Garden with Insight v1.0 Help: Nitrogen  Losses Through Leaching, Surface Runoff, Lateral Subsurface Flow
The amount of NO3N lost when water flows through a layer is estimated by considering the change in
concentration. Thus the equation
VNO3 = QT * C(NO3) (Equation 157)
where VNO3 is the amount of NO3N lost from a soil layer and C(NO3) is the average concentration of
NO3N in the layer during the percolation of volume QT through the layer.
At the end of the day, the amount of NO3N left in the layer is
WNO3 = WNO3(o)  QT * C(NO3) (Equation 158)
where WNO3(o) and WNO3 are the weights of NO3N contained in the layer at the beginning and ending
of the day.
The NO3N concentration can be calculated by dividing the weight of NO3N by the water storage
volume:
C'(NO3) = C(NO3)  C(NO3) * QT / (bl * PO) (Equation 159)
where C'(NO3) is the concentration of NO3N at the end of a day, PO is soil porosity, and bl is a fraction
of the storage PO occupied by percolating water. Equation 159 is a finite difference approximation for the
exponential equation
C'(NO3) = C(NO3) * exp(QT / (bl * PO)) (Equation 160)
Thus, VNO3 can be computed for any volume, QT, by integrating equation 160 [Equation 161].
Equation 161
VNO3 = WNO3 * (1.0  exp(QT / (bl * PO)))
Code:
same
Variables:
VNO3 = NitrateLeachedFromLayer_kgPha
WNO3 = nitrate_kgPha
QT = volumeMovingThroughLayer_mm
bl = nitrogenLeachingParam
PO = porosity_mm
The average concentration during the percolation of QT (one day since EPIC is a daily time step model) is
[Equation 162].
Equation 162
C(NO3) = VNO3 / QT
Code:
same
Variables:
C(NO3) = MeanNitrateConcInLayerDuringWaterFlow_gPm3
VNO3 = nitrateLostFromLayer_mm
QT = volumeMovingThroughLayer_mm
Amounts of NO3N contained in runoff, lateral flow, and percolation are estimated as products of the
volume of water and the concentration from equation 162. The calculation sequence is:
1. Soil layer 1  Leaching is estimated first by subsituting O
(PercolationForLayer_mm) for QT in equation 161 and solving for VNO3.
Then VNO3 is removed from WNO3(1) and placed into WNO3(2). Next,
runoff and lateral flow losses are estimated simulataneously by
substituting Q (RunoffVolume_mm) + QR (LateralFlow_mm) for QT
in equation 161 and the resulting VNO3 into equation 162. Individual
losses are estimated for runoff (Q*C(NO3)) and lateral flow
(QR*C(NO3)). Finally, these losses are removed from WNO3(1).
2. Soil layers 2M  Leaching and lateral flow losses are estimated
simulataneously by substituting QR (LatFlow_mm) +
O (PercolationForLayer_mm) for QT in equation 161 and the
resulting VNO3 into equation 162. Individual losses are calculated
as in (1)  QR*C(NO3) and O*C(NO3), subtracted from WNO3 and
added to the WNO3 of the next layer. The process is repeated
layer by layer to the bottom of the soil profile.
