WMOD00528

• Title: The Feasibility of Enhancing Streamflow in the Silver Iodide in the Sevier River Basin of Utah bt Seeding Winter Mountain Clouds
• Date: 12/1/1991
• Weather Modification - Doc Type: Report

5. SELECTION OF AN EXPERIMENTAL AREA IN THE SEVIER RIVER BASIN

The first requirement for an experimental area is that the region be at high elevation and provide
significant uplift to the prevailing westerly flow during stonns. Mountain ranges provide the additional
production of SL W that results in snowpack accumulation throughout the winter.

Most mountains exist in the eastern portion of the basin as seen on figure 5-1 which is a landfonn map
taken from Ralls et al. (1991). Going from south to north the major barriers include the Paunsaugunt and
Markagunt Plateaus, the north end of the Escalante Mountains, the Aquarius and Sevier Plateaus, the
Tushar Mountains, the Pavant Range, the San Pitch and Canyon Mountains and the Wasatch Plateau.

5.1 Transferability of Experimental Results

The Sevier Drainage is small enough that the results of experiments on any suitable mountain barrier
should be readily transferable to all other barriers in the basin. Thus, the main factors in selecting the
most suitable experimental area are minimization of logistic difficulties, especially related to aircraft
sampling and high altitude surrace measurements, and maximization of stonn events for study. The fact
that most of the mountain barriers fonh the divide between the Sevier and other drainage basins is not a
concern. The primary purpose of the proposed cloud seeding experiments is to develop and validate the
technology, not to produce additional water in the Sevier Basin. Once an acceptable technology has been
documented, it can be applied throughout tIle basin to increase water supplies. Of course, successful
seeding of the mountains that fonn the divide with other basins would result in additional snowfall in the
adjoining basins because targeting cannot be confined to just one side of a mountain range. But with the
semiarid nature of the entire region additional water likely would be welcomed in neighboring drainages.

5.2 Snowpack and Water Yields Across the Sevier Basin

The average annual water yield from runoff in the Sevier Basin is shown on figure 5-2 taken from Ralls
et al. (1991) who in turn extracted it from the Hydrologic Atlas of Utah published in 1968. This figure
gives an overall portrayal of the high runoff production zones which are essentially the high elevation
zones. Since annual runoff is strongly related to the seasonal snowpack, with snowpack water equivalent
data routinely used to forecast streamflow, figure 5-2 indicates the high snowpack areas. The highest
runoff (and snowpack) regions are in the southwest comer of the eastern portion of the drainage, just
northwest of Navajo Lake (the Cedar Breaks National Monument vicinity), and along the northeastern rim
of the basin, on the Wasatch Plateau. Average annual yields of 20 in are indicated in both areas. The
Tushar Mountains are close behind with a region producing 18 in of runoff. All other mountain areas
produce considerably less runoff with 12-in maximums.

The strong relationship between runoff yield and elevation can be seen indirectly in figure 5-3 which is
a logarithmic plot from all streamflow gauge data in the Sevier Basin published by Ralls et aI. (1991).
The yield per area is much greater for those gauges that monitor small drainage areas; that is, the
unregulated high elevation headwater regions. Yield per area decreases rapidly as measurements are taken
further downstream and more lower elevation terrain with regulated flows is included. A power function
of the fonn RUNOFF YIELD = 2055 X (DRAINAGE AREA)-O.47 provided a good fit to the data of
figure 5-3, resulting in a correlation coefficient of 0.93.

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