Garden with Insight v1.0 Help: Phosphorus - Soluble P Loss in Surface Runoff

The EPIC approach is based on the concept of partitioning phosphorus into the solution and sediment phases as described by Leonard and Wauchope (Knisel, 1980). Because P is mostly associated with the sediment phase, the soluble P runoff equation can be expressed in the simple form [Equation 204] where YSP is the soluble P in kg/ha lost in runoff volume Q in mm, C(LP) is the concentration of AP in soil layer l in g/t, and k(d) is the P concentration of the sediment divided by that of water in m3/t. The value of k(d) used in EPIC is 175.

Equation 204

YSP = 0.01 * C(LP) * Q / k(d)
Code:
the equation used for runoff and percolation of P is very diffferent
in the code. This equation computes not kg/ha of P leached, but the
concentration of soluble p in the leached water. The equation is
meanPConcDuringWaterFlow_gPm3 = AP * min(0.75, Q / SW) / Q where
Q is runoff + percolation for the first layer, or percolation for others
This is then multiplied by runoff or percolation to get kg/ha.
Variables:
YSP = SolublePLostInRunoffForLayer_kgPha
C(LP) = labilePConcForLayer_gPt
Q = runoffVolume_mm
k(d) = kPhosInSedimentOverWater_m3Pt = 175.0