. A new line is then added to the tab displayed at the bottom of the page and the input for the new method can be filled in.
The Irrigation method section is used to specify how and when a given field is irrigated.
A Name should be specified for each irrigation method. A new method is created by clicking on the ‘Append’ button, designated with the symbol . A new line is then added to the tab displayed at the bottom of the page and the input for the new method can be filled in.
Alternatively, predefined irrigation methods can be loaded from a template file by clicking on the ‘Load template’ button, 
, and browsing to the specific template file. An existing template used in another MIKE HYDRO model can also be used, by clicking on ‘Import existing’, 
, and browsing to the previously created .mhydro file.
By clicking on the ‘Delete’ button, 
, the currently displayed Irrigation method is deleted.
To switch between the different Irrigation methods, click on the lines relative to different methods in the tab in the lower part of the Tabular view.
In the following you need to define: Type of irrigation methods
The following three Irrigation methods are available:
The Irrigation method dialogue also contains fields to set the Spray loss time series, the Wetting fraction time series, the Trigger option and the Application option. The Trigger option determines when the irrigation will start and the Application option determines how the depth is calculated when the irrigation has started.
The available options for Trigger and Application options depend on the selected Irrigation method, while the Spray loss and the Wetting fraction time series do not.
All input are specified as time series where the time is relative to the day of sowing.
· Spray loss time series. The Spray loss time series correspond to the fraction of the irrigation water that is evaporated before the water reaches the soil surface. For sprinkler irrigation, this fraction may be relatively high, whereas it is relatively low for drip irrigation, for example. The value has to be strictly less than 1.
· Wetting fraction time series. The irrigation model requires a wetting fraction that determines the fraction of the field surface that is being wetted during irrigation. For example, sprinkler irrigation this fraction will be close to 1, whereas for drip irrigation it may be as low as 0.1. The wetting fraction is also an important factor for determining how much irrigation is required before the surface soil storage is filled and hence when the root zone starts to fill.
Figure 8.3 Illustration of the wetting fraction (after /1/) where I and Iw are the irrigation depth for the field and the irrigation depth for the part of the wetted surface, respectively, and fw is the fraction of the surface wetted by irrigation
When this option is chosen, no irrigation is applied to the fields.
For the FAO 56 irrigation model the following parameters have to be specified for the Trigger and Application options.
Trigger option
· Option. The following four alternatives are available in the Trigger option of the FAO 56 irrigation method:
– Fraction of TAW (Total Available Water). Irrigation starts when the soil moisture content reaches the specified fraction of TAW. TAW is defined as the volume of water contained in the root zone when at field capacity.
– Fraction of RAW (Readily Available Water). Irrigation starts when the soil moisture content reaches the specified fraction of RAW. RAW is defined as the volume of water that can be transpired by the crop without exposing the crop to soil water stress. It is defined as:
where p is related to the crop model (see more details in the Depletion fraction section, presented in the Crops section).
– Specified depletion depth. Irrigation will start when the soil moisture content reaches the specified depletion depth.
– Prescribed. No Time series in the Trigger option box are required as input when this option is selected. Irrigation is only described by the irrigation Application option time series given as Fixed irrigation rate option.
· Time series. The Trigger option values are given as time series when any other Trigger option than the Prescribed Depletion is selected.
Depending on the selected Trigger option, the required time series file must contain:
– Time in the first column, which indicates the number of days from start of crop season, and either
– Fraction [-], which is used for the first two options above, or
– Depletion [mm], which is used for the specified depletion option.
Application option
· Option. The following three alternatives are available in the Application option of the FAO 56 irrigation method:
– Fraction of TAW. Irrigation stops when the soil moisture content reaches the specified fraction of TAW.
– Fraction of RAW. Irrigation stops when the soil moisture content reaches the specified fraction of RAW.
– Fixed Irrigation Rate. The specified irrigation rate is applied to the field.
· Time series. The Application option values are given as time series. Depending on the selected application option, the required time series file must contain:
– Time in the first column, which indicates the number of days from start of crop season, and either
– Fraction [-], which is used for the first two options above, or
– Irrigation rate [mm/h], which is used for the specified depletion option.
For the Rice Crop Irrigation model the following parameters have to be specified for the Trigger and Application options.
Trigger option
· Option. The following two alternatives are available in the Trigger option of the Rice Crop irrigation method:
– Prescribed. No Time series in the Trigger option box is required as input when this option is selected. Irrigation is only described by the irrigation Application option time series given as Fixed irrigation rate option.
– Water Depth. Irrigation starts when the water depth on the ground surface reaches a certain depth.
Application option
· Option. When the irrigation is triggered, the application depth is calculated according to the Application option. The following options are available:
– Fixed irrigation rate. Specified irrigation rate is applied to the field.
– Fraction of Ponded Water Depth: Irrigation stops when the water depth reaches the specified fraction of Maximum Water Depth (MWD). Note: This option is not compatible with Prescribed Trigger option.
The FAO 56 Irrigation method is also compatible with the Rice Crop model. The Rice Crop irrigation method, however, is only compatible with the Rice Crop model.
