Garden with Insight
Garden with Insight v1.0 Help: Soil patch next day functions: calculate percolation and lateral flow
Percolation (downward movement of water through the soil) and lateral flow (horizontal subsurface water flow) are calculated together in a
simultaneous equation because they influence each other.
An amount of rainfall or snowmelt (minus runoff) enters the top soil
layer. Percolation through each soil layer depends on the amount of water entering the layer from
above and the field capacity, porosity, and saturated conductivity of
the soil layer. However, water never flows through a soil layer whose temperature is below freezing.
Percolated water from each soil layer flows into the next layer, and water flowing out of the last layer is
lost to the part of the soil we are simulating.
When the calculation of percolation is finished, the simulation makes two checks. First, it checks that no
soil layer has an amount of soil water above its porosity, because that would be impossible. If any soil
layer has more water than it can hold the extra water is passed back up the soil profile until the top soil layer is reached. If the top soil layer has more
water than it can hold, the extra water is assumed to be sitting pooled on top of the soil. Second, the
simulation accounts for the wicking of excess water up into layers that are less saturated (since the soil
layers are connected, after all). If any soil layer has more water than its field
capacity (the amount it holds after draining) and the layer above it has less than its field capacity,
some of the excess water wicks up into the upper layer.
Lateral flow depends on the porosity and field capacity of each soil layer and on the slope of the soil patch (higher slope means more lateral flow). Lateral flow
moves out of the soil patch and is lost to the system. An adjustment to lateral flow is made for return flow
(water flowing in from soil outside the soil patch) based on the time it takes water to flow out from the soil
patch, to a hillside, and back again. If the soil patches in this simulation interacted in the way they would
in a real garden, lateral flow from one soil patch would enter another soil patch (in the direction the water
was flowing downhill). However, the soil patches are completely separate so far.
calculation of precipitation, snowmelt, runoff, soil temperature
EPIC Percolation and Lateral Subsurface Flow