Estimating Maximum Possible Point Rainfall and Flooding in Western Texas

Comparison of magnitude of record floods in small to medium-sized (<1- 100 km2) rural drainage basins in western Texas suggests that record flooding on most watersheds resulted from 150 to 300 mm of rainfall in 12-24 hours. Rainfall intensity and duration and area of watershed are the most important variables in magnitude of flooding on small watersheds. The probable maximum precipitation for this time interval, however, is most likely in the range of 300 to 600 mm. This estimate is based on point rainfall records in southwestern Texas, central Oklahoma and eastern Colorado. Recently documented point precipitation records suggest that the probable maximum precipitation is much greater than has been recorded at most weather stations during the 100 plus years of weather records in this part of Texas. The greatest calendar day rainfall at Del Rio, Texas, for example, changed in 1998 from 223 mm (recorded in 1935) to 432 mm on August 23, 1998-an increase of more than 93%. Even this new rainfall record is probably significantly below the maximum probable precipitation. Probable maximum precipitation rates are probably determined by physical and meteorological limits, however, regional escarpments, and canyons cut into those escarpments, play a role in localizing and maintaining optimum conditions for precipitation and runoff. Assuming a probable maximum 1-Day precipitation of 300 to 600 mm, the probable peak discharge resulting from such an event would be 4 to 6 times the calculated 100 year flood and 2 to 3 times the calculated 500 year flood on most watersheds.