Drainage Design Manual, Hydrology - Flood Control District of ...

Drainage Design Manual for Maricopa County
Hydrology: Application
9.3.4 Rainfall Losses Example
Compute the area-weighted Green and Ampt rainfall loss parameters for each subbasin shown in
Figure 9.1 (see Section 9.1.1). Soil map units as they occur within the watershed are shown in
Figure 9.7. The majority of the watershed lies within the limits of the Soil Survey of Aguila-Carefree
and Parts of Maricopa and Pinal Counties, Arizona. The remaining portion of the watershed
lies within the limits of the Soil Survey of Yavapai County, Arizona, Western Part. Soil characteristics
for each map unit are provided in Table 9.6. The area of each map unit present within each
subbasin is provided in Table 9.7 along with the corresponding soil characteristics. Vegetation
cover for all natural portions of the watershed is estimated to be 26 percent. Developed areas
within the watershed are shown in Figure 9.8. Land use characteristics are provided in Table 9.8.
The area of each land use type present within each subbasin is provided in Table 9.9 along with
the corresponding land use characteristics.
Solution:
1. Compute the log-averaged bare ground XKSAT for each subbasin using Equation (4.4) and
the data from Table 9.7:
Subbasin S1:
A T
=
3480.4 acres
Log-averaged
XKSAT
=
10 a ; where:
a
=
5.6log 10
0.96 + ( 167.2 + 873.3)log 10
0.44
+1, 723.5log 10
0.33+ ( 50.6 + 2.4 + 137.1)log 10
0.09
+ ( 3.0 + 67.3 + 340.3 + 72.5)log 10
0.14 + ( 36.0 + 1.6)log 10
0.01
-------------------------------------------------------------------------------------------------------------------------------------------------------
3,
480.4
a
=
–
------------------------ 1,
887.43
=
3,
480.4
- 0.54
Log-averaged
XKSAT = 10 -0.54 = 0.29 in/hr
Using the above procedure, the log-averaged XKSAT for each subbasin is shown in the
table in Step 2.
August 15, 2013 9-29