Download Introduction Background history of the DRAINMOD model

DRAINMOD
Introduction
Background history of the DRAINMOD model
DRAINMOD refers to a process based field scale model used to simulate the hydrology of poorly drained
soils (Skaggs et al. 2012). The model was developed by Dr. Wayne Skaggs from North Carolina State
University in the late 1970s. Skaggs (1978) published the first version of the DRAINMOD which included a
description of the basic algorithms and results of an initial field test which was done on four drainage
treatments. FORTRAN language was used with the original model. The model received support from USDA
Natural Resources Conservation Service in 1979 which laid the foundation for its application.
Since then, numerous modifications have been made to the model. While the model was first developed to
simulate hydrological flow in poorly drained and high water table soils, it has over the years been
extended to include predictions of fate of salt, carbon and nitrogen in drained fields (Luo et al. 2000). The
model has changed over the years and has advanced significantly from being a strictly field-scale
hydrologic model to becoming a water quality and watershed-scale model as well (Skaggs et al. 2012)
Description of the model and application
The model is based on water balances in the field surface, soil profile and drainage system in some cases
(Skaggs et al. 2012). It predicts the impact of drainage and management practices on soil water regime,
water table depths and crop.
Input requirements
The main input requirements for the DRAINMOD model are the soil properties, drainage design
parameters, crop parameters, and weather and irrigation data (Reynolds 2008). Water balance is conducted
midway between parallel drains at hourly rates as shown below.
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DRAINMOD
The water balance equation is:
V = D + ET + DLS - F
Where V is the change in air volume
D = Drainage (subirrigation section)
ET = Evapotransipiration
DLS = Deep and Lateral Seepage
F = infiltration
Water balance equation at the soil surface can be found from the following equation
P in this case is the precipitation, F is infiltration, is the change in storage volume and RO refers to the
runoff.
Where thewater table is found below the ground surface, Houghoudt's equation is used to predict the
drainage rates. Radial flow is assumed near the drains and Dupuit-Forchheimer assumptions are made as
the water moves away from the drains (Zhao et al. 2000). While for saturated soil profiles and ponded
surfaces, Kirkham’s equation is applied.
Outputs
Based on the input parameters and water balance which is conducted on hourly basis, the model is able to
calculate for the drainage rate, surface runoff, infiltration, evapotranspiration, water table depth and deep
and lateral seepage. These hydrologic variables are predicted by the model and summary outputs made
available on daily, monthly, and yearly bases. The model can also predict additional outputs such as the
depth of irrigation water, crop yields, and variables indicating wetland hydrologic status (Skaggs et al.
2012)
Model calibration and validation
Calibration is necessary when using the DRAINMOD model. Same input data is required for calibration of
this model including data on soil properties, drainage system parameters, site characteristics, weather
data, and crop data. Although DRAINMOD was originally developed to be used without calibration, the
variability and difficulty associated with simulating the actual field conditions including soil properties,
crop root depths, and surface depressional storage have made calibration necessary (Luo et al. 2000).
DRAINMOD is typically calibrated for a soil series to represent an area much larger than that instrumented
for calibration. Calibration is often done over a one year period which should include both the dry and wet
seasons. Validation is then done by comparing the model predictions of water table depths, subsurface
drainage and surface runoff with field measurements (Skaggs et al. 2012). Comparison is normally made by
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DRAINMOD
graphing the actual field measurements and model predictions and then quantifying the results using
statistical methods such as the normalized percent or the mean absolute error (Skaggs et al. 2012).
Applications
Hydrologic predictions made by this model have been tested and found to be valid under a wide range of
soil, crop and climate conditions (Skaggs et al. 1994). As such, the DRAINMOD model has gained increased
usage in the design of drainage systems. It is currently being used to fit system designs to local soil, site
and climate conditions and to quantify the effect of associated management practices on drainage water
quality (Skaggs et al. 2012). Given its focus on poorly drained soils, it has received wide usage in the
analysis of wetlands hydrology. It is being applied to describe the performance of drained stormwater and
to analyze water table fluctuations and wetland hydrology.
Model limitations
One of the main limitations to the use of this model has been the simulation of snowy conditions. The
DRAINMOD model has often been found to be inefficient in simulation under conditions of snowing and
thawing due to creation of frozen soil profile. The model application has thus been limited to warm
seasons owing to its lack of freezing and thawing algorithm (Luo et al. 2000). Modifications have however
been made to the model to include prediction of the soil temperature and snow accumulation and melting
processses (Zhao et al., 2000). Details of new algorithim can be found in an article published by Luo et al
(2000)
Perhaps another area of weakness with the original DRAINMOD model has been predicting the
denitrification process. A field study done by Madramootoo et al. (1995) found that the model was poor at
predicting the cumulative denitrification rates. First order kinetics is used to model the denitrification
process. While such an approach performs well under NO3-N limiting conditions, it has been found to
overestimate the denitrification processing rates under conditions of high NO3-N concentrations.
Another limitation to the original DRAINMOD model lies in its way of treating soil organic matter. A simple
approach is used to describe the interaction between the organic and mineral N pools (Youssef 2004). The
ON pool is considered as a continuous supply of mineral Nitrogen. Once initialized, the ON pool stays
constant for the whole simulation period. However, the cumulative effects of agricultural practices such as
application of animal manure and plant residues cannot be modeled through such an approach (Youssef
2004).. The long term effects of such practices are not effectively captured. There is need for a more
realistic representation of soil ON for the long term simulations.
Reference
Luo, W., R. W. Skaggs, and G. M. Chescheir. 2000. DRAINMOD modifications for cold conditions. T. ASAE,
3(6):1569-1582.
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DRAINMOD
Madramootoo, C.A., S.R. Broughton, and G.T. Dodds. 1995. Watertable management strategies for soybean
production on a sandy loam soil. Canadian Agricultural Engineering 37(1):1-7. North Carolina Water Resources
Res. Inst., North Carolina State Univ., Raleigh, N.C.
Reynolds, W.N., 2008. DRAINMOD: a simulation model for shallow water table soils. Raleigh: North Carolina
University.
Skaggs, R. W. 1978. A water management model for shallow water table soils. Rept. 134.
Skaggs, R. W., D. Amatya, R. O. Evans, and J. E. Parsons. 1994. ‘Characterization and evaluation of proposed
hydrologic criteria for wetlands’. J. Soil and Water Cons. 49(5): 501-510.
Skaggs, R.W., Youssef, M.A. and Chescheir, G.M., 2012. ‘DRAINMOD: model use, calibration and validation’.
American Society of Agricultural and Biological Engineers, vol. 55 (4), pp. 1509 -1522
USDA, 1994. DRAINMOD: user's guide. North Carolina State University.
Youssef, M.A., 2004. Modeling nitrogen transport and transformations in high water table soils. Raleigh, North
Carolina State University
Zhao, S., S. C. Gupta, D. R. Huggins, and J. F. Moncrief. 2000. ‘Predicting subsurface drainage, corn yield, and
nitrate–N losses with DRAINMOD–N’. J. Environ. Qual. 29: 817–825
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