Assessment of SWAP model in estimating the salinity and soil moisture content (Case study: Birjand)

This study wasconducted to evaluate of the agro-hydrological model SWAP3.03 forsimulating soil moisture content and salinity undercultivation of Wheatvarieties in terms of different qualities andquantities of Irrigation waterduring 2005-2006 growing season in the Research Field of University Birjand.Irrigation water was supplied from three local wells with different qualities namely 1.4, 4.5 and 9.6 ds/m.The model was initially calibrated with respect to the soil layers hydraulic and solute transportation parametersbased on a studyconducted in the research field. The results of this study showed that the seasonal average of soil moisture content and salinity distributionwas more uniformin full irrigation and excess (125% ET) treatmentsin comparison to othertreatments.Maximum nonuniformityin soil moisturecontent and salinity was observed under water and salinity stress treatments. Deficit irrigation and saline treatments reduced the accuracy of model predictions in estimating of soilmoisture content and salinity for various growing stages. Best model predictions were related to full irrigation (R2 = 0.95 for salinity class S1, R2 = 0.94 for salinity class S2,R2 = 0.93 for salinity class S3 in estimating the soil moisture content and R2 = 0.77 for salinity class S1, R2 = 0.82 for salinity class S2,R2 = 0.91 for salinity class S3 in estimating the salinity)and 75%ET treatments(R2 = 0.85 for salinity class S1, R2 = 0.82 for salinity class S2,R2 = 0.78 for salinity class S3 in estimating the soil moisture content and R2 = 0.7 for salinity class S1, R2 = 0.73 for salinity class S2,R2 = 0.87 for salinity class S3 in estimating the salinity). The trend of changes in accuracy of the model to estimating the soil moisture content and salinity was decreased towards deeper layers (R2=0.89-0.72 for soil moisture content and R2=0.87-0.71 for salinity). Despite of soil moisture content and salinity in upper layers of soil,high correlation coefficients (R2= 0.89 and0.87, for soil moisture contentandsalinity,respectively)indicated that SWAP is a dependable model for simulation of soil moisture content andsalinity.