As demand for water in the southwestern United States rises during the next century due to growing population, pressure on limited water resources will intensify. Interannual variations in precipitation P are large throughout Arizona (AZ), and known to be influenced by the global-scale El Ni¿o--Southern Oscillation (ENSO). Mean monthly P was compiled in each of seven AZ climate divisions for composites of 28 El Ni¿o and 23 La Ni¿a years (1895--1994), using 6-month (June--November) mean values of the Southern Oscillation Index (SOI) to assign ENSO phase. Autumn, late winter and spring months in all AZ divisions averaged >50% higher P during El Ni¿o than La Ni¿a years. Management practices related to timing and relative proportions of surface water and groundwater withdrawals, based in part on ENSO conditions, are clearly feasible in this region. Precipitation data from the central highlands of AZ (NE of Phoenix) indicate decadal-scale trends in linear regression correlation of autumn through spring P as a function of SOI. These trends include higher P than the long-term mean since about 1960 for a given value of SOI. ¿ 1999 American Geophysical Union