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<br />Colorado has a complex and sophisticated system of water distribution within and outside <br /> <br />the state. However, there are two dominant pathways of water use that are pertinent to the <br /> <br /> <br />discussion presented here. The first pathway is that precipitation falls on ground and becomes soil <br /> <br /> <br />moisture (SM) and ground water (GW) to support vegetation and other uses locally where it <br /> <br /> <br />oc~urs. A portion may also become streamflow (ST) and reservoir water (RW). This is the <br /> <br /> <br />dominant pathway for all lower elevations of the state and for the higher elevations in the summer <br /> <br /> <br />season. The second pathway is that precipitation falls on the ground at higher elevations in the <br /> <br />winter season, snowpack (SN), and becomes available as streamflow (ST), reservoir water (RW), <br /> <br />soil moisture (SM) and ground water (GW) during the following spring and summer and beyond <br />in time depending on many factors. <br /> <br />These two pathways of water supplied by precipitation lead to a very natural question. <br /> <br />How variable in time and space is the supply of precipitation in Colorado? This is the question <br /> <br />that this report will address. Climate is known to vary on all time scales from years to decades to <br /> <br />centuries to millennia. Locally, at a specific location a general guideline would be that annual <br /> <br /> <br />precipitation varies from one-half the average to twice the average in a period of a few decades. <br /> <br /> <br />This variation by a factor of two from the average is large and can be critical for a relatively dry <br /> <br /> <br />state like Colorado. If the variation were plus or minus 10%, concerns and impacts would be <br /> <br />much smaller. Other questions about variation is whether dry and wet periods occur one year at a <br /> <br /> <br />time or in groups of years and are they predictable? This report will seek to explain how <br /> <br /> <br />precipitation has varied for nearly the past 100 years (snow is less) and where we are now. The <br /> <br /> <br />question of predictability will not be addressed. <br /> <br />2 <br />