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Garden with Insight v1.0 Help: Tillage

Each tillage operation
mixes some materials between the soil layers
changes the bulk density
converts standing residue to flat residue
changes ridge height
changes surface roughness (to input value)

Each rainfall event (day in which it rains)
changes the bulk density due to settling

The EPIC tillage component was designed to mix nutrients and crop residues within the plow depth, simulate the change in bulk density, and convert standing residue to flat residue. Other functions of the tillage component include simulating ridge height and surface roughness.

Each tillage operation is assigned a mixing efficiency (0-1). The tillage mixing equation is [Equation 323] where X is the amount of material in layer l after mixing in kg/ha, EF is the mixing efficiency of the tillage operation, X(o) is the amount of the material before mixing in kg/ha, and M is the number of soil layers in the plow depth, PD, in m.
this is done in other code, and is roughly the same

The change in bulk density in the plow layer is simulated for each tillage operation by using the equation [Equation 324] where BDP is the bulk density in soil layer l after tillage, BDP(o) is the bulk density before tillage, and BD(o) is the bulk density of the soil when it has completely settled after tillage.

Equation 324

BDP = BDP(o) - (BDP(o) - 2/3 * BD(o)) * EF
BDP = BulkDensityAfterTillageForLayer_tPm3
BDP(o) = bulkDensity_tPm3
BD(o) = bulkDensitySettled_tPm3
EF = mixingEfficiency_frn

Between tillage operations, the soil settles with each rainfall event (if it rains on a day) according to the equations [Equation 325] and [Equation 326] where SZ is a scaling factor for soil layer l, O(l-1) is the percolation rate into the layer in mm (R-Q for the top layer) and SAN is the percentage of sand in the layer. The exponential parameters of equation 326 are set to give 0.10 * (BD - BDP(i-1)) when SZ = 5.0 and 0.95 * (BD-BDP(i-1)) when SZ = 100.0. Thus, near the surface soils with little sand will almost completely settle with 100 mm percolation. Equations 325 and 326 cause fast settling when rainfall is large and soils are sandy and have been tilled recently. Also, settling is much faster near the surface (this allows simulation of long-term deep chiseling effects). Of course, settling is relatively slow for soils low in sand content, especially in low rainfall areas.

Equation 325, 326

SZ = O(l-1) / power(Z, 0.6) * (1.0 + 2.0 * SAN / (SAN + exp(8.597 - 0.075 * SAN)))
BDP = BPD(i-1) + (BD - BDP(i-1)) * (SZ / (SZ + exp(3.92 - 0.0226 * SZ)))
SZ = O(l-1) / power(Z, 0.6) * 0.2 * (1.0 + 2.0 * SAN / (SAN + exp(8.597 - 0.075 * SAN)))
otherwise the same
BDP = BulkDensityAfterSettlingFromRain_tPm3
BDP(i-1) = bulkDensity_tPm3
SZ = settlingScalingFactor
R = rainfallForDay_mm
Q = runoffVolume_mm
O(l-1) = waterEnteringLayer_mm
Z = depth_m
SAN = sandContent_pct
BD = settledBulkDensity_tPm3

Another important function of the tillage model, converting standing residue to flat residue, is accomplished with the equation [Equation 327] where SR(o) and SR are the standing residue weights before and after tillage in t/ha and PD is the plow depth in m.

Equation 327

SR = SR(o) * exp(-56.9 * PD * EF)
deltaSR = SR(o) - SR(o) * exp(-56.9 * PD * EF * EF)
SR = StandingCropResidueAfterTillageMultiplier
PD = tillageDepth_m
EF = mixingEfficiencyForOperation_frn

Other functions of the tillage component include simulating ridge height and interval and surface roughness. These variables are specified for each tillage implement. However, the ridge interval and height are computed after each tillage operation to reflect the combined effects of the current and previous operations. The ridge height is estimated by using the equations [Equation 328] and [Equation 329] where HR is the ridge height after the tillage operation k in m, HT is the input ridge height for the tillage operation in m, and k refers to the sequence of operations.

Equation 328

if HT(k) < HT(k-1), HR = HT(k) + (HT(K-1) - HT(K)) * exp(-PD(k) / PD(k-1))
if HT(k) >= HT(k-1), HR = HT(k)
HR = RidgeHeightAfterTillage_m
HT = ridgeHeightForOperation_mm
HT(k-1) = ridgeHeightLastOperation_mm
PD = tillageDepthForOperation_m
PD(k-1) = tillageDepthLastOperation_m

After each tillage operation, the ridge interval is set to the input ridge interval of the operation with the greater HT.

Equation 329

if HT(k) > HT(k-1), RIN = RIN(k)
if HT(k-1) > HT(k), RIN = RIN(k-1)
RIN = ridgeIntervalForOperation_m
RIN(k-1) = ridgeIntervalLastOperation_m
HT = ridgeHeightForOperation_mm
HT(k-1) = ridgeHeightLastOperation_mm

The user specifies the date and depth for each tillage operation. The tillage operation is carried out on the specified date if the soil is dry enough. If not, the operation occurs on the next suitable day. It is also possible to schedule operations by fraction of heat unit accumulations. The heat unit schedule may be user input or automatically developed by EPIC. Various combinations of scheduling (by date, heat units input, or automatic heat units) are also permitted.

The harvest index and harvest efficiency provide adequate flexibility to accommodate almost any harvest strategy. The harvest index (HI) is input for each crop and adjusted during each year of simulation as described in the Crop Yield section. Normally, the adjusted HI dictates the fraction of the above ground biomass removed from the crop. Thus, for a grain crop like corn, about 40-50% is removed. However, if corn is cut for silage, the input HI would be about 0.95. An option to override HI allows single crops to be harvested in two different ways. For example, oats could be harvested for grain by using the model adjusted value of HI ~~ 0.4 and then the straw could be baled by using the appropriate override value (0.5- 0.95). The harvest efficiency (HE, efficiencyFactorHarvestOrPesticide) indicates what portion of the harvested material actually leaves the field. For most operations, HE may range between 0.7 and 0.95. However, it can be set as low as 0.0 to simulate the plowing under of cover crops.

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Updated: March 10, 1999. Questions/comments on site to
Copyright © 1998, 1999 Paul D. Fernhout & Cynthia F. Kurtz.