Study on water resource allocation applying different planning scenarios in Chahnimeh reservoirs using WEAP model

The long history of research on utilization of resources and their allocation to the consuming sectors dates back to the first human efforts in managing his nature in which this exploitation was mostly focused on man (Simonovic, 1992). Now, given the recent droughts in Sistan and due to the fact that the major management of Helmand River occurs in Afghanistan, the Chahnimeh reservoirs in Sistan require special comprehensive management. Therefore, the present study aims to determine the amount of water allocated to the agriculture, potable water, aquaculture, and environmental sectors of quadruplet reservoirs of Chahnimeh.
Chahnimeh reservoirs are located in East and Northeast of Sistan and Baluchistan province between 61° 29' to 61° 44' east longitude and 30° 40' to 30° 54' north. Water management in decision support system (WEAP) deals with proper setting of priorities and preferences in consumption of resources. Factors considered in this study for water resources’ planning include the environmental need, evaporation, potable water, agriculture, and aquaculture. In this study, the environmental water rights are calculated in accordance with the Montana method (Tennant, 2011). Evaporation from the reservoir’s surface is also calculated based on the monthly curve of volume, level, height, and evaporation at different levels. According to the exponential population, the potable water required for 3 regions of Zabol city, Sistan villages and 30% of the water requirement in Zahedan city was calculated as 3% (Statistical Center of Iran, 1390). Parameters required for agriculture were annual cultivation area and the annual rate of water consumption per hectare. In the reference scenario (R), the regional hydrological conditions were considered as the current conditions, and only the irrigation ability of the 1st, 2nd, and 3rd Chahnimeh was used in water resources planning. Scenario (R1) was created based on the reference scenario (R), but the cultivation level of crops according to the appropriate cropping patterns and drought conditions dropped. In scenario (R2), the required water for other sectors (agriculture and portable) was provided and if there is a suitable opening of the Helmand River, the water requirement of Hamoun wetlands will be provided by the Chahnimeh reservoirs. By relying on the natural flow of the river, after implementing the scenario of development (D), the 4th reservoir tank is added to the previous ones and the cultivation level of agricultural products has increased by 13,500 hectares. In scenario (D1), all hydrological conditions and reservoirs are considered in accordance with the scenario of development and the required water provided from these four reservoirs has been studied. Any violation of system performance from a performance threshold or inability of the system in supplying the required water is called system failure. There are several methods to analyze the performance or failure of the system. In this research, the Reliability index is used to compare the scenarios with each other (Hashimoto et al., 1982).
This index is determined by calculating the number of failures per requirements during each simulation period. By comparing the scenarios (R1) with (R), the rate of Reliability index in potable water section rises from 90.6% to 96.9%. In the comparison of the scenarios (R2) with (R), the required rate of environmental need of Hamoun wetland in the scenario (R2) was regarded as 10% of monthly input of the Chahnimeh reservoirs, so the results of the model showed that in comparison with the scenario (R), the Reliability index of the system in other sectors remained constant. In comparing the scenarios (D) and (R), the effect of increasing the area under cultivation from 45,000 hectares in the status quo to 58,500 hectares in the development program and addition of the fourth Chahnimeh reservoir to the utilization system of water resources were reviewed. In the scenario (D), the reduced impact of the Reliability index on agriculture and potable water sectors were ignored, so performing agricultural development program under the status quo with a Reliability index of over 98% would be provided. In comparing the scenarios (D1) and (R), the condition of water rights needed for the sectors of agriculture, The purpose of comparing these two scenarios was to assess and analyze the effect of adding the fourth Chahnimeh on the utilization system of water resources and the agricultural development of the region through taking into account the environmental needs of Hamoun wetland in the region. Storage of surplus water in reservoirs could increase water level subject to evaporation. This case was determined by comparing the scenario (D) with the scenario (D1). The results of the simulation showed that evaporation loss in the scenario (D1) was 50 million m3 less than in the scenario (D) in the 4th Chahnimeh. This result also focused on the amount of water released for meeting the environmental needs of Hamoun wetland. In comparison of the scenarios (R2) with (R), considering the water rights as 10% of the water inlet of the Chahnimeh reservoirs would not be a threat to water allocation to other requirements.Under the scenario (R), which represented the current state of the water resources in the region, in 85% of the years, 57.5% of requirements would be provided in the agriculture sector, whereas in the potable water sector, in 85% of the years 87.3% of the needs were met. Under the scenario (D), representing the management development state of water resources in the region, not only the current land under cultivation would be preserved but also13, 500 hectares of agricultural land would be added to the agricultural lands. In this way, in 85% of the years, it would supply 85% of the requirements. Analysis of these scenarios and the results obtained from the scenario (D1) showed that along with the increased needs of the agricultural sector and the Hamoun wetland, in terms of the Reliability level, the development conditions increased more than other scenarios. Moreover, with the addition of the environmental needs of the Hamoun wetland to the water resources’ planning of the region, the rates of evaporation loss from the 4th reservoir could reduce significantly.