For each crop stage, three vegetation parameters need to be specified:
· LAI - The Leaf Area Index, which is the (Area of leaves)/(Area of the ground), can vary between 0 and 7 depending of the vegetation type;
· Root - The Rooting Depth of the crop. It will normally vary over the season. Consideration about the soil type should be taken because some crops may develop different root distribution depending on the soil characteristics;
· Kc - The crop coefficient.
The leaf area index and the root depth should be specified at the end of each crop stage. The development of LAI and root depth between the specified values are then interpolated linearly by the model. In addition to these parameters, it is often necessary to supply the crop coefficient (Kc), which is used to adjust the reference evapotranspiration relative to the actual evapotranspiration of the specific crop.
By the FAO definition, the reference evapotranspiration represents the potential evapotranspiration for a 8-15 cm high reference grass plane with ample water supply. Most farm crops may differ from this in two ways:
· In the early crop stages, where LAI of the farm crop is lower than the LAI of the reference grass crop, the evapotranspiration of the farm crop is less then the calculated reference evapotranspiration. This is accounted for in the Kristensen & Jensen ET calculation, since a crop LAI is used as input. Therefore, for most field crops it is therefore not necessary to specify Kc values below 1 in the early crop stages.
· In the crop mid-season the opposite situation may occur where crop potential evapotranspiration is larger than the calculated reference evapotranspiration of the reference grass crop. This is not handled in the ET calculations, and Kc values above 1 may therefore be relevant for some crops in the mid-season during the period where crop leaf area index is at its maximum.
A Kc value of 1 means that the maximum evapotranspiration rate will equal the reference evapotranspiration rate.
If pan evaporation data are used in place of reference evapotranspiration data in the model input, it often necessary to apply site specific pan coefficients to convert the pan evaporation to reference evapotranspiration. Pan coefficients are normally in the range 0.5 – 0.85.
