Effect of Variant Irrigation Regimes and Salinity on Wheat yield in Gorgan Region

This research was conducted to evaluate yield and yield components of wheat crop under conditions of water deficit and salinity stress. It was also intended to monitor salt and water distribution in the soil profile in order to develop wheat water-salinity productions and to evaluate existing root water uptake models. The SWAP model was used to simulate solute and water transport under salinity and water stress condition. The treatments consisted of four levels of irrigation water 50(W1), 75(W2), 100(W3) and 125(W4) percent of crop water requirement and with four irrigation water salinity levels of 1.5(S1), 8.5(S2), 11.5(S3) and 14.2(S4) dS/m. The experiment was performed according to a randomized complete block design with split plot layout, which considered water quantity as main plot and water quality as subplot with three replications and lasted two growing seasons. The results revealed that the effects of salinity and water stress were significant on wheat yield, grain weight and grain per spike, but not significant on spike number per m2. The yield reduction due to the use of the drain saline water was not significant enough to make the use of such water completely impossible for irrigation purposes. Application of such saline water would increase soil salinity and therefore, improved irrigation management must be implemented to protected land use sustainability. Wheat yield and WUE did not considerably decreased when some drain saline water was mixed with non-saline irrigation water. The highest variation of soil salt and water contents occurred in the upper soil layer. The salt and water content distributions of the soil profile were uniform in the plots, which were fully irrigated. The soil salinity at the harvest time was increased as compared with the beginning of the season as a result of using saline water. However, it decreased again by the next presowing stage due to autumn rainfall. Various types of production functions including: Linear, Quadratic, Cobb-Douglas and Transcendental were evaluated and it was found that transcendental form gave the best results. The results indicated that marginal production based on water quantity on wheat yield is positive. However, marginal effect of a unit of matric potential on wheat yield was not the same as a unit of osmotic potential. The amount of yield reduction due to a unit increase of matric potential was more than a unit increase of osmotic potential. The marginal rate of technical substitution (MRTS) indicated that each one of the two factors, namely ECe and water content, can be substituted for the other one for a wide range in order to achieve equal amount of yield. The reduction function term was predicted by the nonlinear models more accurately than by the simple additive models. Dynamic simulation solute and water transport with SWAP model revealed that there was a good agreement (R2=0.68) between the experimental and simulated solute and water content in soil depth.