Conjunctive management of surface and groundwater under severe drought: A case study in southern Iran

Transcription

1 Conjunctive management of surface and groundwater under severe drought: A case study in southern Iran Davoodreza Arab, Hamid Sohrabi, Milad Hooshyar, and Aliasghar Elyasi * * Corresponding author, water resources engineer, Rahbord Danesh Pooya Institute, Tehran, Iran, aaelyasi@gmail.com Abstract Hormozgan Province, located in the south of Iran, faces several challenges regarding water resources management. The first one is the discharge of a massive volume of water to the Persian Gulf because of the concentration of the annual rainfalls in a short period of time and the narrow distance between the headwater and the coast. The second one is the unbalanced development of economic sectors in comparison with distribution of fresh water resources. Finally, long-term drought is also common in this area. The construction of a carry-over dam (Esteghlal Dam) and several conveyance pipelines and withdrawing of the surface water and groundwater resources were considered as the solution to deal with those challenges. During recent drought, severe overdraft and inefficient use of recourses confirmed the fact that all done before are not enough. During this period, there was a tendency to store water in reservoir in order to meet the demand of the urban sector. Therefore, the agricultural demand was the first victim of water allocation policy. It caused over exploitation of the groundwater resources (to meet the agricultural demand) and considerable losses (evaporation and leakage) from the reservoir. All of the abovementioned problems confirm the necessity of the development of a conjunctive use policy. In this paper, all demand related to the Esteghlal Dam and the ( City and agriculture in the ) were considered as the case study. The main objective was to find the best applicable conjunctive policy which as well guarantees the conservation of the. Alternative water allocation policies have been developed based on the present capacities and the experience of local operating staff. A Decision Support System (DSS) model called Water Evaluation and Planning system (WEAP) has been used to develop the simulation model. Historical inflow data, water withdrawals and the allocation policies, related to the Esteghlal dam have been simulated in order to verify and calibrate the DSS model. Results showed that, according to the hydrological conditions, storing water in the reservoir during droughts had no significant effect on meeting demands; therefore, the allocation of water from the Esteghlal Dam had priority over the allocation of groundwater resources. This is mainly due to the high rate of evaporation in the area and the qualitative and quantitative limitations of groundwater resources. 1

2 Keywords: DSS Model, conjunctive water system, drought, WEAP Introduction Efficient allocation of water resources is one of the main challenges in water resources palming especially in arid or semi-arid regions with expanding demands. There have been several simulation, optimization [1-8], and hybrid approaches in the literature to tackle this issue [9, 10]. The situation is more complicated when the resources both includes surface [11-13] and groundwater water and they hydrologically interact [14]. Hormozgan Province, with population of 1,403,674 in 2006, is situated in south of Iran at the coast of Persian Gulf. -the capital city- has population of more than 500,000 and consumes about 65% of total allocated municipal water. -one of the most economically and environmentally important agricultural sectors of the province- is 200 km far from. Esteghlal Dam, which is located in across Minab River and has been in operation since 1986, supplies the demand of and water rights of. The main demand sites and water resources of the province are shown in figure 1. As shown in this figure, although the fresh water resources are concentrated in the east and the middle of the province, the main demand sites are located in the west, e.g. there are no fresh water resources close to the most heavily populated city () and it is dependent on just a 25-year-old pipeline from the reservoir to the. The area is a semi-arid region with the average annual rainfall of 180 mm. The rainy season for the west and east of the province is from December to March and July to August, respectively. The precipitation usually falls through a few intensive storms and causes severe run-off during rainy seasons which mainly is discharged into the Persian Gulf. In order to control the floods, two carry over dams were constructed, Esteghlal dam and Jaghin dam, and another one is under construction. Drought analysis using SPI (Standardized Precipitation Index) [15, 16], for two-year periods and based on monthly precipitation, shows the frequency of severe droughts in the area (Figure 2). 2

3 Figure 1- The main demand sites and water resources of Hormozgan SPI Severe Drought Moderate Drought Extreme Drought Figure 2- SPI for two-year periods and based on monthly precipitation in the area During recent drought from 2006 to 2008, there was a tendency to store water in reservoir to meet the demand of, considering social and political issues. However this policy has been succeeded in meeting the municipal demand, it caused some social an environmental problems. has been extremely over 3

