Garden with Insight v1.0 Help: Soil patch next day functions: calculate today's runoff and water erosion from rainfall
Calculation of runoff and water erosion is fairly complex and we won't
go into much detail here. Runoff depends on the intensity of rainfall, the water-holding or retention capacity of the soil (how much it can hold and how much it is
already holding), the slope of the soil, the amount of vegetation cover
(which retards runoff), and the soil texture.
EPIC offers six variations on the Universal Soil Loss Equation (USLE) for calculating water erosion. The
water erosion equation contains several terms. The first term, the energy component, is an index of hard the runoff water strikes the soil. A big factor in water erosion
is how hard the water hits the soil, because this determines whether any of the soil gets broken off and
carried away. The energy component can be calculated by any of six equations, each of which uses slightly
different ways of estimating energy. We won't go into all the equations here, but they all rely to some
extent on runoff volume, peak runoff rate (the fastest speed at which
the runoff water flows) and the watershed area.
The other five terms of the water erosion equation are as follows.
The soil erodibility factor is based on the soil texture (sand, silt and clay) and on organic
matter in the soil: basically how well the soil holds together. Note that this is different from the soil
erodibility factor used in wind erosion.
The crop management
factor is based on the amount of vegetation cover and mean values
for parameters that describe how well each plant holds soil around its
roots. Basically this factor describes how well you are using crops (plants) to manage soil erosion.
The erosion control practice
factor is a parameter describing how well the soil is managed explicitly for reducing water erosion,
using practices like terracing and contouring.
The slope length and
steepness factor uses parameters that describe the shape and slope
of the watershed to determine how the water is flowing through the soil
The coarse fragment
factor (different from the wind erosion coarse fragment factor) is based on the rock content of the top
soil layer (more rocks means less erosion).
All these terms are multiplied together to get the total amount of water erosion from rainfall today.
calculation of wind erosion
EPIC Runoff Volume
EPIC Peak Runoff Rate
EPIC Water erosion