Increasing water demand has become inevitable due to population growth, industrial development and agriculture. On the contrary, water resources limitation and observing sustainability have restricted water allocation. Therefore, supply of water for all demands is impossible. Overuse of groundwater and surface resources, rainfall reduction, drought in most parts of the world, as well as pollution of ground and surface water have severely lessened available water resources. So, using these vital resources, with sustainable manner, requires proper management. Demand management and water production are two effective approaches. But, water production projects are time-consuming and high-cost. So, water management models are appropriate tools for decision making to predict the future situation, with low time and cost. Mathematical techniques are useful tools for keeping the balance between environmental requirements and human demands, specifically for optimal utilization of water resources in complex applications such as watershed or aquifer management. Aiming to more comprehensive decision making, a combination of two powerful operational and research techniques, including simulation and optimization could lead to a possible sustainable and optimal economic management.
This study was aimed to the simulate water policy in the Arazkuse watershed, Golestan province, Iran. In this regard, both system dynamics model and water balance relationships were applied. VENSIM as a system dynamics software was used to capture causal and consultative relationship among controlling factors, while water balance was applied to estimate availability of surface and subsurface water. As four sectors of water consumption (urban, industrial, environmental and agricultural) were considered in the current study, the data about landuse, cultivated crops in the watershed, hydrological information and drinking water consumption were gathered. Then, the water consumption was optimized in the agricultural sector, using linear programming technique with the two objectives of maximizing economic benefits and minimizing water consumption, by Lingo software. Finally, the effect of water optimization in the agricultural sector on the groundwater sustainability index of Arazkuse basin was investigated. The groundwater sustainability index represents the portion withdrawn water from the whole of aquifer reservoir.The infiltrated water volume and deep storage coefficient variables were used to evaluate available groundwater from the total groundwater and the total discharge of the basin during the simulation period. Accounting for as much as 75% of the water consumption in the basin is supplied from groundwater while for surface water this portion was 25%. In general, the total volume of annual available groundwater in the Arazkuse basin was about 75 million cubic meters whereas the total volume of available surface water equaled 155 million cubic meters. These values indicated that a large volume of surface water flows out of reach, which puts extra pressure on groundwater to meet the needs and may endanger the stability of the aquifer. Due to the amount of water supply and requirements by various sectors (including the drinking water, environment, agriculture and industry) the available water to each sector was calculated. A linear programming technique was used to optimize the allocation of water resources with an objective function of maximizing economic profit while minimizing water consumption in the agricultural sector. Flood irrigation is a common strategy for rice cultivation. Hence, irrigation efficiency in the studied watershed is lower than other watersheds which applied the irrigation methods such as drip and sprinkler systems. Considering the fact that about 32% of agricultural lands in Arazkuse watershed are dedicated for rice cultivation, to some extent could be concluded that it would be possible to increase irrigation efficiency, by improving conveyance and distribution coefficients.
The results showed that although the available surface water was approximately double, provided water by groundwater supplied about 75% of the required water, especially for the case of agricultural needs. Despite the fact that runoff may be the source for a part of recharged water, a considerable amount of the available water exits in the form of runoff. Therefore, the supplied water for various sectors was linked to the excessive use of groundwater resources. Hence, the agricultural sector was the main water consumption that needed to be optimized. The assessment of the effect of optimization of agricultural water use on groundwater sustainability index showed under optimal condition of water required for agriculture the index shifted from 14.4 in the present condition to 1.9 for the new scenario.