1-02CW373 Supplemental Data

• Case Number: 02CW0373
• Stream Name: South Fork South Platte River
• Watershed: South Platte
• Water Division: 1
• Water District: 23
• County: Park
• Instream Flow App - Doc Type: Supplemental Data

Table 1. Flow parameters used to determine minimum flow requirements using the
R2CROSS Method (after Nehring 1979).
Stream width Average depth
(feet) (feet)
Average velocity
(ft /sec)
Wetted perimeter
( %)
1 - 20 0.2 or greater 1 50
21 -40 0.2 -0.4 1 50
41 -60 0.4 -0.6 1 50 to 60
61 - 100 0.6 — 1.0 1 70 or greater
Water Availability
The preferred technique to determine water availability is the analysis of observed data from a
continuously operated gaging station upstream of any water development on a stream. This
quality of data often does not exist for many streams. Discharge can be estimated using other
techniques such as statistical analysis. Multiple regression analysis of watershed variables is a
common technique for determining water availability (Gordon et al. 1992). Kircher et al. (1985)
developed relationships between mean monthly flow and drainage area and mean basin elevation
for the mountainous region of Colorado. Due to unique physiography and geology, these
relationships are not considered reliable for the South Park area. Therefore, standard regression
analysis of the available gage data was used to determine water availability.
Hydrology for the South Fork of the South Platte River was provided from the City of Aurora
gage. The data generally included discharge values from 1 April through 30 September for 1989
through 2000. A statistical curve fitting approach on the 12 years of data was used to estimate
water availability from 1 October through 31 March. Average daily discharge was calculated
from the data and the spring and fall baseflow periods were isolated from the data and transposed
so that discharge could be estimated throughout the winter. The periods selected are shown in
Figure 4. Regressions were run using a standard statistical software package. The type of
regression that best fit the available data was a polynomial function (Figure 5). The resulting
regression equation that describes the best fit line through the data is:
y = 0.0007x2 — 0.2098x + 23.897
Where y is the discharge (cfs) and x is the number of the day based on 7 September being day 1,
25 December being day 110, and 20 April being day 226. This modified day should only be
used with the above regression equation to estimate daily discharge from 1 October through 31
March, which is the period with little or no available flow data.
Minimum Flow Study, South Fork of the South Platte River — Final Report Page 7
February 19, 2001