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1__-' "J " ~.,i ~. , histograms for WAU's 130100; 130202 and 130301 in Figure 3. Above Elephant Butte Reservoir in WAU's 130201 and 130202, flows in the Rio Grande follow the same general trend during the year as that indicated by the monthly distribution of outflow from Colorado for WAU 130100. The influences of spring snowmelt and summer thunderstorm activity are apparent, however, the actual magnitudes of flow vary considerably in response to local tributary inflows and irrigation diversions. For example, the mean annual flow in the Rio Grande just below the Rio Chama confluence (outflow from WAU 130201) is on the order of one million acre-feet [11], but downstream in WAU 130202, the combined effects of substantial irrigation diversions, ground water inter- actions, evapotranspiration losses, municipal use, and reservoir evaporation reduce the mean annual flow in the River below Elephant Butte Reservoir to about 600,000 acre-feet [4,11,13,14,15]. Irrigation depletions downstream of Elephant Butte further decrease the flow in the River to 365,000 acre-feet per year at El Paso near the lower end of WAU 130301. The effects of reser- voir storage and regulation of release waters are clearly evident in the monthly streamflow distributions for the Rio Grande immediately below Elephant Butte (WAU 130202) and at El Paso (WAU 130301). I.. ~'f' ~" <, <, ;1 An important point to note with regard to streamflow conditions in the Rio Grande is that flows entering Elephant Butte during the peak months March-October have been steadily decreasing, especially during the last forty years, As noted below, average monthly flows at San Marcial, New Mexico, located just upstream of the impoundment, have been reduced by almost 60% [13,14]. Period Average Monthly Flow of the Rio Grande at San Marcial, New Mexico March-October (acre-feet) 1986-1915 1916-1939 1940-1957 1958-1968 1969-1978 122,943 111,929 74,864 49,641 48,963 '" ~-,.. r '.... IJ'v'G.... u 14