Garden with Insight
Garden with Insight v1.0 Help: Soil patch next day functions: figure the nitrate carried up to the top of the soil during evaporation
As water evaporates from the soil it wicks water lower in the soil profile up through the soil. The simulation of this wicking effect was
explained in the section on percolation and lateral flow. But when
water flows it takes things along with it. In this simulation the only substance with which we are
concerned that has enough mobility in water to be taken along with upward movement is nitrate. (Remember that labile P is
relatively insoluble in water and tends to quickly precipitate out.) Nitrate moves from each soil layer up to the next, but stops at the top soil layer and never moves out
of the soil entirely.
The amount of nitrate moved out of each layer is calculated simply by figuring the nitrate concentration in
the layer before evaporation loss (nitrate divided by the volume of water
in the layer), then using that concentration to calculate the amount of nitrate lost in evaporated water. The
assumption here is that the evaporated water has the same nitrate concentration as the other water in the
soil layer; probably this is valid.
Nitrate lost from each soil layer is removed from the layer, and all of the losses together are added to the
first layer's nitrate amount. You could argue that this method is overly simplistic and that the best way
would be to take nitrate from each layer, add it to the layer above, and continue up the soil profile. But on the other hand, all this movement is simultaneous, so
moving the nitrate sequentially is unrealistic. Perhaps it would be
reasonable to use the nitrate concentration before additions from below to calculate the nitrate movement
out of each layer, and place the nitrate from each layer into the layer above. However, maybe the existing
method is a close enough approximation to what you would get if you used the more complicated method.
calculation of evaporation, percolation
and lateral flow
EPIC Nitrate Transport by Soil Water Evaporation