Adjusting the BEINP Plant Growth Parameter
Figure 3: Canopy Cover and Biomass
Figure 4: Canopy Height and Biomass Relationship
Figure 5: Potential heat units for sorghum-planted two weeks later than corn. (Kiniry et al., 1991
Figure 6: Potential heat units for corn (Kiniry et al., 1991).
Figure 7: Potential heat units for soybeans--planted four weeks later than corn (Kiniry et al., 1991).
Figure 8: WEPP leaf area and vegetative biomass relationship.
Table 17: Suggested values for the cropland plant specific input parameters for the WEPP erosion model (version 95.7).
Table 17(cont): Suggested values for the cropland plant specific input parameters for the WEPP erosion model (version 95.7).
The WEPP crop growth model is a modification of the EPIC crop growth model (Williams et al., 1989) which accounts for water and temperature stresses on biomass production and harvested yield. The WEPP crop component was designed so that parameters may be adjusted for each different crop and for variations within crop varieties. Included in Table 17 and in the WEPP management file builder are estimates of crop parameters for many of the major crops grown in the United States that should provide realistic results. Since the crop growth component was not intended to serve as a crop yield prediction model, the user is advised to use caution when adjusting parameter values in order to overcome errors. In the cases where actual yield/biomass values are vastly different from those predicted by WEPP or crop parameters are not available for a particular crop of interest, the plant parameters may be adjusted WITH CARE. Other sources of errors should be considered before modifying a cropping and management input due to simulation output discrepancies. Crop inputs are best modified for research or sensitivity analysis purposes.
The crop residue decomposition component of WEPP is based on the RESMAN Residue Management model (Stott and Rogers, 1990; Stott and Barrett, 1993; Stott, 1991). This component estimates the amount of residue present daily as standing, flat, or buried, as well as dead roots. It also determines the amount of surface cover provided by the residue.
When the crop of interest is not listed as a choice in the WEPP management file, it is best to start with the crop parameters of a similar crop that currently exist in the crop file. If that option is not feasible, such as the case with many vegetable crops, the Crop Parameter Intelligent Database System (CPIDS) may be consulted for parameterization assistance. Crop parameters may also be refined to better reflect local growing and seasonal conditions. These refinements should better simulate the growing conditions in the field (canopy cover, height, biomass) and not just adjustments in crop yields. The following section provides details on the individual plant parameters, as well as some suggestions on adjustments to these.
Figure 3. Canopy Cover and Biomass
BB - BB describes the relationship between canopy cover and vegetative biomass as shown in Figure 3. This parameter is crop-dependent. Increasing the value of BB in small increments causes two effects in the canopy cover and biomass relationship. In observing a single, constant canopy cover value, the calculated vegetative biomass decreases while the BB increases. When observing a constant vegetative biomass value with an increasing BB, the canopy cover will increase. In other words, as BB increases, the rate of canopy cover development as a function of biomass increases. For example, with a high value of BB (14 for alfalfa, bromegrass, and soybeans), canopy cover approaches 1.0 (100%) very rapidly. On the other extreme, canopy cover for corn increases slowly as biomass increases as shown in Figure 3. When adjusting the BB from a similar crop, if the plant has more canopy cover given less total biomass on the field, increase BB slightly. If the crops have similar canopy covers but the biomass of the crop to be parameterized is less, the BB value may be increased slightly. The crop's biomass and canopy cover, if known, can be plotted as shown in Figure 3 and a linear regression can be performed on the transformed data. Adjustments to this parameter should be made with care and knowledge of the crop under consideration.
BBB - BBB, a canopy height parameter, behaves similarly to BB. BBB defines the relationship between vegetative biomass and canopy height as shown in Figure 4. Note that the Y-axis has been normalized by plotting the ratio of canopy height to the maximum canopy height. Higher BB values indicate greater height for a given biomass. BBB affects the rate that maximum canopy height is reached, not the maximum canopy height (see HMAX). To estimate BBB for a crop not available on the WEPP crop parameter list, values of biomass and canopy cover can be plotted as shown in Figure 4, and a linear regression can be performed on log-transformed data.
