Garden with Insight v1.0 Help: Pesticide Fate

Note: Pesticide fate is not included in this version of Garden with Insight.

Pesticide can be applied to the leaves or the ground. It is lost by:

evaporation during application
moving from the leaves to the soil with rainfall (if applied to the leaves)
"loss from the system" from the leaves (if applied to the leaves)
"loss from the system" from the soil (evaporation?)
runoff (first layer only)
leaching (all layers)
lateral flow (all layers)
eroding out with sediment (first layer only?)

Pesticide lost during application

GLEAMS (Leonard et al., 1987) technology for simulating pesticide transport by runoff, percolate, soil evaporation, and sediment was added to EPIC. Pesticides may be applied at any time and rate to plant foliage or below the soil surface at any depth. When pesticide is applied, some is lost to the atmosphere. Thus, the amount that reaches the ground or plants is expressed by the equation [Equation 219] where PAPE is the effective amount of pesticide applied in kg/ha, PAPR is the actual amount applied in kg/ha, and PAEF is an application efficiency factor (input).

Equation 219
PAPE = PAPR * PAEF
Code:
same except for 1000 multiplier
(which I can't see the necessity of in terms of units)
Variables:
PAPE = PesticideEffectiveApplication_kgPha
PAPR = pesticideApplication_kgPha
PAEF = efficiency_frn

Pesticide that stays on leaves vs. pesticide that falls to ground

To determine how much pesticide reaches the ground, the amount of ground cover provided by plants is estimated with the equation [Equation 220] where GC is the fraction of the ground that is covered by plants, and LAI is the leaf area index on day l.

Equation 220
GC = (1 - ERFC(1.33 * LAI - 2.0)) / 2
Code:
same (equation for function ERFC not given in publication)
Variables:
GC = FractionOfGroundCoveredByPlants_frn
LAI = leafAreaIndex

Therefore, the amount of pesticide that reaches the plants is computed with the equation [Equation 221] where FP is the amount of pesticide that is intercepted by plants.

Equation 221
FP = GC * PAPE
Code:
same
Variables:
FP = PesticideAppliedToPlants_kgPha
GC = fractionOfGroundCoveredByPlants_frn
PAPE = pesticideEffectiveApplication_kgPha

The remaining pesticide falls to the ground and is simply the difference between the effective amount applied and the amount intercepted by plants [Equation 222] where GP is the amount of pesticide that reaches the ground.
equation in other code, very simple

Pesticide washed off leaves by rain storms

Pesticide that remains on the plant foliage can be washed off by rain storms. It is assumed that the fraction of pesticide that is potentially dislodgeable is washed off the plants once a threshold rainfall amount is exceeded. The model uses a threshold value of 2.5 mm and potential washoff fraction for various pesticides have been estimated (Leonard et al., 1987). The appropriate equations for computing washoff and adding to the ground amount are [Equation 223], [Equation 224] and [Equation 225] where WO is the amount of pesticide washed off the plants by a rainstorm of R mm and WOF is the washoff fraction for the particular pesticide.

Equation 225
if R >= 2.5 mm, WO = WOF * FP
else WO = 0
Code:
same (if statement done outside function)
Variables:
WO = PesticideWashedOffPlants_kgPha
WOF = washOffFractionForPesticide_frn
R = rainfallForDay_mm
FP = pesticideOnPlants_kgPha
GP = pesticideAppliedToGround_kgPha

Pesticide on the plants and in the soil is lost from the system based on the decay equations [Equation 226] and [Equation 227] where GP(o) and GP are the initial and final amounts of pesticide on the ground, FP(o) and FP are the initial and final amounts of pesticide on the plants, HLS is the half life for pesticide in the soil in days, and HLP is the half life of the foliar residue in days. Values of HLP and HLS have been established for various pesticides (Leonard et al., 1987).

