WMOD00527

• Title: Quantitative Precipitation Forecasting for Improving Reservoir Operations
• Date: 4/1/1995
• Weather Modification - Doc Type: Report

Each of the 12-unit hydrographs has the same total streamflow volume (in fefs-h). But
streamflow volume will change as the snow level moves up or down on a watershed. The
program achieves this volume adjustment when computing streamflow because the surface
runoff increment is the area-weighted sum of runoff from each zone up to the snow level.
The program properly lags and sums the hydrographs resulting from each surface runoff
increment to obtain the surface runoff hydrograph.

For each period, the area-weighted sum for all elevation zones of liquid water reaching the
ground is output as "MELT + RAIN." The area-weighted sum for all elevation zones of
snowfall plus delayed liquid water is output as "ADDED SNOW."

Only the MELT + RAIN is effective in generating surface runoff. The soil moisture AI and
basin constants determine how much surface runoff is generated by a given amount of MELT
+ RAIN. In effect, these constants determine a curve of cumulative MELT + RAIN versus
cumulative runoff, along which the simulation proceeds, period by period. Thus, two MELT
+ RAIN sequences with the same total water will result in the same surface runoff,
regardless of time distribution.

The hydrologic model program computes base flow for each simulation period and adds it to
the streamflow. To compute the base flow, the program considers base flow in the previous
period and surface streamflow in the current period. Surface streamflow results in gradually
increasing base flow, corresponding to ground-water recharge and increasing discharge. The
combined effect results in a base flow which increases gradually following the surface
streamflow peak, then decays smoothly over many periods.

2.3 Study Area and Topography

The study area selected was the 4820-km2 portion of the ARB (American River Basin) above
Folsom Dam, which is located just northeast of Sacramento. The ARB is located on the
western slope of the Sierra Nevada Mountain Range in northern California. This area
experiences occasional relatively warm winter storm episodes of heavy precipitation, much
in the form of rain, which produce flooding potential. Such storms are associated with basin
average rainfall accumulations exceeding 50 mm (2 in.) of water below the rain/snow level.
The upper-air measurement site used in this study was OAK (Oakland, CA), which is located
about 225 km southwest of the center of the ARB study area.

F'or this study, terrain elevation information was extracted from the National Geophysical
Data Center digital elevation I-minute latitude/longitude data tape on a 2.5-km grid interval.
These data were then averaged to generate smoothed, gridded elevation data with a 5-km
grid interval for model use. The elevation in the ARB ranges from about 150 m (500 ft) to
over 2700 m (8860 ft). Figure 1 shows the model grid points for the ARB plotted on 5 km
topography; the contours are in 500-ft intervals. The grid point numbers are the percent of
grid element area residing within the ARB. The numbers on the outside of the box in
figure 1 are model row and column numbers that define the grid point location indices.

2.4 Data Used in Study

During the 1979-86 period, Reclamation conducted an investigation of cloud seeding as a
means of increasing winter precipitation on the Sierra Nevada. This weather modification
research project, known as the SCPP (Sierra Cooperative Pilot Project), was conducted almost

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