Plants often experience both drought and salinity stress in arid environment. Various mathematical water uptake models exist for plants response to combined drought and salinity stress. The reduction functions are classified as additive, multiplicative and conceptual models. In this study six different macroscopic reduction functions, namely; Van Genuchten (additive and multiplicative), Dirksen et al., Van Dam et al, Skaggs et al and Homaee were evaluated. The experiment was carried out on Roshan variety of wheat crop in a factorial split plot design with 3 replicates in the Research Field of university Birjand. The treatments consisted of four levels of irrigation (50, 75, 100 and 120%of crop water requirement), and three water qualities (1.4, 4.5, 9.6 dS/m). The results of this study indicated that the additive model estimates relative yield less than actual amount. In other word, the effect of combined stresses on wheat yield was less compared to sum of the separate effects due to salinity and water stress. The effect of drought stress on reduction of yield was more than salinity stress. The results also revealed that reduction function of Skaggs et al and Homaee were better fitness to measured data than the other functions.
