Climatic Variability and Flood Frequency of the Santa Cruz River, Pima County, Arizona

Past estimates of the 100-yr flood for the Santa Cruz River at Tucson, Arizona, range from 572 to 2,780 cu m/s. An apparent increase in flood magnitude during the past two decades raises concerns that the annual flood series is nonstationary in time. The apparent increase is accompanied by more annual floods occurring in the fall and winter, and fewer in summer. This greater mixture of storm types that produce annual flood peaks is caused by a higher frequency of meridional flow in the upper-air circulation and increased variance of ocean-atmosphere conditions in the tropical Pacific Ocean. Estimation of flood frequency on the Santa Cruz River is complicated because climate affects the magnitude and frequency of storms that cause floods. Mean discharge does not change significantly, but the variance and skew coefficient of the distribution of annual floods change with time. The 100-yr flood during El Nino-Southern Oscillation conditions is 1,300 cu m/s, more than double the value for other years. The increase is caused mostly by an increase in recurvature of dissipating tropical cyclones into the Southwestern United States during El Nino-Southern Oscillation conditions. Flood frequency based on hydroclimatology was determined by combining populations of floods caused by monsoonal storms, frontal systems, and dissipating tropical cyclones. For 1930-59, annual flood frequency is dominated by monsoonal floods, and the estimated 100-yr flood is 323 cu m/s. For 1960-86, annual flood frequency at recurrence intervals of > 10 yrs is dominated by floods caused by dissipating tropical cyclones, and the estimated 100-yr flood is 1,660 cu m/s. For design purposes, 1,660 cu m/s might be an appropriate value for the 100-yr flood at Tucson, assuming that climatic conditions during 1960-86 are representative of conditions expected in the immediate future.