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<br />, , <br />c <br /> <br />HISTORICAL DRY AND WET PERIODS <br />IN COLORADO <br /> <br />1.0 Introduction <br />Colorado is a dry state with a large spatial variation in precipitation. The annual average <br /> <br />precipitation shown in Figure 1 reveals a maximum of over 50 inches in small areas of the higher <br /> <br />mountains in the southern and northern parts ofthe state. The lowest precipitation near 7 inches <br /> <br /> <br />is in the San Luis Valley in south central Colorado while much of the western part of the state at <br /> <br /> <br />lower elevation from southwest to northwest has below 10 inches. The eastern plains vary from <br /> <br /> <br />near 12 inches to more than 16 inches. The statewide average is near 17 inches as described by <br /> <br />Cowie and McKee (1986). Adequate water is one of the most critical issues which the citizens of <br /> <br />Colorado must constantly address. There are only two sources of water in Colorado. One source <br /> <br /> <br />is water located in underground aquifers which was stored long ago and today is being mined. <br /> <br /> <br />The second source is precipitation in the form ofrain, hail and snow. Other forms of water from <br /> <br /> <br />the atmosphere such as dew are not considered for the present discussion. Precipitation is the <br /> <br />source of water but it is not the form of water used in Colorado. Precipitation falls and then in <br /> <br />some scale of time it becomes one of five sources of usable water - snowpack (SN), streamflow <br /> <br /> <br />(ST), reservoir water (RW), soil moisture (SM) or ground water (GW). In reality, time is an <br /> <br /> <br />important factor because precipitation occurs irregularly in time and then it immediately adds to <br /> <br /> <br />snowpack (SN) and soil moisture (SM) but in a variety of time delays it becomes streamflow <br /> <br />(ST), reservoir water (RW), or ground water (GW). <br /> <br />1 <br />