Excel Spreadsheet
Fill this spreadsheet.
Parameters Required
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Initial Loss (Iâ) – The rainfall depth lost before runoff begins.
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Constant Loss Rate (fc) – Infiltration rate after initial loss, constant over time.
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Impervious Area (%) – Fraction of basin that generates immediate runoff.
How to Calculate the INITIAL LOSS (Iâ)
- Compute Curve Number (CN) for the basin (weighted by land cover and soil group, follow this tutorial).
- Calculate S (maximum retention):
\begin{equation}
S = \frac{CN}{1000} - 10 \text{ (inches)}
\end{equation}
- Compute the Initial Loss (inches):
\begin{equation}
I_a = 0.2 \times S \text{ (inches)}
\end{equation}
How to Calculate the COSTANT LOSS RATE (fc)
- Equal to the Saturated Hydraulic Conductivity (in/hr), calculated also for the Green & Ampt method (follow this tutorial).
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Obtain the raster of saturated hydraulic conductivity (Kâ) for your basin (obtained using the Polygon to Raster ArcGIS Pro tool).
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Open the raster properties in ArcGIS Pro.
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Go to Source → Statistics and read the Mean value.
How to Calculate the Impervious Area (%)
It is calculated also for the Green & Ampt method (follow this tutorial).
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Obtain the raster of Impervious values for your basin (obtained using the Polygon to Raster ArcGIS Pro tool).
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Open the raster properties in ArcGIS Pro.
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Go to Source → Statistics and read the Mean value.
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