FLOOD01413

• County: Elbert
• Title: Use of Rainfall-Simulator Data in Precipitation-Runoff Modeling Studies
• Date: 1/1/1983
• Prepared For: Elbert County
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

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The length of each channel segment was determined and a characteristic shape,
slope, and roughness was assigned. Infiltration and moisture-accounting
parameters of each HRU and flow-routing characteristics of associated
overland-flow plane and channel segments were input to the model. Model
computations then were performed on individual planes, using observed rain-
fall and estimates of potential evapotranspiration. HRUs 1-3 and their
associated overland-flow plane segments were assigned parameter values,
typified by results obtained from simulator plots 1 and 2. Parameters for
HRU 4 were typified by a melding of parameters for plots 1, 2, and 3. Model
results are largely controlled by KSAT, through its dominant influence on
computed volumes of runoff. Hydrograph shape and timing are influenced by
both infiltration computations and the routing specifications described by
the network of planes on channel segments.

Comparison of Observed and Simulated Runoff Events

Three storms that could be modeled occurred on June 25, July 26, and
August 10, 1982. As discussed previously, the hydraulic-conductivity term
in the infiltration equation largely controls the final results. Two sets
of model results were developed for each of these storm events. The first
uses estimates of KSAT values based on re~ults of rainfall-simulator runs
on plots 1, 2, and 3; the second is based on estimates from results of
observed rainfall-runoff events on plots 1, 2, and 3. The value of KSAT
for summer simulation runs on plots 1 and 2 ranged from 1.1 to 1.5 in/h,
and for fall simulation runs on plots 1 and 2 ranged from 0.5 to 0.75 in/h.
An intermediate value of 1.0 in/h was selected to be representative of the
simulator value for HRUs 1-3. The value of KSAT for plot 3 ranged from
1.0 to 1.3 in/h for summer runs and from 0.65 to 1.0 in/h for fall runs.
A value of 1.1 in/h was selected for the simulator value for HRU 4. Values
of KSAT for plots 1 and 2, based on observed rainfall-runoff events, ranged
from 0.4 to 1.0 in/h; a value of KSAT for plot 3 of about 1.2 in/h was indi-
cated. A value of 0.5 in/h was selected to be representative of the natural
value for HRUs 1-3, and a natural value of 0.8 in/h was selected for HRU 4.

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The recording rain gage near the upper end of North Fork Willow Gulch
measured 1.58 in of rain during the storm of June 25; results of modeling
this storm are shown in figures 26 and 27. Peak and volume of runoff are
grossly underestimated by both the simulator and natural KSAT specifi~
cations. Observed runoff is a very large percentage of rainfall (78 percent).

The storm of July 26 produced 1.20 in of rain; model results for this
storm are shown in figures 28 and 29. Parameter specification based on
simulator data slightly overestimated both the peak flow and volume of
observed runoff. Specifications based on natural events more than doubled
the observed peak and volume of runoff.

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