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Because of the excessive use of nitrogen fertilizers, most of the groundwater resources are contaminated in the upstream of the Zayandehrud basin. This study set out to investigate the effect of different levels of water and nitrogen on yield, soil nitrogen, as well as water productivity indices of winter wheat and barley. The experimental treatments were of three nitrogen levels: no nitrogen fertilizer (N0), 50 kg N ha−1 (N1), and 150 kg N ha−1 (N2), as well as two irrigation levels containing deficit irrigation (I1) and full irrigation (I2). Each treatment was repeated three times. Wheat yield for I2 was 5,640 kg ha−1 and for I1 was 3,720 kg ha−1. The yield for barley was 5,620 and 3,850 kg ha−1 for I1 and I2, respectively. The irrigation water productivity (WPi) was increased by 55% for wheat and 83% for barley under deficit irrigation level. We recommend the use of 50 kg ha−1 (instead of 150 kg ha−1), knowing that it reduces the yield and water productivity for wheat by 11.7 and 10.9%, and by 14.5 and 12.6% for barley, respectively. In contrast, the soil nitrogen residual for wheat and barley was reduced by 56.6 and 66.1%, respectively, thus greatly reducing the risk of contamination of groundwater resources. HIGHLIGHTS NO3− concentration in water from 120 wells was measured.; Using high N fertilizer by farmers caused the increase in nitrate in groundwater.; Deficit irrigation is a good strategy to increase WP in a water-scarce arid area, for wheat and barley.; Using 50 kg N ha−1 instead of 150 kg N ha−1 is a good strategy to protect the environment; however, it may slightly reduce yields.;

Considering the plant's positive response to nitrogen, farmers consume nitrogen fertilizers as much as possible. Excessive consumption of nitrogen fertilizers imposes additional costs on farmers, environmental pollution and direct negative impact on human health. Therefore, it is important to determine the amount of nitrogen-nitrate in the soil, plant and irrigation water in order to use nitrogen fertilizers optimally. In addition, according to the nitrogen balance, the amount of nitrogen deficiency is calculated and given to the plant, therefore the overuse of fertilizer is avoided. Most of the usual methods for nitrate measurement are time-consuming and expensive, spectroscopic measurement (spectrophotometer) makes it possible to determine the amount of nitrogen-nitrate in each of the soil, water and plant in the shortest time, inexpensively and accurately. In this research, we introduced a quick method of measuring the amount of nitrate in each of the water, soil and plant. The results are presented in terms of practical units in the field and soil for farmers and students, including kilograms of nitrogen per ton of product and the amount of nitrogen given through irrigation or nitrogen available in the soil in kilograms per hectare.