Herbert B. Osborn

CONVECTIVE STORM PATTERNS IN THE SOUTHWESTERN UNITED STATES

Abstract In the Southwestern intermountain and high plains areas, precipitation is seasonal, with the major part of the rainfall occurring in the summer. Most winter precipitation occurs as low-intensity rain or snow along slow-moving cold fronts. Most summer precipitation occurs as short-duration, high-intensity thunderstorms from purely convective buildup or from convective cells developing along a weak fast-moving cold front. Almost all runoff occurs from the summer convective storms. Since runoff-producing precipitation is of primary interest at the Southwest Watershed Research Center, Agricultural Research Service, Tucson, Arizona, the convective storms have been most thoroughly analyzed. Duration, intensity, areal extent, movement, character, and return frequencies for varying volumes and intensities of these convective storms are analyzed from records from dense networks of recording rain gages in four study areas in Arizona and New Mexico. The primary study areas are the 58-square-mile Walnut Gulc...

Rainfall watershed relationships for Southwestern thunderstorms.

ABSTRACT DEPTH-AREA relationships for thunderstorm rainfall were developed from 20 years of record from dense raingage networks in Arizona and New Mexico, using the National Weather Service method described in NOAA Atlas 2. The relationships are compared with similar previously published ones. Relationships also were developed to indicate the distribution of storm rain-fall over a watershed. This information could be valuable to agencies, groups, and individuals involved in water resources design and evaluation for climatologically similar areas.

EFFECT OF STORM DURATION ON RUNOFF FROM RANGELAND WATERSHEDS IN THE SEMIARID SOUTHWESTERN UNITED STATES

Abstract Almost all runoff from the semiarid rangelands of the Southwestern United States results from intense convective storms of short duration. Depth-duration values for precipitation for this region that are developed through standard procedures may be misleading when used for runoff design. Various combinations of short bursts of rain can, and do, plot on average depth-duration curves, but such curves have little practical meaning for small watersheds (100 square miles or less). For design purposes for small watersheds, depths of precipitation for relatively short periods (15-30-60 minutes) for varying return periods and areas are needed. For runoff design for larger watersheds two probability estimates may be needed—the probability of storms of certain intensities and size falling on tributary watersheds of finite sizes, and the probability of storms developing over a multi-tributary system in such patterns as to produce important volumes and peaks of runoff.

Potential of Convective Cloud Seeding in the Southwest

ADSTRACT WATER users in the Southwest can be divided generally into those who use stored water (reservoir and/or groundwater) and those who depend upon rain-fall for range forage and for livestock watering. These groups may or may not view weather modification simi-larly. The principal purposes for increasing precipitation in the Southwest are: (a) to increase the surface and subsurface water stored for municipal, industrial, mining, and irrigated agriculture users, and (b) to increase range forage for beef production. The former users are most interested in moving the most possible water from the watersheds into reservoirs and ground-water storage; the latter users are most interested in retaining as much water as possible on the land to improve range conditions. Much of the land surface of Arizona and New Mexico is arid or semiarid, and in these lands, thunderstorms are the major runoff source. On rangelands in south-eastern Arizona, for example, about 70 percent of the rainfall and almost all runoff results from intense thun-derstorm rains in July, August, and early September (Dorroh, 1960; Osborn and Hickok, 1968). However, winter storms are significant sources of water supply for mountainous watersheds, such as parts of the Salt River System (the water supply for the greater Phoenix area).