Equation 226
GP = GP(o) * exp(-0.693 / HLS)
Code:
Variables:
GP = GroundPesticideAdjForDecay
GP(o) = amountInLayer_kgPha
HLS = groundHalfLifeForPesticide_days

Equation 227
FP = FP(o) * exp(-0.693 / HLP)
Code:
Variables:
FP = FoliarPesticideAdjForDecay
FP(o) = amountOnFoliage_kgPha
HLP = foliarHalfLifeForPesticide_days

Pesticide leached out (percolation)

Another way that pesticide can be lost from the zone considered in computing runoff (top 10 mm of soil) is through leaching. The GLEAMS leaching component is used here with slight modification. The change in the amount of pesticide contained in the top 10 mm zone is expressed as a function of time, concentration, and amount of infiltration using the equation

dGP/dt = PSTC(w) * f (Equation 228)

where GP is the amount of pesticide in the top 10 mm zone at time t, PSTC(w) is the pesticide concentration in the water in g/t, and f is the water flow rate through the zone in mm/hr.

The total amount of pesticide contained in the top 10 mm zone is the sum of the adsorbed (in the sediment) and mobile (in the water) phases

GP = 0.01 * PSTC(w) * SW + 0.1 * PSTC(s) * BD (Equation 229)

where SW is the amount of water stored in the top 10 mm of soil in mm, PSTC is the concentration of adsorbed pesticide in g/t, and BD is the soil bulk density in t/m3.

The ratio of the concentration of pesticide adsorbed to the concentration of pesticide in the water has been estimated for various pesticides (Leonard et al., 1987) and is expressed by the equation

K(d) = c(s) / c(w) (Equation 230)

where K(d) is the partitioning constant.

Substituting equation 230 into equation 229 gives

GP = 0.01 * PSTC(w) * SW + 0.1 * PSTC(w) * K(d) * BD (Equation 231)

Solving equation 231 for PSTC(w) gives

PSTC(w) = GP / (0.01 * SW + 0.1 * K(d) * BD) (Equation 232)

Substituting PSTC(w) from equation 232 into equation 228 yields

dGP/dt = GP * f / (0.01 * SW + 0.1 * K(d) * BD) (Equation 233)

Rearranging equation 233 and integrating gives the equation expressing the amount of pesticide as a function of the amount of water flowing through the zone

GP = GP(o) * exp(-f / (0.01 * SW + 0.01 * K(d) * BD)) (Equation 234)

where GP(o) is the initial amount of pesticide in the top 10 mm zone in kg/ha, GP is the amount that remains after the amount of flow (f in mm) passes through the zone, SW is the initial water storage in mm, K(d) is the partitioning coefficient in m3/t, and BD is the soil bulk density in t/m3.

To obtain the amount of pesticide leached by the amount of water f, GP is subtracted from GP(o) using the equation [Equation 235] where PSTL (intermediate) is the amount of pesticide leached by f. Pesticide concentration in percolate is computed with the equation [Equation 236] where PSO1 is the pesticide solubility and PSTC(L) is the pesticide concentration in the percolate in g/m3. Finally, PSTL (final) is the product of O and PSTC(L).

Equation 235, 236
PSTL(interm.) = GP(o) * (1.0 - exp(-f / (0.01 * SW + 0.1 * K(d) * BD)))
PSTC(L) = min(PSTL(interm.) / f, PSO1)
PSTL(final) = f * PSTC(L)
Code:
BD -> C * SWT
0.01 * SW -> PO - WP
PSTL(interm.) = GP(o) * (1.0 - exp(-f / (PO - WP) + 0.1 * K(d) * C * SWT))
rest is the same
Variables:
PSTL(final) = PesticideLeachedForLayer_kgPha
PSTL(interm.) = pesticideLeachedByWater_kgPha
GP(o) = amountInLayer_kgPha
f = volumeMovingThroughLayer_mm
SW = waterContent_mm
PO = porosity_mm
WP = wiltingPoint_mm
K(d) = organicCAdsorptionCoeff
BD = bulkDensity_tPm3
C = organicC_frn
SWT = weight_tPha
PSTC(L) = pesticideConcInWaterFlow_gPm3
PSO1 = solubility_gPm3

Pesticide lost in surface runoff

Pesticide loss in surface runoff is estimated with a modification of equation 235 that includes an abstraction coefficient [Equation 237] where PSTQ is the pesticide loss in surface runoff in kg/ha, Q is the surface runoff volume in mm, and ab is the abstraction coefficient.

