Lake Brownwood Watershed - Texas State Soil and Water ...

RESULTS
Model Calibration
The results of reservoir storage calibrations are shown on Figures 3-4 and 3-5. Measured and
predicted mean monthly volumes were within 4.9% (Hords Creek Reservoir) and 3.5% (Lake
Brownwood). The calculated difference between measured and predicted values expressed as a
residual of the means squared is the root mean square error (RMSE). One way to gage the
accuracy of the calibration is to divide the RMSE by the mean measured monthly value. The lower
the result of this calculation, the more precise the comparison. The RSME/actual storage values
were very low (0.3 and 0.1 respectively, for Hords Creek and Brownwood) which indicate that the
model did a good job simulating actual storage volumes.
The results of calibration are shown for the stream gages on Figures 3-6 through 3-9. The
calibration period for each stream gage varied but all fell within the range from 1960 through 1990.
Comparisons of measured flow versus predicted flow (cumulative average monthly flow) yielded
the following differences: 24.7% (08141500; 1960-1990), 3.5% (08142000; 1960 - 1970),
0.2%(08140800; 1965 - 1980), and 6.1% (08140700; 1968 - 1979). The RSME/actual flow
values for different calibration points in the watershed are as follows: 3.3 (08141500), 1.6
(08142000), 2.2 (08140800), and 0.5 (08140700). The calibration for stream gage 08141500
(immediately downstream of Hords Creek reservoir) showed the poorest agreement for measured
and predicted flow and RMSE. This particular calibration was considered acceptable only because
it fell closely between two other calibration points (08140100 and 08142000) that demonstrated a
good correlation.
Brush Removal Simulation
Average annual evapo-transpiration (ET) was 21.57 inches for the brush condition (calibration)
and 19.05 inches for the no-brush condition. This represented 81% and 72% of precipitation
for the brush and no-brush conditions, respectively.
Figures 3-10 through 3-12 shows the cumulative monthly total flow to Lake Clyde, Lake
Coleman, and Lake Brownwood for the brush and no-brush conditions from 1960 through
1999.
The total subbasin area, area of brush treated, fraction of subbasin treated, water yield increase
per acre of brush treated, and total water yield increase for each subbasin are shown in Table
3-2. The amount of annual increase varied among the subbasins and ranged from 82,525
gallons per acre of brush removed per year in subbasin number 38, to 195,281 gallons per acre
in subbasin number 48. The large increase in water yield for the subbasin containing Lake
Brownwood (subbasin 48) was most likely due to the presence of predominantly muck soils
(high runoff potential) associated with water bodies and heavy brush. Variations in the amount
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