A simulation model for evaluating the effect of water availability in the soil and salinity on yield under deficit irrigation has been developed. The model is based on water volume balance concept for scheduling irrigation using different levels of allowable percent water depletion. To take the effect of salinity on yield into consideration, the crop evapotranspiration is linked to the salinity level in the soil water using the procedure described in FAO (1998). The model was used to study the effect of both water deficit and salinity build-up during the growing season on yield of cotton for different scenarios, assuming zero leaching, with different irrigation water salinity levels, and allowable percent depletion of water in the root zone.
To verify the validity of the model, preliminary one year data from experiment conducted in northern Syria during the summer season of year 2004 for cotton are used. The data included four deficit irrigation levels using drip irrigation system: full irrigation (no deficit); applying 80%, 60%, and 40% of full irrigation. The experiment was laid out in three replications. The main outcome of the study included useful relationships between relative yield with relative crop evapotranspiration as affected by different levels of deficit irrigation and water salinity. Given the salinity of irrigation water and selecting a cretin level of percent water depletion, the relative evapotranspiration (Etcadj/ETc) can be predicted. Upon knowing the relative evapotranspiration, the relative yield under the given conditions can be also evaluated.

Keywords: deficit irrigation, irrigation scheduling, soil salinity, simulation, evapo-transpiration, leaching requirements.