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<br />001956 <br /> <br />more restrictive than temperatures. If a cutoff date is required then the ending date would be changed <br />to the cutoff date. <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 first 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/ I <br />(January I) and 12131 (December 31) are used for the dates so beginning and ending date will not be <br />more restrictive than temperatures. If a cutoff date is required then the ending date would be changed <br />to the cutoff date. <br /> <br />The cutoff date or last day of moisture use for crops with limited water supply (shortage lands) was <br />determined annually based on a procedure documented in the Upper Colorado Region, <br />Comprehensive Framework Study (Upper Colorado Region State-Federal Interagency Group, 1971). <br />Documentation from these reports states: <br /> <br />f <br /> <br />"As part of the Type I survey, an average irrigation water cutoff date was determined from <br />past irrigation practice. We selected a representative stream gauge within each subarea and <br />determined the average historical streamflow that had occurred on this date. For each year of <br />the reporting period we used the recessional portion of the annual hydrograph and the <br />aforementioned streamflow to determine the date at which the 'shorted' water supply should <br />be cutoff. This date would serve as the end of the growing season for these lands... The grass <br />pastures and hay crops were the crops principally affected by these shortages. On the <br />computer runs, the Kc curves and other data are identical with the regular grass pasture and <br />hay crops; the shortage lands have been segregated to allow the input of a short season cutoff <br />date." <br /> <br />The final parameter evaluated was the length of growing season, Several annual crops such as corn <br />have a maximum number of days of growth until they mature or quit using water. These length of <br />growing season values are found in TR 21, Table 3, pg. 13. For crops that do not have a maximum <br />number of days of growth, 365 was used so length of growing season would not be the restrictive <br />value. <br /> <br />XCONS examines each of these parameters to determine the start and end of moisture use. The two <br />parameters for the start of growth in the spring are compared to see which occurred latest, that being <br />the most restrictive. The same thing is done for the end of growth. The end of growth period <br />parameters are compared to see which occutred earliest. The difference in days between the <br />beginning and ending dates are calculated and compared to the length of growing season. If the <br />length of the growing season is shorter, the moisture use period is the start of moisture use in the <br />spring plus length of growing season. Otherwise, the moisture use period is that found previously. <br />By using this procedure, the most restrictive moisture use period is found and used in computations. <br /> <br />DRAFT - SJRIP Hydrology Model Documentation <br />June 18, 1999 <br /> <br />17 <br />