Investigating the production functions of water consumption in determining the performance of two watermelon cultivars under water stress

Introduction
Watermelon is a popular fruit that is cultivated in greenhouses and on the ground, and water and fertilizer, as two essential factors for the growth of the product, significantly affect the yield of crops. Watermelon is the twelfth plant in terms of cultivated area in Iran. Due to its importance in export and virtual water, it is necessary to pay special attention to the characteristics of water needs and water consumption. However, water resources are scarce and irrational irrigation and use of fertilizers are common. This caused environmental pollution and waste of resources and also affects the growth and absorption of nutrients. Plant absorption, therefore, affects the yield and quality of the product. China is the first producer of this product in the world with the production of 67% of the total watermelon crop. The next ranks with less than 4% of world production are held by Turkey, Iran, Brazil and Egypt. According to FAO statistics, Iran ranks third in the world in watermelon production. In Iran, 91,000 hectares are cultivated with watermelon, of which 85,400 hectares are irrigated.Considering the special attention given to the watermelon plant as one of the high consumption options in the cultivation pattern, it was necessary to conduct a research in this field to investigate the amount of water required and reduce its yield under deficit irrigation.
 
Materials and Methods
In this study, Charleston variety watermelon was grown with a density of 8000 plants per hectare on May 29, 2022 in the soil and water research farm in Alborz province. Also, Crimson Sweet cultivar with a density of 11,000 thousand plants per hectare was cultivated on Desember 25, 2021 in the southern research center of Kerman province.In this regard, a research was conducted in the form of randomized block design in 4 full irrigation treatments, 75% of water requirement, 50% of water requirement and 30% of water requirement in the research farm of the Soil and Water Research Institute and Kerman Research Center. After applying low irrigations in different treatments, the performance of each treatment was measured. Also, the water requirement values of watermelon were investigated using the water requirement system of the Soil and Water Research Institute of the country under standard conditions. Also, by using two production functions, the sensitivity coefficients of the Charleston variety were determined. Also, the water requirement values of watermelon were investigated using the water requirement system of the Soil and Water Research Institute of the country under standard conditions. Also, using two production functions, the sensitivity coefficients of the Charleston variety were determined.
 
Results and Discussion
The results showed that the standard water requirement of watermelon in Karaj is about 488 mm and in Jiroft area of Kerman is 423 mm and it is in good agreement with the water requirement estimated by the water requirement system. The results show that watermelon in both cultivars reacted to the lowest amount of stress, so it is sensitive to lack of irrigation, and therefore it is necessary to investigate the values of the production function in their case according to the applied treatments. On the other hand, the highest sensitivity coefficient occurred in the period of 60 to 80 days after cultivation in the middle period of growth and the recalibrated production function estimated the yield of the plant with appropriate accuracy in the applied stresses, which the statistical indicators of Charleston variety RMSE, NRMSE, MBE , d and EF were 497, 0.02, -119, 0.99 and 0.98, respectively, and the Crimson number was 568, 0.095, -536, 0.98 and 0.96 respectively, so the sensitivity coefficients and production function It is proposed to simulate the performance of both watermelon cultivars under water stress conditions. Also, the highest productivity was obtained in the low irrigation treatment of 70% and Crimson variety has higher water consumption efficiency. Therefore, this variety is recommended for watermelon cultivation with 70% less irrigation.
 
Conclusion
Summarizing the results showed that the standard water requirement of watermelon in Karaj is about 488 mm and in Jiroft area of Kerman is 423 mm and it is in good agreement with the water requirement estimated by the NIAZAB system. The sensitivity coefficients and the production function presented for simulation The performance of both watermelon cultivars is suggested under water stress conditions. On the other hand, the highest sensitivity coefficient occurred in the period of 60 to 80 days after cultivation in the middle period of growth and the recalibrated production function estimated the yield of the plant with appropriate accuracy in the applied stresses, which the statistical indicators of Charleston variety RMSE, NRMSE, MBE , d and EF were 497, 0.02, -119, 0.99 and 0.98, respectively, and Crimson number was 568, 0.095, -536, 0.98 and 0.96, respectively. Therefore, the sensitivity coefficients and production function It is proposed to simulate the performance of both watermelon cultivars under water stress conditions. Of course, due to the high performance level of the Crimson variety, the amount of water productivity in this variety is twice that of the Charleston variety, so in terms of water consumption efficiency, the cultivation of the Crimson variety is a priority compared to the Charleston variety. Also, the highest productivity was obtained in the low irrigation treatment of 70% and Crimson variety has higher water consumption efficiency. Therefore, this variety is recommended for watermelon cultivation with 70% less irrigation.