4 discharged in order to meet the minimum level of the agricultural demand and severe subsidence in Minab plain occurred. The municipal demand was met at the highest level; nevertheless, just about 50% of agricultural demand was allocated. During those two years more than 25 MCM of the Esteghlal Dam storage evaporated as the result of high temperature and extensive lake area. As mentioned above, unbalanced development, stormy and unreliable rainfall and severe droughts are the main challenges in the area. In order to cope with the unbalanced development, several transmission projects have done or are under construction. The construction of carry-over reservoirs, such as Esteghlal Dam, has been considered as the way out of meteorological challenges. But according to the historical data, efficient allocations and conjunctive use policy are the main ignored principles. In this paper a DSS model called WEAP has been used to develop the simulation model. Several conjunctive use policies have been evaluated, using the simulation model, in 12 hydrological-initial conditions (states) to find the best policy. WEAP model WEAP [17-19] is a Windows-based decision support system for integrated water resources management and policy analysis. WEAP was created in 1988, and continues to be developed and supported by the U.S. Center of the Stockholm Environment Institute. The described area is simulated using WEAP (Figure 3). This model consists of one domestic () and three agricultural demand sectors (Northern, Southern and Mediate ). Esteghlal Dam and are the main water resources in this model. Water demands (cumulative amounts through a 2-year period) were predicted by paying attention to the population and agricultural area (Table 1). Monthly variations of demand (calculated though historical data analysis) are shown in figure 4. sites Table 1- water demand in the area Water Rights () Maximum historical water use () Northern Minab plain Mediate Minab plain Southern Minab plain Sum

5 Figure 3- Minab- WEAP model Percent of annual demand 16% 14% 12% 10% 8% 6% 4% 2% 0% Mediate Northern Southern Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Figure 4- monthly variation of demand sites Some of characteristics of resources in this area are shown in table 2. As shown below the long term average capacity of both surface and underground resources is about 336, but in recent years (during recent drought) it reduced by half. In comparison with the demand, the average available water is enough to meet all the demand even during drought. However, at the worse circumstance all available water recourses reduced to 87 MCM (in 2005). 5

6 Table 2-Some Information about the water recourses in the area Esteghlal dam Minab aquifer Total Ave. Inflow Ave. Inflow (last 10 years) Min. Inflow Max. Inflow Ave. losses Allowable discharge Ave. discharge Max. discharge Current over discharge Available Water Ave. Withdrawal Worse case Water Allocation Policies Considering current capacities and experience of local staffs, the following criteria were used to develop water allocation policies: 1- Which resource (groundwater or surface water) has priority over the others? 2- In Which level dose the water allocation to agricultural demand from Esteghlal Dam have to be stopped, 70 MCM (storing the next 6 months demand of ) or 83 MCM (storing the next 9 months demand of )? Policies, developed to answer these questions, are shown in Table 3. 1st policy describes a conservative policy to meet the domestic demand. The conservation of groundwater resources is concentrated in the 4th policy. Table 3- the water allocation policies Policy Prior Resource Storing water for: 1 Groundwater 9 months 2 Groundwater 6 months 3 Surface water 9 months 4 Surface water 6 months Hydrological conditions A frequency analysis was performed for 2-year stream flow of Minab River to establish hydrological conditions (Shown in Table 4). The historic period considered is from 1963 to The inflow in Wet years is much more than demand, so only Normal; Dry and Very Dry years were considered. The percentile for each hydrological condition is shown in the flow duration curve (figure 5). 6

7 Table 4- the hydrological conditions in the area and the two-year inflow Name Flow Range Average Flow Number of Representative Number of Representative (MCM) (MCM) years during years during Wet 450< Normal 300< <= Dry 160< <= Very Dry 0< <= Percentile % Very Dry Dry Normal Wet Two-year Inflow (MCM) Figure 5- the flow duration curve for the two-year inflow of Minab River Initial Condition The initial storage of Esteghlal Dam, during a 2-year period, has significant effects on allocation policy. So it has been considered as the initial condition of the system. The discrete storage of Esteghlal Dam (initial conditions) is shown in Table 5. Table 5-Initial conditions based on Esteghlal dam initial storage Initial Storage Storage (MCM) Inactive Zone 43 35% 90 70% % 257 7

