Due to minor setback of Web Soil Survey’s tabular data, we push on. In this brute-force method, let’s look at each map unit symbol to get an idea of the soil types (most all are a well-drained loam). We also have some water (MUSYM = 79), rock (MUSYM = 2, 61, 102, 180, and 181), poor-drained gravelly sand (MUSYM = 36). We can also take a look at the depth to permanent layer (bedrock), which varies from 0 inches to >80 inches.
For a first-order approach, let’s assume the following:
| Soil Type | C (kPa) | φ (deg) | γr (N/m3) |
|---|---|---|---|
| Loams (any) | 20 | 32 | 20,000 |
| Rock/poorly drained | 0 | 45 | 20,000 |
| Water | 0 | 0 | 0 |
Using a combo-attack of “Select by Attributes” and “Calculate Field,” it is possible to prescribe values for γr, C, H, and φ.
The last few pieces of the puzzle require some information about water. We know that the unit weight of water is 10,000 N/m3 (γw). To get water depth, typically we would look at a time series of rainfall over the region and calculate event statistics to determine the 10, 50, and 100 year storms.
In our study site, the average annual rainfall is between 400 and 500 mm (16–20 inches) with the winter being the wettest part of the year with an average of about 50 mm (2 inches) of rain per month.
In September of 2013, central Colorado received a reported 200 mm (8 inches) of rainfall. Let’s make this a worst case scenario.
Back in ArcGIS Pro, we have four raster layers: