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0'1173'- J ,L. .J of the San Juan River Above the Animas River Confluence, calculated as the San Juan River at Farmington minus the Animas River at Farmington. The flow is constant at about 270 cfs. The calculated flow above the Animas River confluence compares the same type of flow data as the Archuleta chart, but demonstrates that the mean daily flows were not constant, but ranged from 386 to 99 cfs during the Test period. How this runoff influenced impacts to diverters is the question. The largest drainage area in this section of the river is Canyon Largo, and it enters the river below the Hammond Diversion, Since all the canal diversions are above Canyon Largo, it is unlikely that thunderstorm runoff would have had a large factor in providing a divertible water supply. So, in spite of the rainfall events, results of the Low Flow Test indicate that dam releases of 250 cfs will provide sufficient water in the river to meet water rights in the San Juan River between Navajo Dam and the confluence with the Animas River. Though shortages were encountered at three locations during the Test, these were the result of inadequate diversion facilities, not insufficient water supply. The lowest river flow measured approached 60 cfs below the Hammond Diversion Dam. The remainder of the river is a gaining reach from irrigation return flow and canal waste. River flow above the Animas River confluence was measured at 218 cfs, probably containing a large portion of thunderstorm runoff. .METHOD One way to ass~ss the reliability offield measurements and to check our understanding of how this section of river operates was a mass balance analysis. The stretch of the San Juan River at Archuleta to the Animas River confluence was divided into six reaches for mass balance analysis. Figure 4 shows the total measured flow at each of these reaches. Diversions from canals to irrigated fields, irrigation depletions and return flows were calculated by river reach using New Mexico's 2000 acreage information and average July 1929-1993 consumptive use rates as calculated for the San Juan River Basin Hydrology Model. Table 3 summarizes monthly information extracted from the San Juan River Basin Hydrology Model to calculate irrigation diversions, depletions and return flows rates on a per acre basis. Monthly volumes are divided by modeled acres to determine the average and maximum July rates by acre-feet/acre/day. This report uses the average July rates to calculated diversions, depletions and return flows from irritation, The maximum July rate was included as a comparison. Table 4 displays irrigation diversions, depletions and return flows for each irrigation company by river reach, using consumptive use rates calculated for the area, Irrigation efficiencies for Hammond District was increased from the SJRB Model value of 30% to 60% to reflect recent concrete canal lining and to account for district reported canal wastes. Mass balance analysis was performed on the Citizen Ditch and Hammond Canal to estimate unmeasured losses (spills and seepage) that occur in each canal reach and are assumed to return to the river by the following equation: Canal Loss =Canal Flow(in)-Canal Flow(out)-estimated farm diversions- wasteways, Table 5 shows calculations that determines unmeasured canal losses by river reach, The procedure was to find the flow reduction between two canal measurements, accounting for the canal diversions measured in Table 2 or calculated in Table 5, 2