Rainfed cultivation is one of the best ways to rich the sustainable development with optimal use of green water. About 75 percent of the world's cultivated areas are rainfed, which forms important part of the international economy. For a long time rainfed cultivation has been a common method in low-rainfall areas of Iran and mentioned method is one of the main ways of producing crops yet. Rainfall is an important factor in rainfed agriculture, so its limitations and non-conformity with planting season can reduce the production efficiency. Wheat growing period consists of three seasons of the year. Rainfall in spring, fall and winter is effective on wheat growth. Cereal grains, especially wheat, are the most important food source in the world. After cereals, legumes are considered to be the main human food source and have significant nutritional and agricultural importance. In addition to being high in quality and valuable, suitable and complementary protein for cereals in the nutritional pattern, lentils are also among the legumes that stabilize the air nitrogen in the soil, which lends itself to crop rotation. Thus, intermittent cultivation of rainfed wheat with rainfed lentil is an important factor in stabilizing production in the developing countries. Given that Iran is in arid and semi-arid regions and its population is growing, it is important to plan, and evaluate the past in order to predict the future. Zanjan province is the fifth country with 6.9% of total grains production. In this study, the effect of precipitation in water requirement of these two important products and their water stress has been studied in the climatic conditions of Zanjan plain.
Zanjan province is located at the longitude of 47º 10' to 50 º 5' East and the latitude of 35º 25'37º10' North. The area of Zanjan province is about 39369 square kilometers. According to the De Marten climate classification, the tropical, temperate and subtropical climatic zones can be identified at the applied level. Average rainfall over the past decade has been reported as 301 mm. For conducting this study, statistical data including meteorological and crop yield data 2004-2014 obtained from the Meteorological and Agriculture-Jahad Organization, respectively. Zanjan synoptic station with the latitude and longitude of 48º 31' East and 36º 39' North, respectively, is located 1663 meters above the sea level. In the last 50 year, the annual precipitation trend has been decreasing and this trend is affecting the water availability of rainfed plants in the region and can decrease yield of crops. Although in the long term the average temperature trend is slightly constant, the yield has also declined with decreasing rainfall. Regarding the importance of wheat and lentil in arid lands of Zanjan plain, these two main rainfed crops were investigated in this study by determining the regression relationship between yield and evapotranspiration with precipitation and water stress coefficient.
The results showed that the average evapotranspiration values of wheat and lentil for Zanjan plain during the study period were 398 and 262 mm and the mean temporal stress coefficients were 36 and 33%, respectively. Regression analysis showed the highest correlation between actual evapotranspiration and yield. The highest correlation was observed between temporal stress coefficient and yield of rainfed wheat. But, in lentils this trend was different and the correlation between water stress coefficient (Ks) and yield was more than the precipitation and lentil yield. For 25% decrease in precipitation, wheat yield reduced almost 18%. Generalizing this issue, the response to water stress coefficient of the product or Ky become 1.14. The regression equation also indicated that in case of being precipitation about 300 mm, wheat yield in the area will be near one tone. For 25% decrease in precipitation, wheat yield reduced about 22%. According to figures, the fitted model was significant and well-acted; and during the growth period of these crops, lack of water supply caused water stress as well as severe yield loss. The occurrence of high tension time percentage of 25% for wheat and 20% for lentil in all years of the study period confirms that the climatic potential of the region does not reach the standard requirements of plants, so it is not possible to achieve optimal performance. Hence, new methods of tillage, conservation agriculture and supplemental irrigation are recommended for achieving high yield, as well as the expected cultivation method for crops.