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Zones->ET Parameters->Crop parameters

”Crop parameters” along with ”Soil parameters” are required in the calculation of evapotranspiration . There are quite a few parameters in these two tabs. However, in many cases the default values are suitable for a starting point. The boxes with a light yellow background contain computed values based on other inputs and cannot be changed directly.

The first area in ”Crop parameters” is ”Growth stages”. Irrigation requirements vary according to the growth stage. In early stages, evaporation from the soil surface is dominant. In later stages, transpiration from the plant leaf structure becomes more important. The definitions of these stages are:

• Planting date is the date seedling are set out or the green-up date for perennials.
• End initial is the date the plant vegetative growth reaches about 10% ground cover or initial leaf unfurling for trees and shrubs.
• End development is the date effective full cover is achieved. This is generally at the point of flower development for most plants or full leaf development in trees and shrubs.
• End mid is the date the plants reach maturity. Yellowing or senescence signals the end of the mid season.
• End late is the date full senescence is reached or harvesting begins.

The ”Species parameters” area describes some characteristics of plants.
Note: Root minimum and maximum depth is a very important variable in determining irrigation frequency and amount. When specifying these values, it is recommended to be conservative. The depth of the predominance of roots is the key factor rather than the maximum depth of a few ambitious roots.

• Min root is the minimum depth of root system. For seedlings, this is quite shallow, perhaps a few inches (.05 - .15 m). For perennials, this is typically 12-24 inches (.3 - .5m).
• Max root is the maximum depth of root system. This may or may not be restricted by the depth of soil. For perennials, this would be the same as Min root. Always use the plants with the shallowest root depth or these plants may not receive adequate water.
• Max height is the average height of the plants.
• LAI is leaf area index. The definition of LAI is the area of leaf/unit area of ground surface from a bird’s eye view and for a mature plant. This can be difficult to determine but using 3.0 is a good compromise.
• Dfactor is depletion factor. This is the minimum fraction of potential soil water that can be used up before irrigation is initiated. Typically, this is 0.45 (45% depletion).

The methodology RHS uses to determine crop water consumption is based on crop coefficients.

The”ET factors” area is where these coefficients are specified. A full discussion of crop coefficients can be found here

• Kcb initial applies to the initial growth stage.
• Kcb mid applies to the main growth stage and is calculated from information supplied in the ”Landscapes” dialog.
• Kcb end applies during the final growth stage to full senescence.
• Kcb min represents a minimum evapotranspiration level and is usually set to (0.15).

”Mid season weather” is the average weather conditions with respect to wind speed and humidity during the main crop growth stage or mid season. If this is not known, an AVG wind speed of 2 meters/second and an RH of 45% are good initial values for typical (not very arid and not very humid) climates.