Development and evaluation of a model for water use in deciduous fruit orchards and scheduling of irrigation with the aid of meteorological data

The calibration of the SWB FAO-based crop factor model was approached in two ways.  Firstly, to determine if sap flow derived transpiration coefficients could be used instead of basal crop coefficients in combination with measured soil water deficit to calibrate the model.  Secondly, to perform SWB simulations and fit SWB predicted soil water deficit to measured soil water deficit from orchards, until the best statistical fit was obtained.

Comparison of transpiration coefficients to SWB derived basal crop coefficients indicated that the former cannot necessarily be used interchangebly with the latter for the SWB model.  Statistical output parameters and/or visual fit indicated reasonable agreement of SWB predicted to measured soil water deficit for six of the eleven plots where the fitting procedure was used.

Comparison of seasonal transpiration and evapotranspiration, indicated that the transpiration was undererestimated for one specific plot and overestimated for another.  It follows that the Kcb values determined by the fitting procedure for the two cultivars was too low and too high, respectively. It is important to note that the model could underestimate evaporation grossly for warmer areas where the canopy cover fraction exceeds the irrigated fraction of the soil.