Figure 4. Canopy Height and Biomass Relationship
BEINP (kg/MJ) - BEINP is the biomass energy ratio of a crop. This crop parameter reflects the potential growth rate of a given crop per unit of intercepted photosynthetically active radiation. BEINP can greatly change the rate of growth, incidence of stress during the growing season, and yield in the model. This parameter should be adjusted only if absolutely indicated and then only based on research results. Data for BEINP should reflect unstressed cropping conditions, i.e., no nutrient, temperature, or water stresses.
In terms of erosion, perhaps the most important factor related to plant growth is the amount of biomass produced by the crop. The BEINP parameter is the biomass energy conversion factor. Increasing the value of BEINP will increase the amount of biomass that the crop produces, which will increase both the residue left at harvest and crop yield. The relative amount of yield to total biomass produced may be adjusted using the harvest index. If the user knows that a particular variety of corn, for example, produces 8000 lbs/acre of residue and 120 bushels per acre of grain on the average, he/she may adjust the BEINP and HI parameter values until the model calculates those amounts over a long-term (e.g., 10 years) simulation. If a variety of corn was bred to have a thicker stalk, so as to produce 12,000 lbs/acre of residue and 120 bushels per acre on the average, the BEINP parameter could be increased and the HI value decreased to reflect that difference. The grain yield does not directly influence erosion calculations, but residue left at harvest will have a significant effect on erosion. The WEPP interface management file builder contains crop parameter data to represent low, medium, and high productivity corn and soybeans, as well as a lodging-resistant corn variety.
BTEMP (°C) - BTEMP reflects the minimum or base daily air temperature required for plant growth. When the average daily air temperature exceeds the base temperature of the plant, growth is initiated for the simulation. Base temperatures are stable for cultivars within a species. It is not recommended that this parameter be changed. To compensate for crop varieties with longer or shorter growing seasons and different geographic locations, the sum of growing degrees to maturity (GDDMAX) may be modified.
CF (m2/kg) - parameter used to convert residue mass to percent surface cover [NSERL #10, equation 9.3.2]. Crop-specific CF represents the amount of soil surface covered completely by a kilogram of residue. This parameter is extremely important because the WEPP erosion routines are quite sensitive to percent surface cover.
CRIT (°C days) - CRIT represents the accumulation of growing degree days from planting to emergence. When the accumulation of growing degree days after planting has reached this value, the plants emerge and above ground biomass appears. A higher daily average temperature will cause the plant to emerge faster due to a quicker accumulation of growing degree days. The WEPP model will consider the plants emerged when CRIT is reached or at 14 days after planting, whichever comes first.
CRITVM (kg/m²) - Critical live biomass value of a perennial crop below which grazing is not allowed. If the live biomass value falls below CRITVM, no grazing is allowed on that day. If the live biomass is greater than CRITVM, grazing is allowed and the total biomass removed is calculated by equation 8.3.3, NSERL #10. This is used to 'update' the remaining amount of biomass.
CUTHGT (m) - Height of post-harvest standing residue; cutting height; or cutting height for harvest of perennial crops. This should reflect the amount of standing residue available for conversion to flat residue cover for annual crops. For perennial crops at a cutting harvest, the cutting height determines the amount of plant material harvested.
DECFCT - Fraction of the canopy cover remaining after senescence. If the crop does not reach senescence before harvesting, DECFCT is 1. DECFCT is used to compute the daily decline in canopy cover after senescence begins.
DIAM (m) - Diameter of the stem (stalk, trunk, etc.) at plant maturity. In the case of crops that do not reach maturity before harvest, the maximum stem diameter is used. This value should reflect the portion of the stem at the base of the plant near the soil surface. DIAM is used to initialize residue amounts.
DLAI - DLAI reflects the fraction of the growing season that must be reached before the leaf area index begins declining. The cumulative growing degrees or heat units from planting to leaf area index decline is divided by the total growing degrees accumulated between planting and crop maturity. For vegetables and other annual crops that may be harvested before the leaf area index begins to decline, DLAI is set to 1.0.
DROPFC - DROPFC represents the fraction of live biomass remaining after senescence. It is used to update the decline in crop biomass during senescence.
