates were up to four times as large as the largest observed runoff event of record. These analyses offer insights into the benefit of long-term watershed research with spatially dense and high temporal resolution observations.