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EXPLANATION Precipitation. in inches D Ot010 D 10 to 20 .Jr",~'\ D 20 to 30 107' ~ \ ~ 30 to 40 ;70"'''/ 1 - greater than 40.. '. ., J 109' .r--'~' . " ~ ('" .. ~,rJ ~9~ ...... ..,\...~~ ,\1 ? ~ \ "' .) ~ I 38%e"~ o 20 40 60 MILES b 20 '40 6~ KI~OMETERS Figure 2. Average annual precipitation (1951-80) in the UCOL (Colorado Climate Center, 1984). during the winter (fig. 3). The primary data-collection period for the ucaL study was water years 1996-98. Streamflow during water years 1996-97 was above average for most of the basin. Streamflow during water year 1998 was about average. High streamflows will dilute many water-quality constitu- ents, resulting in lower concentra- tions. Suspended sediment and constituents associated with sedi- ment may have larger concentra- tions during periods of high flow than during low flow. I rrigation is the Principal Water Use Within the UCOL Irrigation accounts for 97 per- cent of the water use in the UCaL (fig. 4). Ninety-nine percent of the water used in the Study Unit is derived from surface-water sources (U.S. Geological Survey, 1995). Ground water accounts for 1 per- cent of water use and is an impor- tant resource in remote and rural areas where the water is used pri- marily for domestic purposes. Water diverted eastward from the UCaL, through transmountain diversions, is used by many munic- ipalities in the eastern plains of Colorado. This diverted water from the UCaL constitutes about 35 per- cent of the water supply for the city of Denver (Denver Water Depart- ment, 1999) and about 65 percent of the water supply for Colorado Springs (Scott Campbell, Colo- rado Springs Water Utility, oral commun., 2000). In addition, the Colorado Big Thompson project, using water diverted from the UCaL, provides complete or '0 70,000 ~z 00 60,000 --'u LLW :::;:00 ~CI: 50,000 Ww g:(L 40,000 OOf- ZW ~w 30,000 wLL :::;:S,2 ~~ 20,000 <;u o~ 10,000 . partial supply for more than 30 cit- ies and towns in northern Colorado (Northern Colorado Water Conser- vancy District, 2000). Transmoun- tain diversions can affect the water quality in the basin because the diversions can account for a sub- stantial portion of the local stream- flow in upstream areas. In addition, the diverted water commonly has low salinity that is no longer avail- able to dilute more mineralized water in the downstream part of the Study Unit. Mean of historical daily streamflow 1970.95 - Daily mean streamflow .. '1," C"~ (m'7r~.~m(. "If "'.., I r I Colorado River at State Line r 1~ f ~ I 'f I ~rt \ " ~ \ .. _. p""OV ''l-~.. -......J .... I o Oct Jan Apr July Oct Jan Apr July Oct Jan Apr July Oct I WATER YEAR 1996 I WATER YEAR 1997 I WATER YEAR 19981 Figure 3. Streamflows in the UCOL were above average in water years 1996 and 1997. Water year 1998 streamflow was near average. B.5urface water o Gro~pd V>'.~ter .. .'11 (38) Within basin population served, in thousands . . 100 >- 3,800 ~ o .0:: ~ ~ 100 ~oo ~ S 80 0--' J:~ f-C> ~ LL 60 0::0 wOO ~ ~ 40 ~::::i ~ 20 ~ --' ~~ ~~ LL~ 00 2 ~~ ~~ Zo:: ~w 1 o::~ ~S' o o ESTIMATED WATER WITHDRAWALS IN 1995 Figure 4. Water use in the basin is primarily derived from surface water; however, ground water is used for some domestic and public water supplies. Transmountain diversions to eastern Colorado from the UCOL accounted for about 451 million gallons per day in 1995 (Upper Colorado River Commission, 1999). 4 Water Quality in the Upper Colorado River Basin