When the user selects auto-application of irrigation, the application can be triggered by a water stress threshold or a soil water deficit threshold. The water stress threshold is a fraction of potential plant growth. Any day actual plant stress falls below this threshold fraction due to water stress, the model will automatically apply water up to a maximum amount per application as input by the user. An irrigation application can also be triggered by a soil water deficit threshold. When total soil water in the profile falls below field capacity by more than the soil water deficit threshold, an irrigation application occurs. As with a manual application, a maximum irrigation amount, irrigation efficiency and surface runoff ratio are applied to each application.

Table 6:2-1: SWAT+ input variables that pertain to irrigation.

Accurately reproducing water management practices can be one of the most complicated portions of data input for the model. Because water management affects the hydrologic balance, it is critical that the model is able to accommodate a variety of management practices. Water management options modeled by SWAT+ include irrigation, tile drainage, impounded/depressional areas, water transfer, consumptive water use, and loadings from point sources.

Impounded/depressional areas are simulated as a water body overlying a soil profile in an HRU. This type of ponded system is needed to simulate the growth of rice, cranberries or any other plant that grows in a waterlogged system. The simulation and management operations pertaining to impounded/depressional areas are reviewed in Chapter 8:1.

To simulate tile drainage in an HRU, the user must specify the depth from the soil surface to the drains, the amount of time required to drain the soil to field capacity, and the amount of lag between the time water enters the tile till it exits the tile and enters the main channel.

Tile drainage occurs when the perched water table rises above the depth at which the tile drains are installed. The amount of water entering the drain on a given day is calculated:

$tile_{wtr}=\frac{h_{wtbl}-h_{drain}}{h_{wtbl}}*(SW-FC)*(1-exp[\frac{-24}{t_{drain}}])$ if $h_{wtbl}>h_{drain}$ 6:2.2.1

where $tile_{wtr}$ is the amount of water removed from the layer on a given day by tile drainage (mm H$_2$O), $h_{wtbl}$ is the height of the water table above the impervious zone (mm), $h_{drain}$ is the height of the tile drain above the impervious zone (mm), $SW$ is the water content of the profile on a given day (mm H$_2$O), $FC$ is the field capacity water content of the profile (mm H$_2$O), and $t_{drain}$ is the time required to drain the soil to field capacity (hrs).

Water entering tiles is treated like lateral flow. The flow is lagged using equations reviewed in Chapter 2:3.

Table 6:2-2: SWAT+ input variables that pertain to tile drainage.

Irrigation in an HRU may be scheduled by the user or automatically applied by SWAT+ in response to a water deficit in the soil. In addition to specifying the timing and application amount, the user must specify the source of irrigation water.

Water applied to an HRU is obtained from one of five types of water sources: a reach, a reservoir, a shallow aquifer, a deep aquifer, or a source outside the watershed. In addition to the type of water source, the model must know the location of the water source (unless the source is outside the watershed). For the reach, shallow aquifer or deep aquifer, SWAT+ needs to know the reach number or subbasin number, respectively, in which the source is located. If a reservoir is used to supply water, SWAT+ must know the reservoir number.

If the source of the irrigation water is a reach, SWAT+ allows additional input parameters to be set. These parameters are used to prevent flow in the reach from being reduced to zero as a result of irrigation water removal. Users may define a minimum in-stream flow, a maximum irrigation water removal amount that cannot be exceeded on any given day, and/or a fraction of total flow in the reach that is available for removal on a given day.

For a given irrigation event, SWAT+ determines the amount of water available in the source. The amount of water available is compared to the amount of water specified in the irrigation operation. If the amount available is less than the amount specified, SWAT+ will only apply the available water.

A manual irrigation application can be scheduled by date or by heat units. Irrigation amount (mm), input by the user, is the amount of water applied that reaches the soil. An irrigation efficiency factor is applied to account for losses from the source to the soil including conveyance loss and evaporative loss. The surface runoff ratio is the fraction of water applied that leaves the field as surface runoff. The remainder infiltrates into the soil and is subject to the soil water routing algorithms described in Section 2, Chapter 3. This allows for more realistic simulation of the soil water profile and application of excess irrigation for leaching salts.

While water is most typically removed from a water body for irrigation purposes, SWAT+ also allows water to be transferred from one water body to another. This is performed with a transfer command in the watershed configuration file.

The transfer command can be used to move water from any reservoir or reach in the watershed to any other reservoir or reach in the watershed. The user must input the type of water source, the location of the source, the type of water body receiving the transfer, the location of the receiving water body, and the amount of water transferred.

Three options are provided to specify the amount of water transferred: a fraction of the volume of water in the source; a volume of water left in the source; or the volume of water transferred. The transfer is performed every day of the simulation.

The transfer of water from one water body to another can be accomplished using other methods. For example, water could be removed from one water body via consumptive water use and added to another water body using point source files.

Table 6:2-3: SWAT+ input variables that pertain to water transfer.

Consumptive water use is a management tool that removes water from the basin. Water removed for consumptive use is considered to be lost from the system. SWAT+ allows water to be removed from the shallow aquifer, the deep aquifer, the reach or the pond within any subbasin in the watershed. Water also may be removed from reservoirs for consumptive use.

Consumptive water use is allowed to vary from month to month. For each month in the year, an average daily volume of water removed from the source is specified. For reservoirs, the user may also specify a fraction of the water removed that is lost during removal. The water lost in the removal process becomes outflow from the reservoir.

Table 6:2-4: SWAT+ input variables that pertain to consumptive water use.

SWAT+ directly simulates the loading of water, sediment and other constituents off of land areas in the watershed. To simulate the loading of water and pollutants from sources not associated with a land area (e.g. sewage treatment plants), SWAT+ allows point source information to be read in at any point along the channel network. The point source loadings may be summarized on a daily, monthly, yearly, or average annual basis.

Files containing the point source loads are created by the user. The loads are read into the model and routed through the channel network using rechour, recday, recmon, recyear, or reccnst commands in the watershed configuration file. SWAT+ will read in water, sediment, organic N, organic P, nitrate, soluble P, ammonium, nitrite, metal, and bacteria data from the point source files. Chapter 2 in the SWAT+ User’s Manual reviews the format of the command lines in the watershed configuration file while Chapter 31 in the SWAT+ User’s Manual reviews the format of the point source files.