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Irrigation Demand |
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Conditions: |
Irrigation selected in Land Use and UZ and ET simulated |
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dialogue Type: |
Integer Grid Codes with sub-dialogue data |
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Grid Code |
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The Irrigation Demand is used to describe when the water will be applied in the model.
The Irrigation Demand data item is divided into two dialogues. The first is the distribution dialogue for the Demand Areas and the second contains the information on when the water will be applied in each Demand area.
Demand Type
User Specified - For the User Specified demand type, the demand is not calculated. Rather it is simply specified as a constant value or as a time series.
Crop stress factor - The crop stress factor is the minimum allowed fraction of the crop specific reference ET that the actual ET is allowed to drop to before irrigation starts . That is, the minimum allowed (Actual ET)/(Reference ET x Kc) relationship. This should be a value between 1 and 0. If the actual Crop Stress factor falls below the given value, irrigation will be added.
Ponding depth - When using this option, the demand will be equal to the difference between the actual ponding depth and specified ponding depth. The option is typically used for modelling irrigation of paddy rice. If the ponding depth falls below the specified value then more irrigation water is added.
Max allowed deficit - The available water for crop transpiration (AW) is the difference between the actual water content and the water content at the wilting point for the root zone. The maximum available water for crop transpiration (MAW) is then the value of AW for the reference moisture content, where either the saturated water content or the field capacity can be specified for the reference moisture content. The deficit can then be defined as the fraction of the MAW that is missing and is a value between 0 and 1, where 0 is the deficit when the actual moisture content is equal to the reference moisture content, and 1 is the deficit when the available water for crop transpiration is zero, which is when the water content drops below the water content at the wilting point. When using the Max Allowed Deficit method, irrigation is started when the deficit exceeds the moisture deficit start value and stops at the moisture deficit end value.
If, for example, the reference moisture content is the field capacity and irrigation should start when 60 % of the maximum available water in the root zone is used and cease when field capacity is reached again, the value for the Start should be 0.6, and the value for the Stop value should be 0
Reference Moisture Content
If the Maximum allowed deficit is used, then the reference moisture content can be either for calculating the deficit is either the saturated water content or the field capacity water content.
Temporal Distribution
In each of the demand types the demand factor can be specified as a constant value or as a time series. However, an additional option in this combobox is to use the Vegetation Properties file. To use this option, you must include a vegetation properties file when you specify the vegetation. Further, you must specify the irrigation properties in the vegetation properties file itself. In this case, the Vegetation properties file, will contain all of the values needed by each of the different methods and the demand values cannot be input in this dialogue.
