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<br />66 <br /> <br />JOURNAL OF WEATHER MODIFICATION <br /> <br />Volume 38 <br /> <br />APPLICATION OF A HYDROLOGIC MODEL TO ASSESS THE EFFECTS OF CLOUD SEEDING <br />IN THE WALKER RIVER BASIN OF NEVADA <br /> <br />Douglas P. Boyle, Gregg W. Lamorey, and Arlen W. Huggins <br />Desert Research Institute <br />Reno, Nevada <br /> <br />Abstract. The focus of this study is to use a physically-based, distributed hydrologic model to estimate the <br />impacts of cloud seeding efforts on the streamflow generated within the areas of the Walker River Basin <br />targeted by the Nevada seeding program. The hydrologic model is calibrated using GIS information, model <br />default values, and manual calibration to fit observed streamflow at a USGS surface water station within the <br />Walker River Basin. The calibrated model is then used in two case studies that are designed to simulate a non- <br />seeded condition and a seeded condition with a 10% increase in precipitation on the five target areas. The <br />results from the two modeling case studies indicate that the additional precipitation applied in the seeded case <br />results in increases in evaporation and runoff from the target areas but does not significantly impact the storages <br />of moisture in the groundwater and soil zone for all of the five target areas. The fraction of seeding-increased <br />precipitation that resulted in streamflow varied from 49% to 89% among the different target areas. The <br />remainder of the additional precipitation resulted in evapotranspiration from the target areas. <br /> <br />1. INTRODUCTION AND SCOPE <br /> <br />There are more than a dozen wintertime cloud <br />seeding programs in the western U.S. whose <br />primary goal is to increase snowfall over specific <br />drainage basins in order to subsequently increase <br />stream runoff in the spring and summer months. <br />An accurate assessment of the impacts of any cloud <br />seeding operation on streamflow runoff requires <br />detailed knowledge of the spatial and temporal <br />increases in precipitation due to the cloud seeding <br />activities and the watershed response to the <br />additional precipitation. Obtaining this knowledge <br />is often difficult or impossible due to the budget <br />and time constraints associated with the field effort <br />required to collect the necessary data. For projects <br />in the western U.S. the documentation of seeding <br />effects has been accomplished by assessments of <br />randomized experiments such as in Mooney and <br />Lund (1969), and by highly focused <br />nonrandomized experiments. Latter experiments <br />studied in-cloud microphysical changes, snowfall <br />characteristics and precipitation rate changes at the <br />surface, and evidence of the seeding material in <br />snow layers in seeding target areas (e.g., Super, <br />1999, Deshler et al., 1990 and Warburton et aI., <br />1996). Some operational projects have compared <br />stream runoff from seeded and unseeded basins as <br />part of their evaluation (Henderson, 1966). Also, <br />McGurty (1999) used snow chemistry and a <br />relationship between snow density and silver <br />concentration to estimate increases in snowfall and <br />runoff in a Sierra Nevada target area. While the <br />prediction of additional runoff due to seeding <br />efforts has not been routinely attempted, there have <br />been preliminary studies conducted in the Upper <br />Colorado River Basin (Super and McPartland, <br />1993 ). <br /> <br />- Reviewed - <br /> <br />Within Nevada's Weather Damage Modification <br />Program (WDMP), a cooperative research effort <br />with the U.S. Bureau of Reclamation (Hunter et al., <br />2005), hydrologic modeling has been incorporated <br />into the research to predict how changes in the <br />snowpack from cloud seeding will alter runoff in <br />the affected streams of targeted basins. This <br />research involves the application of a hydrologic <br />model to simulate watershed response to additional <br />precipitation from cloud seeding activities through <br />the different hydrologic processes (snowpack <br />evolution, evaporation and transpiration, <br />infiltration, soil moisture movement, runoff, and <br />streamflow). This research component was "piggy- <br />backed" onto cloud seeding operations that are <br />routinely conducted in the Walker Basin by the <br />State of Nevada. <br /> <br />This paper presents results from the hydrologic <br />modeling of the Walker River Basin whose <br />headwater region is on the eastern (mainly <br />downwind) side of the Sierra Nevada just north of <br />Yosemite National Park. During the winter of <br />2003-04 this headwater region was targeted by <br />several ground seeding generators and occasionally <br />by aircraft seeding. The hydrologic model is <br />initially calibrated during a period when there were <br />no known cloud seeding operations. Next, the <br />hydrologic model is used to investigate the impacts <br />of cloud seeding in the Walker River Basin through <br />two case studies. In the first case study. the model <br />is run forward in time through the 2003-04 winter, <br />assuming the target areas are not impacted by <br />ground or aircraft cloud seeding activities. In the <br />second case, cloud seeding activities are assumed <br />to increase the total precipitation on the target areas <br />during the 2003-04 winter by 10%. Although <br />storms over the Walker Basin were routinely <br />seeded in 2003-04 and snow profiles verified the <br />