Equation 237
PSTQ = GP(o) * (1.0 - exp(-Q * ab / (0.01 * SW + 0.1 * ab * K(d) * BD)))
Code:
BD -> C * SWT
0.01 * SW -> PO - WP
PSTQ = GP(o) * (1.0 - exp(-Q * ab / ((PO-WP) + 0.1 * ab * K(d) * C * SWT)))
Variables:
PSTQ = PesticideLeachedByRunoffAndLatFlow_kgPha
GP(o) = pesticideInLayer_kgPha
Q = volumeMovingThroughLayer_mm
ab = abstractionCoeffForPesticideRunoff = 0.1
SW = waterContent_mm
PO = porosity_mm
WP = wiltingPoint_mm
K(d) = partitioningCoeffForPesticide_m3Pt
BD = bulkDensity_tPm3
C = organicC_frn
SWT = weight_tPha

The calculation sequence is:
1. for first soil layer
a. Leaching is estimated first by substituting O for f in equations 235 and
236.
b. PSTL is removed from layer 1 and placed into layer 2.
c. Runoff and lateral flow losses are estimated simulataneously by
substituting Q+QR for Q in equation 237.
d. The average concentration PSTC(Q+QR) is obtained by dividing the rsulting PSTQ
by Q+QR.
e. Individual losses are estimated for runoff (Q*PSTC(Q+QR)) and lateral
flow (QR*PSTC(Q+QR)).
f. Finally, these losses are removed from layer 1 (to nowhere).
2. for all the other layers
a. Leaching and lateral flow losses are estimated simultaneously by
substituting O+QR for f in equations 235 and 236.
b. The average concentration PSTC(O+QR) is obtained by dividing the resulting
PSTL by O+QR.
c. Individual losses are calculated as in the step for the first layer:
for lateral flow: QR*PSTC(O+QR)) and for leaching: O*PSTC(O+QR)).
d. These amounts are subtracted from the layer in question and added
to the next layer. This process is reapeated layer by later to
to bottom of the soil profile. (What happens at the last layer?)

The total amount of pesticide lost in the runoff is estimated by adding the soluble fraction computed with equation 237 to the amount that is adsorbed to the sediment. Pesticide yield from the adsorbed phase is computed with an enrichment ratio approach. The enrichment ratio equation is [Equation 238] where PSTY is the pesticide yield adsorbed to the sediment in kg/ha, Y is the sediment yield in t/ha, and ER is the enrichment ratio (concentration of pesticide in the sediment divided by the pesticide concentration in the top 10 mm of soil) computed with equation 167.

Equation 238
PSTY = Y * PSTC(s) * ER * 0.001
Code:
same except the 0.001 is lost and an upper bound of 0.9 is placed on ER * Y / SWT
Variables:
PSTY = PesticideLostInSediment_kgPha
Y = totalErosion_tPha
PSTC(s) = pesticideConcInSoil_gPm3
ER = enrichmentRatioForNPPest
SWT = surfaceLayerSoilWeight_tPha

The pesticide concentration in the soil is calculated by substituting equation 230 into equation 232 and solving for PSTC(s) [Equation 239].

Equation 239
PSTC(s) = K(d) * GP / (0.01 * SW + 0.1 * K(d) * BD)
Code:
BD -> C * SWT
0.01 * SW -> PO - WP
PSTC(s) = K(d) * GP * 0.001 * C / ((PO - WP) + 0.1 * K(d) * C * SWT)
Variables:
PSTC(s) = PesticideConcInSoil_gPm3
K(d) = partitioningCoeffForPesticide_m3Pt
GP = pesticideInLayer_kgPha
SW = waterContent_mm
PO = porosity_mm
WP = wiltingPoint_mm
BD = bulkDensity_tPm3
C = organicC_frn

Soil layers with low storage volumes have high leaching potentials not only because percolation is greater, but also because storage volume displacement is greater (higher concentration). Pesticides with low K(d) values and high solubility are transported rapidly with water. Conversely, high K(d) value pesticides are adsorbed to soil particles and travel largely with sediment.