Effect of Irrigation Scheduling on Indicators of Water Productivity and Sustainable Management of Groundwater: Case Study in Neyshabour Plain

Access to fresh groundwater plays an important role in stable crop production and secure livelihood of people living in the Neyshabour plain. Decline in groundwater table and annual aquifer abstraction of more than 200 million cubic meters are the most important challenges in this plain, where about 110,000 ha of agricultural land are cultivated annually and 96% of groundwater resources are used in the agricultural sector. In this study, Soil-Water-Atmosphere-Plant (SWAP) model was calibrated and validated using measured data from six different fields located in the Neyshabour plain. For this purpose, field information and other SWAP required data were collected in the six farm conditions. Calibrated and validated SWAP model was then used to quantify the effects of existing irrigation practices on water balance components and different water productivity indicators and to determine improved irrigation schedules for wheat, barley, sugar beet, cotton, silage corn, and tomato. Estimation of the net water saving as a result of improved irrigation schedules is also discussed. The results showed that under the current irrigation practices, soil evaporation reduced WP_ET (Y_act/ET_act) over WP_T (Y_act/T_act) by 24%, 26%, 27%, 21%, 8% and 16% for wheat, barley, sugar beet, cotton, silage corn and tomato, respectively. The reduction in WP_ETQ (Y_act/ET_act+q_bot) over WP_ET because of deep percolation was even higher: 50%, 44%, 33%,  37%, 14% and 56% for wheat, barley, sugar beet, cotton, silage corn and tomato, respectively. The substantial differences in WP values emphasized the need to control non-beneficial soil evaporation and deep percolation losses, and change traditional irrigation system by a more efficient one. Model simulations by improved irrigation schedule revealed that a seasonal irrigation amount of 520 mm for wheat, 440 mm for barley,  1010 mm for sugar beet, 930 mm for cotton, 870 mm for silage corn and 1050 mm for tomato would be enough to get the maximum yields. The optimized irrigation schedule uses 26% less irrigation water as compared to current irrigation practices and increases WP_I (Y_act/Irrig) by a factor of 1.2 for wheat, 0.7 for barley, 1.6 for sugar beet, 6.7 for silage corn and 8.3 for tomato because of reduced soil evaporation and moisture storage in the root zone. Considering the total area of cultivation of the six studied crops in the Neyshabour plain (80000 ha), it is estimated that adoption of optimal irrigation schedules can save up to 165 million m³ of water.