Spatial averaging of land use and soil properties to develop the physically-based green and ampt parameters for HEC-1

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Abstract

The computer hydrologic model HEC-1, developed by the Hydrologic Engineering Center (HEC), has been used for many years by hydrologists and engineers to estimate surface water runoff caused by rainfall events. Besides the rainfall data itself, the most sensitive set of input parameters in a runoff model are the rainfall loss parameters. The SCS (NRCS) Curve Number approach is commonly used to compute rainfall losses. Another method available for computing rainfall losses in HEC-1 is the Green and Ampt approach. However, this method is used infrequently because of the difficulty in obtaining the soil data needed to derive the Green and Ampt parameters. The purpose of this study is to present an automated method of computing Green and Ampt parameters using digital soil and land use data. Studies were performed with data that compare this automated method with another non-automated method of computing Green and Ampt parameters. Parameters computed using the automated method were consistent with the parameters computed using the non-automated method. Furthermore, by taking advantage of Geographic Information System (GIS) overlay capabilities, significant time was saved in computing the Green and Ampt parameters.

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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