Garden with Insight v1.0 Help: Nitrogen - Losses Through Leaching, Surface Runoff, Lateral Subsurface Flow

The amount of NO3-N 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 NO3-N lost from a soil layer and C(NO3) is the average concentration of NO3-N in the layer during the percolation of volume QT through the layer.

At the end of the day, the amount of NO3-N left in the layer is

WNO3 = WNO3(o) - QT * C(NO3) (Equation 158)

where WNO3(o) and WNO3 are the weights of NO3-N contained in the layer at the beginning and ending of the day.

The NO3-N concentration can be calculated by dividing the weight of NO3-N by the water storage volume:

C'(NO3) = C(NO3) - C(NO3) * QT / (bl * PO) (Equation 159)

where C'(NO3) is the concentration of NO3-N 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 NO3-N 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 2-M -- 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.