Spatial averaging of land use and soil properties to develop the physically-based green and ampt parameters for HEC-1
Section snippets
Nomenclature
- E
actual retention after runoff begins (L)
- S
potential maximum retention (L)
- P
rainfall (L)
- Q
flowrate (L3/T) or runoff (L)
- CN
the NRCS curve number
- Ia
initial abstraction (L)
- f
infiltration rate (L/T)
- i
rainfall intensity (L/T)
- Ks
the soil’s hydraulic conductivity, wetted zone, steady-state rate (L/T)
- ψ
average capillary suction in the wetted zone (L)
- θ
soil moisture deficit (dimensionless), equal to the effective soil porosity times the difference in final and initial volumetric soil saturations
- F
depth of rainfall
Methods
Sabol et al. (1995) described a method of computing the input data for the Green and Ampt model using readily available geographic land use and soil type data as an overlay to a watershed model. While this method produces accurate results, it is time-consuming and has not been fully automated. This study presents an algorithm used in the WMS and based on Sabol’s method for automatically determining the Green and Ampt parameters required for each sub-basin in an HEC-1 watershed model (the method
Results and discussion
In order to test the spatial Green and Ampt computations, a watershed model with 17 sub-basins was successfully delineated for the Gavilan Peak Watershed in the vicinity of the community of New River in northern Maricopa County, Arizona. Hydrologic parameters, including Green and Ampt loss parameters, were defined and the HEC-1 model was run for the Gavilan Peak Watershed.
Using current county practices, basic GIS operations were used to determine the values required for the computation of Green
Conclusions
The Green and Ampt method is one of the methods available for computing sub-basin loss rates for a watershed model. Besides the rainfall data itself, the most sensitive set of input parameters for a hydrologic model are the rainfall loss parameters. The SCS (NRCS) Curve Number approach is used most frequently to compute rainfall losses because it provides a simple approach for computing losses. The ability to compute losses using the physically based Green and Ampt method is also available in
Acknowledgements
The authors are grateful for the Flood Control District of Maricopa County for providing resources for this research.
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