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Modernize Hydrologic Prediction Processes by Creating Custom Statewide SSURGO Green and Ampt Parameter Database Tasks 2 & 3: Green and Ampt Rainfall Loss Parameters

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    This section is a continuation of work by Arizona Department of Transportation (ADOT) to develop Green and Ampt loss parameters for soils using methods developed by Saxton and Rawls (2006). Unless otherwise noted, methods for determining rainfall loss parameters are consistent with those developed by Mohave County and are included in the Drainage Design Manual for Mohave County, 3rd Edition, December 2018. In 2014, ADOT developed Green and Ampt rainfall loss parameters statewide for the top 6 inches of the soil horizon. At the time, 6 inches was consistent with the previous ADOT Hydrology Manual and current hydrology manuals for Maricopa County Flood Control District. In a recent presentation hosted by the Arizona Floodplain Management Association (AFMA), a staff member of the Maricopa County Flood Control District Special Projects Branch discussed ongoing research into the controlling horizon for Green and Ampt modeling. This ongoing research indicates that the use of the top 3 inches of the soil’s horizon yields the best comparison with measured precipitation gage hyetographs and stage gage hydrographs. This depth is considered a reasonable infiltration depth for 100-year storm rainfall in Nevada and for transportation projects with lower recurrence intervals. A greater depth could be considered for infrequent storms such as the 500-year storm event or Probable Maximum Precipitation storm used for dam safety purposes. For the purposes of developing statewide Green and Ampt rainfall loss parameters in support of transportation projects, the top 3 inches of the soil horizon will be used. Green and Ampt rainfall loss parameters were determined for Nevada Department of Transportation (NDOT) using data derived from Natural Resources Conservation Service (NRCS) soils surveys. The data was calculated using the most restrictive layer within the top 3 inches of the soil column.
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