WSP12518

• File Number: 8282.400
• Description: Colorado River Operations and Accounting - Deliveries to Mexico
• State: CO
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 9/1/2001
• Author: Various
• Title: The Journal of Arid Environments - Special Issue-The Lower Colorado River Basin and Delta - Volume 49-Number 1 - September 2001 - RE-Colorado River-Mexican Delta Issues - 09-01-01
• Water Supply Pro - Doc Type: Report/Study

46

M, }, COHEN ET AL

to inundation of the floodplain, outflows via evaporation and infiltration also increased
dramatically during flood years.
The most consistent source of water to the delta, both as a whole and for each of the
three sub-systems, was agricultural drainage. Such drainage provided the overwhelming
majority of discharge to the Cienega de Santa Clara and El Indio wetlands, and more
than 40% of total inflows to the Colorado River-Rio Hardy mainstem complex in
non-flood years. Particularly notable for the mainstern was the contribution of waste-
ways (22%) during non-flood years. With the exception of EI Doctor wetlands, the
contribution of local sources of water was negligible.
The contribution of agricultural drainage is consistent with its historic role of provid-
ing a baseline against the variability of mainstem Colorado River discharge. Since 1960,
agricultural drainage and returns from irrigation canals have provided greater discharge
(310 x 106m3 year-I) than median discharge from the mainstem (180 x 106m3 year-I)
(IBWC, 1998). However, the location of this discharge, primarily into the Rio Hardy
wetlands, limited the potential benefits of agricultural drainage to native riparian vegeta-
tion in the upper reaches of the delta.
Of potentially greater benefit to this vegetation was the contribution to the
mainstem from the two wasteways and from San Luis Rio Colorado effluent.
Luecke et ai. (1999) estimate the annual discharge requirements for the upper
100 kIn reach of the mainstem at 40 x 106 m3. This total was not met in either 1994
(31 x 106 m3) or 1996 (14 x 106 m3), though in other non-flood years the combination
of mainstem discharge through Morelos Dam, wasteways, and municipal effluent
exceeded this target below the SIB. Water quality was not assessed in this study,
though it is assumed that the effluent is of lower quality than the wasteway
discharge. It should be noted that wasteway discharge varied markedly on a monthly
basis, from 0 in July to more than 47 x 106 m3 in October, 1998 for the KM 27 wasteway
(IBWC, 1998).
Flood stage was exceeded for the mainstem in three of the seven years of the study
period. Inundation of the floodplain has important implications both for recruitment of
native riparian vegetation and for recharge of the alluvial aquifer. Flood stage for the
mainstem below Morelos Dam has not been determined definitively. A review of
recorded mean daily discharge at the SIB indicated that discharge of 100 m3 s - I or less
may be sufficient to promote inundation of the floodplain, as was reported for 21
February 1997 (Luecke et ai., 1999). By this date, records of mean daily discharge had
exceeded 100 m3 s - I on seven occasions, with an additional nine occurrences exceeding
80 m3 s - I. Inundation of the floodplain has not been reported for 1995, which witnessed
a maximum instantaneous discharge of 80 m3 s - I and a maximum mean daily discharge
of 77 m3 s - I. Further investigation of flood stage for the mainstem, and how this stage
varies across space, is warranted.
While records existed for many of the sources of inflow, sources of outflow were
generally estimated from other records. Evaporation and infil~tion for. the mainstem
varied markedly between flood and non-flood years, due to the 10crease 10 surface area
of open water, Evapotranspiration rates varied across the sub-regions, reflecting the
greater proportion of emergent wetland vegetation at the Cienega relative to the main-
stem complex. The estimated discharge at the downstream boundary for the mainstem
suggests that recorded discharge at SIB may not be a reliable indicator of total discharge
through the system.
The study was hampered by the limited availability of outflow data, challenging
efforts to balance the water budget. Most notable was the lack of information on
depth to the alluvial aquifer, ground-water movement, infiltration rates, and discharge to
the Laguna Salada. The magnitude of shallow ground-water cOl'!sumed.bY non~wetland
vegetation in the delta, estimated at 210 x 106 m3, suggests that 1Ofiltratlon dunng flood
years may recharge the aquifer. These data gaps are significant. Filling these gaps will
improve understanding of discharge through the system.

47

We thank Dr Jorge ~yarzabal of .CNA for assistance in obtaining data, Daniel F. Luecke
and an anonymous reVlewe: for their helpful comments, Eric Connally for statistical assistance,
and the Compton Foundation and the Oracle Corporate Giving Program for general financial
suppon.

WATER BALANCE FOR THE COLORADO RIVER DELTA

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