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<br />The computed response for plot 3 (fig. l7A) is in close agreement with <br />observed runoff. In addition, the fitted value for hydraulic conductivity <br />of 1.2 in/h is in line with the fitted value for hydraulic conductivity <br />determined from the simulator runs of 1981. Results for plot 4 (fig. l7B) <br />are very poor, and in no way representative of those previously experienced. <br />The observed runoff data show a subtle but possibly significant flattening <br />of the rising hydrograph, starting at about 25 minutes into the run. From <br />this point on, the deviation between observed and computed runoff gives the <br />general appearance of the action of a sink, or diversion of flow. Surface <br />features of plot 4 are very irregular; large dish-shaped clumps of yucca <br />dot the ground surface. These yucca areas are characteristically porous, <br />and are interlaced with rodent holes and rotted'root chambers. In several <br />instances, these clumps of yucca are located on rather flat drainage. 'divides <br />between gentle swales. Under high rates of surface runoff, these low divides <br />may become inundated and the yucca area may act as a sink. One large area of <br />yucca is located directly upslope from the measuring flume. Surface runoff <br />normally divides just above this clump and flows laterally into the training <br />dikes. It is quite possible that a large diversion of low and loss of runoff <br />occurred into the area of yucca during this run. The lower than anticipated <br />runoff prompted a sampling of the soil profile a few feet downslope from the <br />suspected intake area. The soil was uniformly wet (near field capacity) to <br />the depth of the auger handle (40 in). Sampling on the general plot area <br />showed much shallower depths of water penetration, rarely exceeding 12 in. <br />Other reasons for the discrepancy between observed and computed response <br />could be invoked; however, the fact remains that adjustment of model para- <br />meters cannot account for apparent threshold effects exhibited in observed <br />runoff. <br /> <br />Observed results for plot 5 show the pronounced influence of high ante- <br />cedent soil moisture and the absence of surface cracks on runoff (fig. l7C). <br />In contrast to the dry conditions of 1981, plot 5 is reasonably well behaved <br />under wet conditions and produces runoff that is comparable to that from <br />plots 3 and 4. As shown in figure l7C, the computed results are reasonably <br />close to the observed runoff. The large value of surface roughness, required <br />to fit the cracked surface condition of 1981, has been reduced to a more <br />realistic value. <br /> <br />Results of simulation runs made on plots 1 and 3 during October 1982 are <br />shown in figure 18. The antecedent soil-moisture condition for these fall <br />runs was higher than the previous year, especially so for the run on plot 3. <br />The near surface (0 to 2 in) moisture content for the run on plot 1 was <br />similar to that of 1981, but the soils were more moist at depth in 1982. <br />The run of plot 3 was scheduled for October 7, but it had to be repeatedly <br />postponed because 'of wind, rain, and snow and was not made until October 29. <br />Antecedent soil moisture at this time was the highest observed for any of the <br />runs on plot 3. In addition, soil temperature was quite low, 340F at the <br />surface, and increased slightly with depth. <br /> <br />Fitted results for both runs are in close agreement with observed runoff <br />(fig. 18). The value of KSAT for plot 1 is comparable to that found the <br /> <br />32 <br />