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<br />.Water in the Balance. <br /> <br />Measuring and Assessing Drought <br /> <br />Because drought can be defined in so many ways, based <br />on both the causes (lack of supply) and u'1e effects <br />(adverse impacts to water users). several methods have <br />evolved to measure and assess drought. A variety of <br />monitoring and analytical tools are available to help track <br />precipitation and current water supply anomalies, and to <br />identify and describe droughts that have occurred in the <br />nast. In narticular. several indices are used to help simplify <br />~omplex' data to provide information for planners and <br />decision makers. Paleoclimatic techniques. such as <br />measurement of tree rings. ice cores, pollens, and ancient <br />lake levels, are also employed to study drought patterns <br />and frequencies over the past several centuries. <br /> <br />Instrumental data are used extensively for monitoring <br />precipitation. snowpack, streamflow, and reservoir levels. <br />Precipitation is measured daily at several hundred <br />locations across Colorado. Some National Weather Service <br />stations have data collected for 100 years or more, and are <br />used extensively by the Colorado Climate Center at <br />Colorado State University for drought research. Snowpack <br />data, critical for predicting runoff and surface water <br />supplies, are collected at higher elevations by the Natural <br />Resources Conservation Service, U.S. Department of <br />Agriculture. A few of these sites date back more than 60 <br />years. Precipitation and snowpack data have been <br />analyzed in a recent study (summarized in this report) to <br />determine the patterns of wet and dry periods and their <br />hydroclimatic impacts in Colorado over the last 100 years. <br />Monitoring this data is very important to predict near- <br />future drought potential. Streamflow is the net result of <br />precipitation, snowmelt, evapotranspiration. infiltration and <br />groundwater recharge. as well as man-made influences <br />such as irrigation diversions and reservoir storage and <br />releases. The combination of streamflow readings and <br />reservoir levels provides the best direct indication of <br />available surface water supplies in each of our river basins. <br /> <br />These climate observation networks provide important data <br />to analyze current and historic droughts and relate water <br />availability to the observed impacts. Years of experience, <br />along with common sense, have shown that the types and <br />levels of drought impacts are directly related to the <br />following drought characteristics: <br /> <br />. Magnitude (how large the water deficits are in compari- <br />son with historic averages) <br />. Duration (how long the drought lasts) <br />. Arent Extent (what area is impacted by the drought) <br /> <br />Severity. the most commonly used term for measuring <br />drought, is a combination of the magnitude or "dryness" <br /> <br />and the duration of the drought. This combination can be <br />linked to actual impacts from a drought. Traditional maps <br />and graphs of precipitation, snowpack, and streamflow <br />patterns continue to be used extensively for identifying <br />drought. The following set of indices are also used in <br />Cnlorado: <br /> <br />The Palmer Drought Severity Index is a complex soil <br />moisture calculation that is used by federal agricultural <br />agencies to determine when to provide drought assistance. <br />Since this index- was developed for areas of the country <br />with more homogeneous climates, Colorado adapted the <br />index by separating the state into 25 climatically similar <br />regions. <br /> <br />The Crop Moisture Index was developed from the Palmer <br />Index, and was designed to evaluate short-tenn moisture <br />conditions across major crop producing regions. It uses <br />the average temperature and total precipitation for each <br />week and compares the calculated index with the previous <br />week. This is a better index to measure rapidly changing <br />conditions and for comparing different locations. <br /> <br />The Surface Water Supply Index (SWSI, pronounced <br />"swazee") was developed in Colorado, and is used in many <br />Western states to provide a weighted index of snowpack, <br />streamflow, precipitation, and reservoir storage. The SWSI <br />is calculated independently for each basin due to differ- <br />ences in climate and reservoir capacities. The weighting <br />factors for this index change from winter to summer. <br /> <br />The Standardized Precipitation Index (SPI), also developed <br />in Colorado, appears to be the simplest yet most robust <br />index for describing drought patterns. The SPI is based on <br />current and historical precipitation data for a particular <br />location. The SPI is proportional to precipitation deviation <br />from the "average" (surplus or deficit) for that location, and <br />has a unique probability that the deviation would occur at <br />that location. The SPI can be computed for different time <br />scales, can provide early warning of drought and help <br />assess drought severity, and is less complex than the <br />Palmer Index. The SPI identifies a beginning and end for <br />each drnught, as well as an intensity level for each month <br />in which the drought occurs. The following table shows <br />the values for the SPI index. <br /> <br />SPI Values <br />extremely wet <br />very wet <br />moderately wet <br />near normal <br />moderately dry <br />severely dry <br />extremely dry <br /> <br />2.0 + <br />1.5 to 1.99 <br />1.0 to 1.49 <br />-.99 to .99 <br />-1.0 to -1.49 <br />-1.5 to -1.99 <br />-2 and less <br /> <br />An excellent discussion of different drought indices is available on the National Drought Mitigation Center web site at <br />http://enso.unl.edulndmdenigmalindices.htm <br /> <br />6 <br />