EXTNCT - EXTNCT is the radiation extinction coefficient. It is used to calculate intercepted photosynthetically active radiation from daily solar radiation and leaf area index.
FACT - Adjustment factor to account for the effect of wind and snow on standing to flat residue conversion. FACT is the fraction of the previous day's residue that remains standing for the current day. This factor is set to a default value of 0.99 in the WEPP Version 95.7 interface file builder for all crops, but the parameter has no effect when all biomass is removed from a field.
FLIVMX - Maximum friction factor (Darcy-Weisbach) for living plant. Used to account for hydraulic roughness for crops such as cotton, small grains, alfalfa, and grasses. Most generally crops are assigned values based on whether they are planted (or drilled) perpendicular or parallel to water flow. For the case of wide-row crops such as corn or crops planted parallel to the flow of water, FLIVMX should be set to 0.0. Crops that are drilled or grown in narrow rows perpendicular to the flow of water, e.g. wheat, should be assigned a FLIVMX of 2.0-3.0. For perennial grasses and pasture situations, FLIVMX should be set to 12.0. When a furrow or rill has more than 50% of the flow impeded due to living plant stems and leaves, set FLIVMX to at least 3.0.
GDDMAX (°C days) -Potential accumulation of growing degree days or heat units from planting to maturity. The growing degrees begin accumulation with the planting date and once GDDMAX is reached, the plant growth is stopped and no updates are made until the start of leaf drop or harvest occurs. If the user does not know the growing degree days to crop maturity, entering a value of 0.0 will cause the model to calculate GDDMAX based on the crop planting date and harvest date. For perennial crops GDDMAX should be set to 0.0. Growth of a perennial crop stops when the average daily air temperature is less than the plant base temperature (BTEMP), and the plant becomes dormant once the five-day average daily temperature drops below the critical minimum temperature (TMPMIN)., Figure 5, Figure 6, and Figure 7show growing degree days and growing season days for corn, sorghum, and soybeans (Kiniry et al., 1991).
Figure 5. Potential heat units for sorghum-planted two weeks later than corn. (Kiniry et al., 1991
Figure 6. Potential heat units for corn (Kiniry et al., 1991).
Figure 7. Potential heat units for soybeans--planted four weeks later than corn (Kiniry et al., 1991).
HI - HI is the normal harvest index of the unstressed crop (dry crop yield/dry above ground biomass). This crop parameter should be based on experimental data where crop stresses have been minimized to allow the crop to attain its potential. The WEPP crop growth component uses the harvest index and adjustments for water stresses to estimate crop yield. The harvest index concept was originally developed for grain crops and has been extended to tuber crops and crops where vegetative biomass is harvested. WEPP does not use the HI for perennial crops with multiple cuttings. Instead, harvested biomass is estimated as a function of the cutting height and the canopy height.
HMAX (m) - The maximum canopy height (HMAX) of the crop is used in an empirically-based equation with BBB and the above ground biomass to calculate a current canopy height. HMAX may be adjusted after observation of the crop.
ORATEA - ORATEA represents the maximum rate of residue decay that occurs under conditions considered optimum for the soil microbial population. Within the WEPP model, ORATEA is adjusted by an environmental factor (EF) to account for the daily changes in the temperature and soil water content. Since the EF differs for standing, flat, and buried residues, these three pools are kept separate for estimating residue mass. Increasing ORATEA will increase the rate of residue mass lost from a field. The ORATEA value should not be adjusted, however, unless specific field data verifying the need for change exist.
ORATER - ORATER is similar to ORATEA, but is specific to the dead root biomass.
OTEMP (°C) - OTEMP is the optimal temperature for plant growth and is stable for cultivars within a species. It is not recommended that this temperature be changed once it is determined for a cultivar. Differences in varieties and maturity lengths will be accounted for in the growing degree days to emergence (CRIT) and maturity (GDDMAX). Temperature stress is a function of OTEMP. Temperature stress occurs when the air temperature is significantly higher or lower than OTEMP.
