عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Deficit irrigation was investigated for major crops in the west of Qazvin plain irrigation network in order to reduce the water consumption in the agricultural section, and provide suitable method for the conjunctive use of surface and groundwater resources in the irrigation networks. Solving the optimization problem of conjunctive management models for surface and groundwater resources using meta-heuristic algorithms can be considered as one of the efficient solutions. In this regard, comprehensive studies have proved the ability of these algorithms. The purpose of this study is to manage the conjunctive use of surface and groundwater using Gravitational Search Algorithm (GSA) with the objective of maximizing annual Net Benefit (NB) in water scarcity conditions.
The study area is the west region of the Qazvin plain’s irrigation network. The scenarios consist 30 items, combination of 6 levels of deficit monthly irrigation percentage (0, 0-10, 0-20,0- 30, 0-40 and 0-50%) and 5 levels of groundwater allocation percentage (80, 85, 90, 95, 100% of maximum extraction).
Based on SPI drought index, the water year 86-87(2007-2008) was selected as the base year of drought. In this year, the allocation of surface water was about 30.1 million m3 and the total groundwater extraction for agriculture was estimated to be about 126.7 million m3. The optimization model was prepared based on the GSA. The crop pattern in the study area was selected based on the crops in the latest pattern suggested by Jihad Keshavarzi, including fall crops of wheat, barley and rapeseed, and summer crops of sugar beet, tomato, corn- and fodder-maize. The gardens’ composition maintained and their water demands were subtracted from the total water consumption.
The decision variables in this optimization model were 82 items, which include 7 cultivation areas, 12 monthly surface water extractions, 12 monthly groundwater extractions and 51 monthly irrigation demands for different crops that were separately defined by specific limits in the program. The dimension and iteration numbers were 100 and 200,000, respectively. The amounts of water demand for the selected crops were calculated separately. The potential evapotranspiration was calculated from the data of Qazvin Synoptic Station using CROPWAT8 program with the calculation method of modified Penman-Monteith FAO.
The limits of the optimization model include a limit related to the maximum annual volume of surface water allocation, a limit related to the maximum annual volume of groundwater allocation, 12 limits related to the amount of monthly allocation of surface and groundwater resources, and a limit related to the maximum area of optimal crop areas. In order to define the yield function of the main crops, the results of Tafteh research (2014) in Qazvin plain were used. The optimization model was executed for different scenarios and the results were analyzed.
In comparison between scenarios with equal level of deficit irrigation, the percentage of fall crops increases along with decreasing the groundwater extraction, and in comparison between scenarios with equal groundwater extraction, the percentage of fall crops increases along with increasing the deficit irrigation percentage, Also, in scenarios with equal level of deficit irrigation, the percentages of wheat and tomato increase slightly along with decreasing groundwater extraction.
Increasing the percentage of wheat and rapeseed cultivation in fall crops and decreasing the percentage of tomato, corn and fodder maize cultivation are other changes in the cultivation pattern in the condition of deficit irrigation. Decreasing the volume of groundwater extraction and increasing the deficit irrigation levels cause decrease and increase in the area of cultivation, respectively.
In full irrigation treatment by selecting the optimum crop pattern, groundwater extraction reduces from 5 to 20% and groundwater resources saved up to 25.3 million m3 while NB decreases from 1.7% to 10.9%. Also by increasing the amount of deficit irrigation to 50% in scenarios with equal groundwater extraction, the NB decreases by about 6.4% to 10.7%. In comparison of the NBPD (Net Benefit Per Demand) between different scenarios, the scenarios with 20% reduction in groundwater extraction and the one with full irrigation obtained the highest value of NBPD, and the scenarios with deficit irrigation up to 40 and 50% obtained the lowest value of NBPD.
The deficit irrigation operations caused an increase in cultivated areas from 41 to 54% at deficit irrigation percentage of 50% compared to the full irrigation treatments, although it reduces the NBPD by the maximum amounts of 5.9 to 10.3%. In general, the rate of decrease in the NBPD percentage is less than the rate of decrease in the groundwater extraction percentage and the rate of increase in the deficit irrigation percentage, due to the high productivity resulted from deficit irrigation and optimal water use.
In all scenarios, most of the water demands were supplied from groundwater resources (about 77.0 to 80.8% of the total demand) and the rest from surface water resources. Almost in all scenarios about 63 to 100% of the annual demands were supplied and in the most critical months at least 50 percent of the demands were supplied.
The highest percentage of annual deficit irrigation was 37% and observed in scenario including reduce groundwater extraction equal to 20% and deficit irrigation up to 50%. In scenarios with the same percentage of deficit irrigation, with a 20% reduction in groundwater extraction, the water supply was reduced by a maximum amount of 5%, and in scenarios with the same groundwater extraction volume and different percentages of deficit irrigation, the water supply was reduced by a maximum amount of 37%. Therefore, the GSA optimization model was able to solve the problems of conjunctive optimization of surface and groundwater resources by providing a model of optimal crops in water scarcity conditions and flexibility in different percentages of irrigation, while increasing the area of cultivation.