the objective of this paper is to present a mechanistic model to estimate the available soil water storage to forecast yield for nonphotosensitive annual crops. in this study, concern is devoted to the movement and retention of water within the agricultural system. crop yield depends upon the basic processes of photosynthesis and respiration. the yield also depends on crop species, crop nutrition, available energy, plant population, weeds and parasite populations, mainly. in addition, actual evapotranspiration also depends on the same attributes and processes. therefore, the basic hypothesis of this model is that it is possible to forecast crop yield with estimates of actual evapotranspiration. knowing empirically the process of soil water loss, but without knowing all attributes and their complex relations ruling this behavior, a cosine model was established to estimate available soil water storage. the cosine model was constructed and checked together with other models and the conclusion is that the cosine model best estimates soil water storage.