8 Results coverage was considered as the main criteria for evaluating policies. As a national criterion, the municipal and agricultural demand coverage must not be less than 95% and 80%, respectively. There is an attitude toward groundwater conservation which has been over discharged during recent droughts, so those policies which caused less groundwater discharge were considered as more efficient. The results of simulation model for applying each policy to all possible Hydrological-Initial Conditions (called states) are shown and the best policy, based on the mentioned criteria, is highlighted in table 10. Only 3 out of 12 states policies, allocation from Esteghlal dam, showed better performance. The shortage of rainfall and/or low initial storage caused exhaustion of all surface and groundwater recourses in these states. However in other states the high rate of evaporation makes storing water in the reservoir inefficient. The policies which intend to allocate surface water, cause low level of the storage and less evaporation. For example, applying policy 4 to the state dry-35% caused the demand coverage 3% more than demand coverage by applying policy 3 to the same state. However the change in the storage of the system was the same for both policies. The change in the storage is the sum of the change in the storage of Esteghlal Dam and the change in the storage of. For these two discussing policies is as fallows Policy 3 in state dry-35% CChaaaaaaaa iiii SSSSSSSSSSSSSS = (89 90) 34 = 35 MMMMMM Policy 4 in state dry-35% CChaaaaaaaa iiii SSSSSSSSSSSSSS = (80 90) 25 = 35 MMMMMM Policy 4 tends to withdraw water from the reservoir first, so the evaporation decreases throughout the applying this policy and more water will be available to allocate. According to the results, in spite of few severe states, the allocation of water from the reservoir has priority over the groundwater in most of the states. This policy tends to store water in the aquifer instead of the reservoir which leads to high level of evaporation and other losses. The aquifer is capable to store water for longer periods with less loss. The risk associated with uncertainty of rainfall and long-term droughts is reduced through storing water for long periods. From the environmental point of view, storing water in the aquifer causes notable qualitative and quantitative improvement of the groundwater resources. 8

9 Table 6- Very Dry-Inactive zone Discharge (MCM) Table 7- Very Dry-35% Policy 1 69% 51% Policy 1 94% 52% Policy 2 69% 51% Policy 2 92% 55% Policy 3 69% 51% Policy 3 94% 51% Policy 4 69% 51% Policy 4 91% 53% Table 8- Very Dry-70% Table 8- Very Dry-100% Policy 1 100% 77% Policy 1 100% 100% Policy 2 100% 81% Policy 2 100% 100% Policy 3 100% 72% Policy 3 100% 85% Policy 4 96% 75% Policy 4 100% 89% Table 9- Dry-Inactive zone Table 10- Dry-35% Policy 1 100% 66% Policy 1 100% 87% Policy 2 100% 75% Policy 2 100% 93% Policy 3 100% 62% Policy 3 100% 78% Policy 4 100% 68% Policy 4 100% 81% Discharge (MCM) 9

10 Table 11- Dry-70% Discharge (MCM) Table 12- Dry-100% Policy 1 100% 100% Policy 1 100% 100% Policy 2 100% 100% Policy 2 100% 100% Policy 3 100% 96% Policy 3 100% 100% Policy 4 100% 100% Policy 4 100% 100% Table 13- Normal-Inactive zone Table 14- Normal-35% Policy 1 100% 84% Policy 1 100% 94% Policy 2 100% 84% Policy 2 100% 100% Policy 3 100% 84% Policy 3 100% 90% Policy 4 100% 84% Policy 4 100% 94% Table 15- Normal-70% Table 16- Normal-100% Policy 1 100% 100% Policy 1 100% 100% Policy 2 100% 100% Policy 2 100% 100% Policy 3 100% 100% Policy 3 100% 100% Policy 4 100% 100% Policy 4 100% 100% Discharge (MCM) 10

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