El Nino Southern Oscillation (ENSO) effects on hydro-ecological parameters in central Mexico

The impacts of El Nino Southern Oscillation (ENSO) on precipitation, reference evapotranspiration, and vegetation in a three-state region of central Mexico were investigated using daily weather data from 20 weather stations for the years 1970 through 1990, which included 5 El Nino years, 5 La Nina years, and 11 Neutral years. In addition, two years, 1997 (El Nino), and 1998 (La Nina) of 10-day NDVI composites were analyzed during the growing season (May-Oct) along with precipitation and reference evapotranspiration (ETo) over central Mexico. Regional precipitation trends were analyzed using the normalized rainfall departures. The interannual variation of vegetation cover was analyzed using the NDVI on 10-day and monthly bases. The Food and Agricultural Organization (FAO) Penman-Monteith method was used to calculate ETo. The dynamics of the soil water balance in central Mexico was evaluated according to the method proposed by Thornthwaite and Mather. Analyses indicate that driest conditions occurred within the northern part of the region and during neutral ENSO years. Rainfall amounts during El Nino and Neutral years were not statistically different however, La Nina years were about 30% wetter than N and EN years (0.05 level). The correlation coefficient between NDVI and precipitation was 0.79 in 1997, and 0.52 in 1998, in June and July, respectively. Negative correlation was found between NDVI and reference evapotranspiration during the rainy months of July and August. The spatio-temporal variability of NDVI showed that there was significant statistical difference in NDVI between regions, but not between years. Regional soil water balance determinations indicated that conditions were most favorable in the Southern part of the region for crop growth during La Nina years. In general, soil water deficits were reduced by about 50% during the growing season compared to the annual soil water deficits