DRAINMOD simulates the performance of a given system for a long period of weather record.  It predicts water table depth, drainage rates, surface runoff, ET, etc.  on a continuous basis.  Alternative system designs may be simulated to select the one that best satisfies the system design objectives.  Several drainage and associated water management practices can be analyzed with DRAINMOD.  A short  discussion of these are given below.

Conventional Drainage

Subsurface drainage is considered to be that drainage provided by evenly spaced parallel drains with a free (not submerged) outlet.  Either drain tubing or drainage ditches may be considered.  Surface drainage is characterized by  the average depth of depressional storage, with poor surface drainage for large depressional storage and good surface drainage when the amount of water that can be stored in surface depressions is small.

The performance of a drainage system consisting of both surface and subsurface components may be analyzed.  By conducting simulations for several combinations of surface and subsurface drainage, the best depthspacing combination of subsurface drains can be determined in terms of the drainage objectives.

Controlled Drainage

Controlled drainage is used to conserve water and reduce pollutant loading from drained lands.  It is implemented by placing a control structure in the drainage outlet such that the water level in the outlet must rise to a set level (weir elevation) before drainage can occur.  The water level in the outlet raises to the weir elevation due to surface runoff and subsurface drainage during wet periods.  During dry periods water stored in the outlet flows through the drain into the soil profile to satisfy ET demand.  This lowers the water level in the outlet and provides potential storage for subsequent drainage events.

The effectiveness of controlled drainage depends on the elevation and timing of the water level control (weir depths), depth and spacing of the drains, soil properties, and other system parameters such as layout and the dimensions of the outlet ditch or canal.  The effects of these factors can be determined by conducting simulations for a range of parameter values.

Subirrigation

This is similar to the controlled drainage mode.  The main difference is that water is pumped into the drainage outlet to maintain the outlet water level at the set point, or weir elevation.  The effects of depth and spacing of the drains, elevation and timing of the outlet water levels (weirs) and surface drainage intensity can be determined by conducting simulations for a range of parameters.  In addition to hydrologic components and yields, the volume of water pumped for subirrigation is computed.

Surface (sprinkler) Irrigation (Wastewater irrigation)

Sprinkler irrigation is an option for the model.  This option was originally intended to analyze drainage for wastewater irrigation.  Conventional irrigation can be analyzed for some cases.  Two periods can be specified in  which irrigation will not be simulated.  The effect of depth and spacing of the subsurface drains and surface drainage intensity on number of days of irrigation and loading rates may be determined.

The minimum drained (air) volume required for irrigation is specified as an input.  This is the minimum air volume in the soil profile for irrigation and is a function of the water table depth and the soil porosity.  For safety, this should be set   at 1 cm more than irrigation amount. The amount of rainfall sufficient to cause postponement of irrigation is also an input.

Hydrologic Analysis of Wetlands

Since DRAINMOD describes the soil water balance for high water table soils and predicts water table depth on a day-by-day basis, it can be used to determine whether or not a particular site meets the wetland hydrologic criteria.  DRAINMOD was modified in Version 4.6 to facilitate these types on analyses.   Four inputs were added to the model to define the beginning and end of the growing season, the threshold water table depth, and the number of consecutive days with the water table above the threshold required to meet a wetland criteria.  When DRAINMOD is used with these inputs, it will calculate and report the number of years during the simulation period that the specified criteria were met.

DRAINMOD was further modified in Version 5.0 to calculate the runoff volume from surrounding areas that drain to a wetland and add that runoff volume to the wetland water balance.  This is a two step process. In the first step, files of  runoff hydrographs from the surrounding areas are created  using DRAINMOD and instantaneous unit hydrograph method .  In the second step, the runoff hydrograph files are used as input (similar to rainfall input) for the DRAINMOD analysis of the wetland.  Inputs for calculating the runoff volume include the area of the land draining to the wetland, the time of concentration for the lands, and  an instantaneous unit hydrograph adjustment value.  Inputs for the analysis of the wetland receiving the runoff are the runoff hydrograph files generated in the first step and a ratio of the contributing area to the receiving wetland area.  

Analysis for Nitrogen Movement and Loss

DRAINMOD-N can be used to predict nitrogen concentrations in the soil profile and in surface and subsurface drainage.  This is particularly useful for determining the effect of water management of fertilizer management on nitrogen loading from agricultural fields.  The model simulates a simplified version of the nitrogen cycle.  It uses a quasi two-dimensional model for describing the movement and fate of nitrogen in shallow water table soils with artificial drainage.  The main nitrogen pool considered is nitrate-nitrogen. The controlling processes considered by the model are rainfall deposition, fertilizer dissolution, net mineralization of organic nitrogen, denitrification, plant uptake, and runoff and drainage losses.

Analysis for Soil Salinity

DRAINMOD-S can be used to predict soil salinity as affected by irrigation water quality and drainage system design.  This is particularly useful in arid regions for designing irrigation and drainage systems that will minimize yield reduction due to excess soil salinity.  In DRAINMOD-S, the overall relative yield function also includes the effect of salinity stress on crop yield. The salinity option includes sections to enter the dispersion coefficients,  precipitation limit of salt, soil depth use to calculate average salinity for crop yield, initial salt concentration in the profile and additional output options.