The effects of El Niño on the weather of southern Arizona and Sonora, Mexico include: precipitation increases, temperature cooling, increased forest fires from the dry summers following the winter rains, and a 25% increased possibility of flooding. It is thought that El Nino may stop the anthropogenic effect of greenhouse gas increasing in the atmosphere. Although an El Niño event can make the general climate, wetter, drier, warmer, or colder than normal, southern Arizona generally receives more precipitation than normal during El Niño years. Heavy precipitation tends to occur in December, with more precipitation occurring later in the spring. Historically, strong El Niño episodes have featured above-normal precipitation over the state during December-March. For this period, totals have averaged about 180% of normal precipitation in the southern part of the state, with actual precipitation departures of 2- to 3-inches. During February through April, Arizona tends to be cooler than normal, but only slightly in the western part of the state.

   El Niño events in the past average about 7-in. of precipitation over southeast Arizona. Reviewing the 12 El Niño years from 1914 to 1997, one report found that increased precipitation caused flooding to occur only during three of those years or 25% of the time. However, the occurrence of El Niño is not synonymous with flooding. Floods in Arizona result from various conditions including heavy summer thunderstorms, strong winter low pressure systems and remnants of tropical storms. El Niño, however, increases the likelihood of strong winter storms and causes tropical storm remnants to move into Arizona. It is not surprising that some of the largest floods in Arizona history occurred during El Niño events. Any long-term increase in precipitation and runoff in the arid southwest potentially leads to rising water levels of terminal lakes. With no outlet to the sea, water leaves the lake only via evaporation.

   Although inversely related to precipitation increases, corresponding changes in the temperatures occur; temperatures are expected to be a couple of degrees below average during El Niño events. With the likely occurrence of cooler temperatures and heavy precipitation in the Southwest, deeper snowpack can be expected in the higher elevations. Snowpack is the source of much of the streamflow in the West. Since there is a delay between snow fall and melt, the effects of El Niño on stream flow may not be apparent until spring or summer.

   It has also been shown that El Niño precipitation is linked with the occurrence of forest fires. The precipitation associated with El Niño years means the wildfire season tends to be relatively quiet. In the southwest borderlands region, the El Niño periods have produced a string of wet winters and springs. Summer rainfall has been erratic, producing more frequent dry summers. This climatic shift might explain recent increases in the number of fires and the annual area burned.

   More specifically, research of El Niño's impact on winter precipitation in south-central Arizona shows winter precipitation during El Niño years to be almost double than among non-El Niño years. Particularly in southern Arizona, the last 20 years have been exceptionally wet due to very frequent El Niño events (NOAA, 1996; Glantz, 1985; NOAA, website_9). Although this time span has been a period of rapid growth, the water needed to support this growth may have come, at least partly, from El Niño events that may not continue with the same frequency in the future. Because of El Niño events since the early 1980's, Arizona may have a false picture of its available water resources (Liverman and Bales, 1997).

   After Lake Powell reached capacity in 1981, there have been water releases to the delta in 1983-1988, 1993 and 1997-1999. These recent flood releases, combined with agricultural drain flows, have prompted the re-emergence of ecologically valuable riparian and wetland habitats (Glenn et al., 1992a,b, 1996, 1999). More cottonwood-willow (Populus fremontii and Salix spp.) habitat exists in the delta than in the riparian forests upstream (Ohmart et al., 1988), as is similar with wetland plant species (Reed, 1988). The El Niño of 1982-83, which was particularly strong and brought much snow to the headwaters of the Colorado River and flooded the drainage basin, sent down 16 maf to the delta, wiping-out the Mexicali Valley crops; but, this flooding was the first in approximately 50 years and was a beneficial event for the native vegetation (Glenn et al., 1996). This resurgence of native cottonwood and willow trees in the riparian corridor below Morelos Dam now contains three times as much cottonwood and willow habitat as the U.S. stretch of river below Davis Dam, which is five times longer than the stretch in Mexico (Glenn et al., 1999, 2001a, 2000b; Valdez et al., 1998). This greenness brought the attention of bi-national scientists and environmental groups, who have since been studying the delta, starting about 1990. The volume of water reaching the Gulf of Californa in the years of flow (in 10 of 20 years (50% of the time) since filling is approximately 20% of the river's base flow. These releases are associated with 3-7 year El Niño cycles, and thus, are expected to continue into the future (Glantz, 1995).