عنوان مقاله [English]
Introduction: The level of aquifers has decreased as a result of the decrease in the water level of Urmia Lake in Iran. In addition, the infiltration of both salt water and excess salts, especially nitrates, caused by the use of chemical fertilizers have reduced the quality of groundwater in Urmia plain. Saline waste water, return water, agricultural activities, evaporation and infiltration of sea water are among the salinity factors of water resources. On the other hand, in modern agriculture, the yield and productivity of the product is improved by the use of chemical fertilizers. For this reason, agriculture is known as the most important source of non-point pollution of groundwater nitrates caused by human activities. The nitrate range of plain varies from 0 to 115 mg/l. Also, the level of EC varies from 0.3 to 2.9 ds/m. Both ranges are in warning points, which should be prevented from increasing excessively by adopting management measures. This study mainly aimed at investigating quantitative and qualitative changes of irrigation water in agro-economic terms.
Materials and Methods: The present study was conducted in Urmia plain located in Urmia city of Iran. The area of the plain is 962 km2 and is located on the western side of the lake. The yields of irrigated wheat, forage corn, sunflower and tomato were simulated using the AquaCrop model in the cropping year of 2022-2023; then, the options of reduced irrigation, reduced fertilization and increased groundwater salinity were applied in all stages of crop growth. Also, four groups of climatic, crop, soil and management data were used for the simulation. In proportion to the fact that decreasing crop yield has economic effects, the effects were investigated by calculating water and nitrogen fertilizer physical and economic productivity indexes. Both consumption amount of each input per hectare and the purchase cost of each input were obtained through the interviews with 140 farmers using random classification sampling method. By adding all the costs, the cost of each product was calculated and the farmers' income was obtained according to the yield and price of the products. Then, changes in costs were examined by changing the amount of irrigation and fertilization. Changes in income were also calculated by applying each option, and net profit was obtained by changing the costs and revenues. By carrying out yield simulation and applying options, the impacts of yield reduction on farmers' productivity and profit were calculated and accordingly, the potential areas for growing each crop in each region were determined.
Results and Discussion: Decreased fertilization demonstrated the utmost effect and increased salinity of irrigation water and groundwater revealed the least effect on the yield of crops. Among the two indexes, the economic productivity index of water IRI rials/m3 and fertilizer IRI rials/kg was chosen, based on which tomato (209580, 3005000) and sunflower (177630, 2115720) in region 5, forage corn (94900, 1323500) and irrigated wheat (5620, 454570) in regions 3 and 6 had the highest rate, respectively. Furthermore, the rate of fertilizer economic productivity index was higher than water productivity. After calculating these indicators, the value of the water and fertilizer economic productivity index was recalculated after applying the options and the changes were analyzed. With the reduction of irrigation, the water economic efficiency index for some crops decreased in some areas and increased in some other areas, but with the decrease in fertilization, the economic efficiency of fertilizer decreased in all areas and all crops. In both options, tomato and wheat had the highest and lowest amount, respectively.
Conclusions: Considering the obtained results and the quantitative and qualitative effects of water reduction in the region, it is necessary to reduce the effects of excessive use of chemical fertilizers before reaching a critical stage. In addition, reducing the excessive use of surface and underground water is vital. If the current trend continues, the cultivation pattern and the type of land use should be changed according to the conditions, or additional costs may be imposed on the farmers to eliminate the effects of excessive consumption, and all these cases require proper management of water resources to maintain their quantity and quality. Given the high impacts of the fertilizer reduction option and the economic productivity index of fertilizer, the possibility of replacing organic fertilizers with nitrate ones should be technically and economically investigated and the high price for using more than the recommended amount of nitrate fertilizers should be determined. These policies are also effective for reducing salinity because the main source of groundwater salinity is excessive fertilization. It is also suggested that appropriate irrigation technologies are introduced to the farmers and the correct cultivation patterns are presented in each region of the whole area. Finally, strict management actions should be taken to moderate the destructive effects of excessive fertilization and to reduce the over-exploitation of surface and groundwater.