DRAINMOD was developed at North Carolina State University in the mid 1970's (Skaggs, 1978, 1980).  It is based on a water balance in the soil profile and  uses climatological records to simulate the performance of drainage and water table control systems.  The model was developed specifically for shallow water table soils. Approximate methods are used to quantify the hydrologic components: subsurface drainage, subirrigation, infiltration, evapotranspiration (ET) and surface runoff.  For example, equations developed by Hooghoudt (Luthin, 1978), Kirkham (1957) and Ernst (1975) are used to calculate drainage and subirrigation rates, and infiltration rates are predicted by the Green and Ampt (1911) equation.  Complex numerical methods are avoided by assuming a drained to equilibrium state for the soil water distribution above the water table.  Inputs to the model include soil properties, weather data, crop variables and site parameters.  Soil property inputs include the saturated hydraulic conductivity (by layers), the relationships between drainage volume and water table depth, and information concerning upward flux from the water table.  The effective root zone depth as a function of time is also an input.

Hourly precipitation and daily maximum and minimum temperatures are read from weather records and the water balance is conducted on an hour by hour basis.  Summaries of the model predictions for hydrologic components such as rainfall, infiltration, drainage, ET, etc., are available on a daily, monthly or annual bases.  The performance of a given system design or management alternative may be simulated for a long period of climatological record, say 20 to 40 years to consider the effects of the year by year and seasonal variability.  The effects of water management system design on yields may also be evaluated.  Trafficability and planting date are predicted and stress index methods (Hiler, 1969) are used to calculate yield response to excessive and deficient soil water conditions (Hardjoamidjojo and Skaggs, 1982).  Use of the yield components is an option and can be omitted if desired.

The reliability of DRAINMOD has been tested for a wide range of soil, crop, and climatological conditions.  Results of tests in North Carolina (Skaggs, 1982), Ohio (Skaggs et al., 1981), Louisiana (Gayle et al., 1985; Fouss et al., 1987), Florida (Rogers, 1985), Michigan (Belcher and Merva, 1987), Virginia (McMahan et al., 1988) and Belgium (Susanto et al., 1987) indicate that the model can be used to reliably predict water table elevations and drain flow rates.  The model also performed well for irrigated California soils when seepage losses were considered (Chang et al., 1983).

DRAINMOD has been extended to predict the movement of salt (DRAINMOD-S) and nitrogen (DRAINMOD-N) in shallow water table soils.  DRAINMOD-S is able to predict soil salinity distribution, salt concentrations of drainage water, and the effects of salinity on crop yield.  DRAINMOD-N is able to predict nitrogen concentrations in the soil profile and in surface and subsurface drainage.  Both models use a modified version of DRAINMOD to determine average daily soil water fluxes and water contents.  Solute transport is determined by an explicit solution to the advective-dispersive-reactive equation.  DRAINMOD-N uses functional relationships to quantify rainfall deposition, fertilizer dissolution, net mineralization, denitrification, plant uptake, and runoff and drainage losses.