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<br />- 73 <br /> <br />tor of difference to 1. 38 and the standard deviation to 0.36. The <br />adjusted radar measurements now have an approximate gage density equiva- <br />lence of 10 mi2/gage (30 km2/gage), ~ihich is a considerable improvement <br />over unadjusted radar measurem,ents. <br /> <br />Table 6: Radar Estimate of Rainfall in Mesonet <br /> <br />Before Cluster Adjustment After Cluster Adjustment <br /> . <br />n FD cr n FD cr <br />40 1.53 . 0.43 40 1.38 0.36 <br /> <br />The results of this test suggest that gage-adjusted radar <br />estimates of precipitation might be used to evaluate weather modification <br />experiments. However, the method has its problems. Although gage ad- <br />justment does improve the overall acc:uracy of radar-rain estimates, <br />several instances when gage adjustment actually degraded the accuracy of <br />the radar-rain estimates have been documented. Furthermore, it must rain <br />in the adjustment cluster or clusters in order to determine an adjustment <br />factor. . These considerations :plus disagreement as to the optimum method <br />of adjusting radar-rain estimates make gage measurement of rainfall the <br />more desirable method of rain measun~ment in weather modification experi- <br />ments. Radar should still be llsed to provide the types of measurements <br />described earlier that cannot be made with a network of raingages. <br /> <br />4.0 Hydrologic Evaluation of Weather Modification Experiments <br /> <br />It may be possible to evaluate weather modification experiments <br />using hydrologic measurements such as streamflow. Kahan (1977) reviews <br />the hydrological aspects of evaluation of precipitation enhancement in <br />PEP report No.4. Hydrological evaluation is attractive because it pro- <br />vi des a single number which is the r€~sul t of the integrated rainfall over <br />a drainage basin over which precipitation enhancement operations might be <br />conducted. However, in many instances the disadvantages outweigh the <br />advantages. The disadvantages include inaccuracy of discharge measure- <br />ments, the high variability of natural flows and the time dependence of <br />successive river flows. <br /> <br />The ideal situation for hydrological evaluation of precipitation <br />enhancement is one in which the precipitation occurs as rain over a basin <br />in which most of the precipitation runs off into a stream or river that is <br />well-instrumented with flow meters. However, such conditions are rarely <br />met in actual practice, making such evaluations less useful than rain-' <br />gages or radar. For more details the reader is referred to PEP report <br />No.4. <br />