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t.:! .; !hydrographs of wells in the southern end of the valley (fig. 17) show two Itrends. The only long-term record (well 2-57-5CCC) shows a rather large lwater-level decline until about 1960. The short-term records generally show 'slight increases in water levels in this part of the valley since 1967. . In addition to the temporal data indicated in the hydrographs, spatial !data collected in 1947 and 1978 were used to plot water-table maps. Water- Itable maps of Beaver Creek valley (fig. 18) and Badger Creek valley (fig. 19) iwere drawn from data published by Bjorklund and Brown (1957). As part of this Istudy, water levels in approximately 200 wells were measured in the spring of :1978 (table 19, at back of report). Water-table maps drawn from these data lare shown on figures 20 and 21. Comparison of the figures (18 with 20 and 19 :with 21) indicates some of the problems of declining water levels faced by :the irrigators of this area. I I Hydraulic-conductivity data are not generally available for the modeled ~areas. Transmissivity and saturated thickness maps have been publ ished by ~Hurr, Schneider, and others (1972a, 1972b) for the South Platte River allu- !vial aquifers north of this study area. Estimates of hydraulic conductivity :obtained from these maps range from 100 to 500 ft/d. Bjorklund and Brown :(1957, p. 3ll reported the results of two aquifer tests in Beaver Creek val- !ley; computed values of hydraulic conductivity were about 90 and 380 ft/d. 'Specific-capacity data reported by Bjorklund and Brown (1957, p. 89) ranged :from 19 to 65 (gal/min)/ft. Computing transmissivity from the specific- Icapacity values and dividing by the probable range of saturated thickness :gives hydraulic-conductivity values ranging from about 30 to 500 ft/d. There 'are no specific-yield data for either of the alluvial valleys. Bjorklund and :Brm'ln (1957, p. 58) and Hurr, Schneider, and Hinges (1975, p. III assumed a , ' ,value of 0.20 for the specific yield of the entire South Platte River valley ~lluvium. Altitudes of the bedrock surface were obtalned from maps published by Bjorklund and Brown (1957). Recontouring was necessary in the area of the 'sand dunes between the two valleys due to discrepancies in altitudes of bed- 'rock and water-level data. Altitudes of the land surface were obtained from p.S. Geological Survey 7!-minute quadrangle maps. j Data describing boundary fluxes or recharge were not available for either modeled area. On the basis of information presented in the section on Hydrogeologic Setting, the recharge due to precipitation was assumed to be zero on the valley floor. Recharge in the sand dunes was assumed to enter the modeled areas as boundary fluxes. Recharge in the Badger Creek valley is complicated by the fact that the quantity of surface water used for irriga- t i on is unknown in much of the mode I ed a rea. , 38