Precipitation Probability Distribution, Percent Runoff, and Trend Analysis for 25 Reservoir Basins in Texas from 1980–2019

This study analyzed precipitation probability distributions, percent runoff, and trends in precipitation from 1980 to 2019 in the Brazos, Colorado, Guadalupe, Neches, Red, and Trinity river basins in Texas for 25 U.S. Army Corps of Engineers reservoirs and 15 other reservoirs for the purpose of flood risk and water-resource management. Probability distribution analyses include annual and monthly precipitation depths, maximum 2-day monthly precipitation for 5-, 10-, 20-, and 50-year return periods, monthly frequency of 2-day events exceeding these thresholds, and the mean and maximum day intervals between such events. Precipitation depths for a selected return period can be used with computed monthly percent runoff values during wet and dry conditions to estimate inflow volume. Inflow volume from different precipitation intensities, focusing on a return period and wet and dry conditions, serves a crucial role in providing guidance to make decisions regarding water storage and discharge. Results indicate mostly statistically significant upward trends in maximum 2-day monthly precipitation and in the number of events per year where 2-day precipitation totals exceed 5-, 10-, and 20-year return period amounts. Mostly downward trends were indicated in the average and maximum number of days between events, indicating the time between large precipitation events is decreasing.