Exercise 8 - Critical Dominate Soil

Skill Building: WEPS version 1.0 is able to estimate only one soil map unit in a field at a time. It is recommended that the user select the most erodible soil of a "manageable size". This is called the "Critical Dominate Soil".

The most erodible soil can be considered to be the one with the highest percentage of sand. Calcareous soils will be more erodible than non-calcareous (<15% calcium carbonate) at the same texture class. For example, a fine sandy loam will be more susceptible to erosion than a loam. Bottom-line: Take a look at the soils first when running WEPS.

Example: Consider the soil map of a field in Grant Co., Washington. There are three map units in the half circle (Figure 1.82). A good rule of thumb would be to use the most erodible soil greater than 10% of the field or greater than 10 acres in size (i.e., a manageable size).

The total of the field is 39.3 acres.

Question: What soil should be used to estimate the soil loss?

Answer: From Figure 1.83 we see that the Royal loamy fine sand (map unit 113) and Taunton loamy fine sand (map unit 151), comprise the majority field with 88.5% of the area. The Ekrub fine sand (map unit 36) makes up 11.5% of the area. However, the Ekrub fine sand should be used because it is the most erodible and greater than 10% of the field and it is upwind of the damaging westerly spring winds.