PLTOL - Plant specific tolerance to moisture stress. PLTOL is the fraction of total soil porosity that soil moisture must decrease to before water stress occurs, and water uptake is reduced. For example, for PLTOL = 0.25, water uptake by the plant is not reduced until soil water falls below 0.25 times soil porosity. If the user inputs a value of 0.0 for PLTOL, the WEPP model will set PLTOL to 0.25. WEPP internally limits the value of PLTOL to the range of 0.1 to 0.4.
PLTSP (m) - Normal in-row plant spacing. PLTSP is used to calculate the plant population and basal area. PLTSP may be observed and changed to reflect common planting practices. Values for in-row plant spacing may be found in seed catalogs or reference materials such as Lorenz and Maynard (1988).
RDMAX (m) - Maximum rooting depth for a crop. RDMAX may be drawn from research or observed in the field. The depletion-level of soil moisture is updated for the current rooting depth which is calculated from RDMAX and the ratio of current growing degree days to GDDMAX.
RSR - Root to shoot ratio is the ratio of root biomass to above ground biomass (both dry weights). This ratio is used to update total plant root biomass for all crops using the increase in the current day's biomass value.
RTMMAX (kg/m²) - RTMMAX is the maximum root biomass for a perennial crop. Live root biomass will be accumulated until the maximum value is reached. Once this point is reached, the growth and death of the root are assumed equal. RTMMAX should be set to 1.0 for annual crops.
SPRIOD (days) - Number of days over which senescence occurs, i.e., the senescence period for a particular crop. During this time the canopy cover and biomass are linearly decreased using DECFCT and DROPFC, respectively.
TMPMAX (°C) - Maximum temperature that inhibits growth of a perennial crop. Since this parameter is not used for annual crops, 0 may be entered. The growth of a perennial plant will be stopped until the average daily temperature drops below this upper limit.
- Minimum critical temperature that causes dormancy in a perennial
crop. Plant growth stops when the average daily temperature is
at or below TMPMIN. This parameter is not used for annual crops,
and a 0 may be entered.
Figure 8. WEPP leaf area and vegetative biomass relationship.
XMXLAI - XMXLAI is the maximum leaf area index potential for a specific, unstressed crop. Once a canopy cover exists, the current leaf area index is adjusted using XMXLAI and vegetative biomass. This value is obtained through research data. The maximum leaf area index for many crops such as corn, soybeans, grain sorghum, cotton, and alfalfa is 5.0. Some crops have higher XMXLAI such as 8 or 9 for wheat, oats, and barley. A typical leaf area index development curve as a function of biomass is shown in Figure 8. XMXLAI affects the rate of biomass development. Also, LAI affects evaporation and transpiration until LAI exceeds 3.0 and the plant transpiration rate equals the potential evaporation rate.
YLD (kg/m²) - YLD is the optimum yield for the specific crop under unstressed conditions. The crop growth model in WEPP does not account for biomass and yield variation due to nutrient, pest, and other management factors. WEPP estimates an unstressed crop yield and compares it to YLD. This ratio is then used to adjust biomass accumulation to simulate unstressed yields equal to YLD. During the simulation the model applies water and temperature stresses to the potential daily increase in biomass. YLD will reflect the sum of multiple harvests or cuttings when applicable (e.g., multiple harvests for vegetable crops). If a 0.0 is entered for YLD, WEPP will calculate and use its internal optimal yield value. For the current version of WEPP (v95.7) it is recommended that the user enter a value of 0.0 here, and control biomass production and yields by altering the BEINP and HI parameters.
Table 17. Suggested values for the cropland plant specific input parameters for the WEPP erosion model (version 95.7).
|-||CRITVM (kg m 2 )|
|-||RTMMAX (kg m -2 )|
Table 17 (cont.). Suggested values for the cropland plant specific input parameters for the WEPP erosion model (version 95.7).
|-||CRITVM (kg m -2 )|
|-||RTMMAX (kg m -2 )|
* Three values of BEINP have been provided for most crops illustrated. These values represent the crops grown under Low/Medium/High fertility levels.
** Growing degree days for crops to reach maturity varies by variety and region. Values here are typical for varieties grown near Indianapolis, IN. Values of 0.0 should be input for perennial crops.
*** Values for ORATEA and ORATER are tentative and
based on wheat=0.0085, corn=0.0065, soybeans=0.0130 .