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42 M, J. COHEN BT AL Ground-water as a distinct source was not assessed as part of this study. Ground- water, applied as irrigation and delivered for municipal use, contributes to the w,ater balance in the form of agricultural drainage and municipal effluent. These contnbu- tions are included within tile estimates of agricultural and municipal discharge to the delta but are not identified separately. Actual records of groundwater extraction and recharge for the study area were not available. Several on-going studies seek to better assess the source, extent, quality, and discharge of ground-water in the region. Such information will greatly improve understanding of the quantity and movement of water in tile region, In flood years, water from tile Colorado River mainstem has discharged into the Laguna Salada basin (Q'h) (Luecke ct at., 1999). The Laguna Salada is also the drainage basin for the Sierra de Juarez range to the west, and for the Sierra de los Cucapas range to tlle east, challenging eiforts to account for the source of standing water in the basin. A review of Landsat 4 Multispectral Scanner Satellite images (path 39 row 38) revealed standing water in the Laguna Salada in 1993, 1997, and 1998. Personal observation (November, 1998) by one of the authors noted water flowing through a drainage canal to the Laguna, but discharge was not determined, Discharge to the Laguna Salada was estimated based on anecdotal observations, an unpublished report (Compean-Jimenez ct at., no date), that estimated the 1984 extent of the inundated area at 40,000 ha, with a maximum depth of 4 m and a volume of 730 x 106 m\ and ii'om Valdes-Casillas at at. (1998), who estimated the extent of the Laguna Salada in 1997 at 10,000 ha, El Indio wetland was constrained by the levee on the left bank of the river, tllough water drained to the mainstem through a gate in the levee until 2000. Discharge at its downstream boundary (Q".) was calculated. Zengel at at. (1995) note that half of the inflow water to the Cienega exited the vegetated portion, though Glenn ct at, (1992) note that the southern portion of the Cienega was essentially an evaporative basin with connection to the Upper Gulf of California only during periods of highest tides, indicating that flow at the downstream border for the Cienega was effectively zero, For the mainstem, discharge at the downstream border (tile river's mouth, at tile Upper Gulf of California) has been estimated in the literature to be equivalent to discharge at the SIB (Lavin & Sanchez, 1999; Galindo-Bect ct al., 2000); no gauge currentlv records this discharge. The last gauging station on the Colorado River, at EI Maritim"o, was destroyed by the 1983 Colorado River floods. Flow data at this station are onlv ~\Vailable for the period Janmlry 1960 through July 1968, when it was determined that tidal influences distorted the readings (CILA, 1968). Station records subsequent to 1968 are limited to mean daily gage height, which reflect tidal influence as well as mainstem discharge and agricultural drainage. Table 1 compllres annual records from El Maritimo Witll those from the SIB. Linear regressions were run for daily discharge records (nT'S - ') for the SIB and, El Maritimo gauging stations for tile years 1960 and 1965, The st~ongest. correlation (r~ = 0.944) for 1960 occurred with a 2-day lag between the two statIons, Wltl, ~ regres- sion line described by y = O.834x + 1.42, The strongest correlation (1'~ = 0'66) tor 1965 occurred with a 2-day lag between the two stations, with a regression line described by y = 0'516x + 0.94, Several factors, including the construction of the levees, Table 1. AII/llwl Jisclu.lIxe of tlte Colol'ado RilleI' at the SIB dTlJ <It EI M"rilitl/() (1 06 m~) 1960 1961 1962 1963 1964 1965 1966 1967 ~--_. -_...'-- SIB 671 218 380 228 121 126 126 115 EI Maritimo 621 105 337 147 95 126 112 73 WATER BALANCE FOR THE COLORADO RIVER DEI.'I"/\ I' ., 140 bll ~ 120 <J '" :a 100 ~ 80 ti .., 60 8 c 40 .... 0 '" 20 ~ 0 0 1992 1993 56/47 649/646 Cd W ~.", ("""- _.,f 1997 1998 187/174 3081306 1994 1995 1996 25,7/22,5 80m 0/0 Max. instantaneousl mean daily discharge [',j HJ{lIre 3. Total days of flood stage discharge at the SIB, 1992-1998, with annual maxi- 11111111 instantaneous and daily discharge, in m3 S-I (lBWC, 1992-1998). > IOOm3s-1 (.); '150m3s-1 (llII); >200m3s-1 (IS). .han~ing irrigation practices, the variability of discharge to the Laguna Salada, and the 'illl'suonable accuracy of the records themselves, challenge efforts to use these ':Ikulated values to estimate discharge at the mouth of the Colorado River. However, 1 "1.SC correlations did provide a value to compare the magnitude of discharge at the I ! I'per Gulf of California calculated by using the water balance equation. Results I ,llccke et at. (1999) estimate that releases of 100-200 m3 s - I are necessary to achieve 11"od stage on the Colorado River mainstem below Morelos Dam. Records of mean ,Iaily discharge at the SIB (IBWC, 1992-1998) were compiled to determine whether the I<,ar was a flood or non-flood year. Figure 3 displays the total number of days, by year, wil hin the study period in which mean daily discharge equaled or exceeded thresholds of 100, 150, and 200 m3 s - I. Along the x axis, below each year, are the annual maximum IlIslantaneous and maximum mean daily discharge, in m3 s - I. Note that no measurable discharge was recorded at the SIB in 1996. Maximum daily discharge occurred in January to March of each year. . '('able 2 shows the values of the factors aggregated as return flow for the mainstem, disaggregated into flood and non-flood years. The Colector del Sur drain, reflecting ,,'ntributions from numerous secondary and tertiary drains serving the lower Mexicali \'alley, discharges to the Rio Hardy. The Principal del Sur drain, reflecting contributions In>m the Carranza drain and other secondary and tertiary drains, discharges to the Rio Ilardy-Colorado River delta wetland complex (Valdes-Casillas et at., 1998). Thl' 11 larked increase in discharge through the KM 27 and KM 38 wasteways between flood .md non-flood years was particularly notable. The similarity in mean annual return flow 1>l'Iween flood and non-flood years reflects increased ground-water extraction during linn-flood years. Table 3 shows the values for the variables in the water balance equation for thl' (:nlorado River mainstem for the years 1992-1998, disaggregated into flood and 11' In-flood years. Qd. was calculated from the EI Maritimo regression described pl'l'. liollSly, generating a large undistributed residual, reflecting markedly greater inl'lows Ihan outflows. This discrepancy may be due to one or more factors, including grl':lIl'l' "\'aporation and infiltration than calculated, unrecorded diversions from the mainsll'll1