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Zones->ET Parameters->Soil parameters

This dialog is used to define the ”Soil parameters” required for the calculation of evapotranspiration . There are quite a few parameters in this tab. However, in many cases, the default values are suitable for a starting point. Default values are set based on the soil ”Classification”. The values with a light yellow background are computed values based on other inputs and cannot be directly altered.

”Water availability” parameters describe the quantity of water in the soil that is available for plants.

• fw(irrig) is the fraction of soil surface wetted during irrigation. It is set in ”Landscapes” when the type of irrigation system is chosen.
• Initial fw sets the initial fraction of soil surface wetted on startup. If irrigation is started after a significant rain event, this value would be 1.0. If the irrigation system was used for the initial watering cycle, the value would be close to fw(irrig).
• Initial De is an estimate of the startup soil water depletion level. For a heavy rain event or irrigation, this value can be set to zero. The closer it is to RAW, the sooner the first scheduled irrigation event will occur.
• RAWis the readily available soil water within the root zone. It is calculated by multiplying TAW by Df as set in ”Crop parameters” tab.
• TAW is total available soil water in the root zone.
• MAD init is the percentage of soil water depletion that can be accepted during the initial growth stage.
• MAD later is the percentage of soil water depletion that can be accepted after the initial growth stage.

”Water content” parameters describe the water holding capacity of the soil and the water content point at which plants will show stress and begin to wilt. The parameter displayed in white is active and can be altered using the slider. To choose a parameter, click on it. The range of values for the slider is determined by the soil classification. If in doubt, choose conservative amounts but try the default as a starting point.

• Field capacity is the volume of water that can be stored per unit volume of soil.
• Wilting point is the point at which the volume of water per unit volume of soil will stress the plants.

”Evaporation” parameters describe the quantity of soil water available to be evaporated. The parameter displayed in white is active and can be altered using the slider. To choose a parameter, click on it. The range of values for the slider is determined by the soil classification. If in doubt, choose conservative amounts but try the default as a starting point.

• TEW is the maximum depth of water that can be evaporated from the soil surface layer.
• Ze is the depth of surface soil layer subjected to drying by evaporation. This is usually set at 0.1.
• REW is the maximum depth of water that can be evaporated from the soil surface layer without restriction.