WMOD00301

• Title: Precipitation Pattern Analysis - Uinta Baisn - Wasatch Front
• Date: 10/1/1990
• State: UT
• Weather Modification - Doc Type: Report

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preCIpItation 192 times (16 percent) in the Uinta Basin and 329 times (28 percent) along the
Wasatch Front.

The southeast flow occurred only 4 times. This flow pattern produced precipitation each time (100
percent) in the Uinta Basin and produced precipitation only once along the Wasatch Front (25
percent). It is significant to note that this type of storm pattern which was favorable to the Uinta
Basin occurred only 4 times in the 2,441 events studied.

Of the 2,441 storms examined, 251 (10 percent) produced precipitation in the Uinta Basin and 506
(21 percent) produced precipitation along the Wasatch Front (i.e., twice as many storms produced
precipitation along the Wasatch Front as in the Uinta Basin). This ratio is verified another way by
comparing the same two areas on the annual normal precipitation map (Figure 3) and noting that
the Wasatch Front receives about twice the annual normal precipitation as does the Uinta Basin.
These values also verify the schematic of the influence of the large-scale flow patterns and storm
dynamics shown in Figure 7 above.

Present Water Deficit

A water deficit or drought is normally perceived in terms of its problems and impacts. Generally,
drought is spoken of as a function of one or a combination of many variables. These data may range
between specific point measurements and averages of data for large areas. Variables that have
historically been used to measure drought include radiation, precipitation, evapotranspiration,
effective precipitation, stream flow, tree rings, varves, natural storage, artificial storage, and
economic, social and psychological indicators.

In Utah, anyone or a combination of these variables may be used to describe conditions in and
around a drought stricken area (Jensen, 1978). One water deficit indicator is the Palmer Drought
Index (palmer, 1965). This drought indicator is a function of meteorological parameters and soil
moisture. This index I:' the most accepted drought indicator in the United States and much of the
world (Orville, 1990). Although it does not address many of the water deficit parameters in Utah,
it is a good objective indicator of the general status of drought. It has been used in this study to
show the relation of the present water deficit in the State with past wet and dry periods.

14

Both the Department of Agriculture and the National Weather Service have used the terms "mild,"
"moderate," " severe" and "extreme" to describe droughts. The severity of drought is a function of
both the value of z, (short-term severity index) and also of the duration of the drought, because a
drought situation tends to deteriorate with time. The Palmer Drought Severity Index (PDSI) is a
long-term index. Its value, X, is given by the equation

~
~ Zt
Xi=LJ O.309t+2.691
t=l

where t is time in months. Palmer assigned values of X to different categories of drought, listed in
Table 5. These values may be used to determine the start and finish of a drought and its severity
at any time during its occurrence.