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<br />G00204 <br /> <br />. <br /> <br />Description of Model Runs <br /> <br />. <br /> <br />Runs of the mathematical model of the Lower South Platte Basin reach <br /> <br /> <br />described above were made in several series. The first series used his- <br /> <br /> <br />torical data in an attempt to duplicate the hydrologic situation which <br /> <br /> <br />existed in the 1947 through 1961 period as closely as possible. Series <br /> <br /> <br />II through V runs were designed to evaluate the sensitivity of the effi- <br /> <br /> <br />ciency of water use in the reach to various components of the irrigation <br /> <br /> <br />systems. The following sections describe the runs in more detail. <br /> <br /> <br />Series I runs--historica1 data <br /> <br /> <br />The purpose of the Series I runs was to duplicate the historical <br /> <br /> <br />return flow situation in the city area as close as possible for the study <br /> <br /> <br />period of 1947 through 1961. It was particularly of interest to duplicate <br /> <br /> <br />the situation during times of inadequate water supply to meet the irriga- <br /> <br /> <br />tion needs, such as the 1954 through 1956 drought period, <br /> <br /> <br />The actual measured weekly volume of streamflow at Balzac was used <br /> <br /> <br />in all Series I runs as well as most of the later series runs. The volumes <br /> <br /> <br />of water diverted by ditches and estimates of ground water pumped under <br /> <br /> <br />each ditch system by months during the study period were obtained from <br /> <br /> <br />the USBR Farm Water Utilization Study (1965), The estimates of canal <br /> <br /> <br />seepage losses and reservoir losses described earlier as developed by <br /> <br /> <br />Dr. Skinner were used, as were the estimates of deep percolation from <br /> <br /> <br />on-farm irrigation activities developed by Dr. Danielson and Dr. Hart. <br /> <br /> <br />Weekly estimates of irrigation requirements by crops during the study <br /> <br /> <br />period were used as discussed earlier. <br /> <br /> <br />The principal calculation of interest in the Series I rUns was the <br /> <br /> <br />estimated weekly flow at Julesburg. The estimated or calculated values <br /> <br /> <br />were compared with actual measured values in the 1947 through 1961 study <br /> <br /> <br />period. Minor adjustments were made and a final Series I run was con- <br /> <br />ducted which was used as the comparison run for the later series. <br /> <br />Series II runs--varying canal losses <br /> <br />. <br /> <br />The purpose of the Series II runs was to evaluate the sensitivity of <br /> <br /> <br />water-use efficiency in the reach to changes in canal seepage losses. <br /> <br />-47- <br />