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.

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.

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.