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<br />001953 <br /> <br />These Task Memorandums evaluated the representativeness of the stations and their proximity to GIS <br />crop acreage to support the appropriate use of climate data in the consumptive use estimates. <br /> <br />The Theissen Polygon Method was used to detennine which stations would represent irrigated lands <br />within a Unit, Weighting factors were developed if two or more weather stations were selected for <br />use within a Unit. Stations selected to represent a Unit are listed in Table 2-4. Figure 2-5 shows the <br />location of weather stations and the areas they represent in the Basin, <br /> <br />Missing temperature data were estimated by reviewing daily minimum and maximum temperature data <br />for missing month. If over 75 percent of the daily data were available, the monthly average <br />minimum and maximum temperatures were calculated based on the mean of the minimum and <br />maximum daily values for those months, If less than 75 percent of the daily data were available, <br />correlations were developed against nearby weather stations to detennine missing mean monthly <br />temperature. Missing mean monthly precipitation were estimated using a ratio and proportion <br />technique developed by the Office of Climatology (Paulhus, 1952), The correlation method relies on <br />comparing ratios of the monthly precipitation values to the long-tenn nonnal (or mean) between <br />different weather stations, All temperature and precipitation data are tabulated in Appendix A. V and <br />A.VI. <br /> <br />Crop Growing Seasons <br />Crop growing seasons in the San Juan River Basin were detennined by considering three growing <br />season parameters: <br /> <br />. beginning and ending mean air temperature of moisture use <br />. beginning and ending day of growing season <br />. length of growing season. <br /> <br />The first set of values are the mean air temperature of earliest moisture use and the mean air <br />temperature of latest moisture use, These values are found in TR 21, Table 3, pg 13, In the case of <br />alfalfa, 280F is used as the ending temperature, Usually, daily minimum temperature records are <br />reviewed to detennine the first date in which a 280F fall freeze occurred, This date is entered as the <br />ending date of last crop growth for the year evaluated, Since a long period of record was evaluated, <br />the 50 percent probability date for 28"F freeze date, from Freeze/Frost Data, Climatography of the <br />U,S. No. 20 Supplement No, 1, for the identified Unit weather station was used to represent the last <br />growing date for alfalfa. For annual crops, the same procedure was used to detennine the last date <br />of growth, <br /> <br />The second set of values are beginning month and day of growing season and ending month and day <br />of growing season, Normal basin growing season starting and ending dates were used to represent <br />the beginning and ending dates, These values were obtained from states of Colorado and New <br />Mexico NRCS Irrigation Guides, Usually, the beginning day is the planting date. Alternatively, it <br />can be the last day at a certain temperature, such as a 28"F frost, The ending day of growth was <br />assumed to be either the cutoff date, harvest date, or the frrst day of a certain temperature. The <br />cutoff date represents the last day in which a crop would have received an adequate supply of <br />irrigation water, In cases such as pasture where temperature controls the growing season, 1/1 <br />(January 1) and 12/31 (December 31) are used for the dates so beginning and ending date will not be <br /> <br />DRAFT - SJRIP Hydrology Model Documentation <br />June 18, 1999 <br /> <br />14 <br />