RAINFALL POINT INTENSITIES IN AN AIR MASS THUNDERSTORM ENVIRONMENT: WALNUT GULCH, ARIZONA¹

Abstract

Point rainfall intensities for a given return period are often used to formulate design storms for rainfall/runoff models to simulate design floods. These design floods are in turn used to design bridges, culverts, and a variety of drainage and flood control structures. The projected rapid growth in the southwestern United States will require very substantial monetary investments in drainage infrastructure. Accurate estimates of point rainfall intensities are critical to ensure both safe designs while not wasting dollars in overdesign. Rainfall point intensities (accumulated rainfall depth over a specified duration) for 5‐, 15‐, 30‐, and 60‐minute durations for the 2‐, 5‐, 10‐, 25‐, 50‐, and 100‐year return periods were determined for southeast Arizona. Thirty‐five years of rainfall record (1961 to 1995) were used in this study. The records came from 20 stations that were grouped into five sets of four independent stations to extend the rainfall records. The stations are in the USDA‐ARS Walnut Gulch Experimental Watershed (WGEW), which is representative of large portions of the Southwest whose runoff generation is dominated by air‐mass thunderstorms. The 5‐, 15‐, 30‐, and 60‐minute maximum intensities per year followed log‐normal distributions. The mean point rainfall intensities of the five sets of gages are very close (between 0 and 11 percent) to the NOAA values of the 5‐, 15‐, 30‐, and 60‐minute durations for all return periods. Much larger differences between the mean point rainfall intensities for all durations were found when these results were compared to those of a previous study done with a shorter rainfall record (between 14 and 33 percent for the 25‐, 50‐, and 100‐year return‐periods). The difference between the largest and the smallest values of point rainfall intensities recorded by each group, for all durations, usually increases as the return period increases.

Land cover controls on summer discharge and runoff solution chemistry of semi-arid urban catchments

“…0.3 mm) likely resulting from the spatial averaging of highly spatially heterogeneous rainfall. The rainfall intensities that we report here are comparable to those reported by Mendez et al (2003) for a 60 min rainfall event, which was the approximate mean duration of rainfall at our sites. Because rainfall characteristics were not significantly different between years, we pooled runoff and rainfall observations made during runoff monitoring to examine spatial controls on runoff quantity and quality.…”

Spatial Variability in Rainfall Erosivity versus Rainfall Depth: Implications for Sediment Yield

“…Large portions of these areas are experiencing accelerated erosion as a result of cumulative impacts of past land management (e.g., overgrazing), drought, and fire (Allen and Breshears, 1998; Davenport et al, 1998; Wilson et al, 2001) or recent land development. For all of these cases, accurate estimates of runoff and sediment yield are important for understanding ecosystem response and cost‐effective planning and design (Mendez et al, 2003). Furthermore, as climate change progresses, more extreme precipitation events are expected (IPCC, 2001), leading to increased erosivity (Nearing, 2001).…”

Understanding uncertainty in distributed flash flood forecasting for semiarid regions

“…For example, Fulton et al [1998] reported values ranging between 75 and 150 mm/h. Mendez et al [2003] analyzed maximum gauge intensities at different durations and frequencies for summer thunderstorms over Walnut Gulch. They found that the maximum 5‐min intensities (i.e., at the radar data time step) frequently exceeded 103.8 mm/h, and sometimes were near 250 mm/h.…”

“…Using radar data, Morin et al [2006] modeled rain cells as circular Gaussian elements with a maximum intensity and a decay factor: these can give theoretical maximum pixel‐scale intensities greater than 103.8 mm/h. From the latter two studies, and because flash flood forecasting mainly deals with intense rainfall over short return periods, we raised the hail threshold to a reasonable 146.3 mm/h, which has been estimated as the mean of the maximum 5‐min gauge intensities for a 10‐year return period [ Mendez et al , 2003]. Note that the actual WG11 peak storm intensities are rarely close to, and are mostly less than, this new value (Table 2).…”