Optimization Analysis for Utilization of Rainwater in the Lake Land Plain, China
Sha Lusheng, Fang Hongyuan, Cai Shouhua
Yangzhou University , Jiangsu, China
Abstract
The utilisation of rainfall water is an essential measure taken to overcome the crisis of water resources in dry areas. This paper has analysed how to fully utilise the stream-flow water and reduce the pumped water by using the simulation technology of system analysis. The Weishan Lake-Luoma Lake water resources system’s different probability of the pump-store curve has been obtained based on the historical and generated stream-flow series. The system response values of each operating alternative has been calculated and the Pareto-solution collection have also been generated. The optimum solution could be determined based on the priority of system objective importance and considering the local economic base and the national economic developing demand.
INTRODUTION
The development and utilisation of rainwater resources is one of the important measures to make up for water resources shortage in the arid regions. In North Jiangsu’s lakeland plain, P. R. of China, the shortage of water resources is serious, the contradiction of supply-demand exists everywhere, so the annual pumping costs are enormous. It is worthy to be researched how to improve the regulation effectiveness of lakes (reservoirs) and heighten the utilisation ratio of runoff to satisfy water demand through planning and management of the water resources system. The present work seeks to be the operation policy of the Lake Weishan-Lake Luoma water resources system for making full use of the local runoff and reducing pumping water.
OUTLINE OF STUDY SITE
The Lake Weishan-Lake Luoma water resources system is located in the northwestern part of Jiangsu, the north latitude is , the east longitude is . It provides Xuzhou City and its six counties with industrial water, agricultural water, navigation use of water and domestic water. The total area of the system is 11258, it currently has a population of 8.67 million and the cultivated area is 61.54* ha.. The researched system is situated in the temperate, semi-humid and monsoon climate zone. The climate is changeable and the disaster caused by the drought and excessive rain is serious. The average annual temperature is 14.2,and average annual rainfall is 852.6 mm, the maximum annual rainfall is 1360 mm and the minimum is 259.3 mm. Water resources consist of surface water, ground water, passenger (flowing?) water and pumping water, the average annual water resources available is 104* and the per capita average is 427, it is less than one-fifth of the national per capita average of water resources.
(1) Surface water: It results from runoff, and the average amount of annual surface water of the system is 21.8* and 20.36% of the total water resources. The surface water available monthly and yearly is quite variable and 85% of the annual runoff is concentrated in June, July, August and September.
(2) Ground water: The quantity of shallow groundwater in the system is 25.8* and 24.85% of the total water resources. The average annual extract is 16.7* and 35.5% of the local water resources.
(3) Passenger (flowing?) water: The passenger water into the system is about 57* and 54.8% of the total water resources. The passenger water from external sources is abundant, but also it concentrates in June, July, August and September.
(4) Pumping water: The sum of the above mentioned three parts of available water is only 30.35*, but the demand for industrial and agricultural water is about 50.55*. It is obvious that the local available water resources does not meet the needs of the industrial and agricultural developments. According to the long-term statistics, the water shortage in a moderate year is 10*, and 30* in a drought year. But the water shortage of the system can be made up by water pumped from the Yangtse River and Luoma Lake using 4 step pumping stations to obtain about 10*per year. The control management of the pumping stations is complicated and the pumping expenses are enormous.
SYSTEM CONSTITUTION AND OPERATION PRINCIPLES
System Constitution
The Lake Weishan-Lake Luoma water system consists of the storage facilities ( Lake Weishan and Lake Luoma ), the pumping stations ( Zhaohe, Liushan, Xietai and Yanhu ), the natural and artificial channels and all kinds of water users ( i.e. domestic, industrial and agricultural uses and the navigational use of the water courses). Based on the existing specific operating situation of the system, the system can be divided into four components. Figure 1 represents the optimal use of the system structure. In the system operation program, it is a rule that Lake Weishan ought to supply water only for Regions 1, 2, 3, and the shortage of water supply can be compensated by the step pumping stations when the available storage of Lake Weishan is insufficient.
Fig 1. Idealisation drawing of the system
Operation Principles
The operation policy not only can ensure that the system is kept under the normal operating conditions, but also can achieve the optimal regulating effects of the system. The operation principles of the system are: First of all, the water storage in Lake Weishan ought to be used as completely as possible, and a reasonable water supply priority of use is, first, domestic water, followed by navigational, industrial and agricultural uses; the amount of pumped water by the Liushan and Xietai stations is dependent on the total shortage of Region’s 1, 2, and 3 water demand, and the compensated water of each region is in proportion of its water shortage; the navigational use of water and the regenerated water from the upstream region to the downstream region are considered as recirculating use water; the spillage (overflow) from Lake Luoma is not available to the system and is the real water loss, but spillage (overflow) from Lake Weishan is regarded as inflow to Lake Luoma; the Zhaohe station’s pump capacity is 60 at present and will be 100 in the near future.
MATHEMATICAL MODEL
Constraint Conditions
Water mass balance of the lake reservoirs
(1)
(2)
in which the superscript 1 and 4 represent Region’s 1 and 4 , respectively; and are the storage of lake at the begin and the end of a time interval ,month; is the inflow; is the pumping water; is the release from upstream; is the spillage from Lake Weishan; is the lake evaporation; is the lake seepage; is the real water supply to the region .
The other constraints are the restrictions of the water level, pumping capacity, channel capacity, water supply probability and so on.
Objective Function
The water supply benefit is the key objective for the operation of the system, so minimising the sum of the pumping costs and the water shortage loss costs in the long-run should be a target of the system. Because it is difficult to calibrate accurately the economic parameters of the water shortage loss without a vast amount of data, the system response function, which is composed of the physical parameters is employed. These parameters include the water shortage, the pumping water, the spillage of lakes and the lake surface evaporation, and it is easy to obtain these based on the computing results. The system response function of the stochastic simulation is
(3)
in which G is the value of the response function; is the function of the water shortage; is the function of the pumping water; is the function of the evaporation loss; is the function of the lake spillage; are the weight of the functions; is the variables collection for the simulation trial, and is the normal water level of Lake Weishan and is the probability to guarantee the different kinds of minimum water supply for the system.
STOCHASTICAL SIMULATION
Simulation Trial
To reduce the water shortage resulting from the pumping capacity, it is effective that an appropriate amount of water is pumped and stored in Lake Weishan in advance, with help of the remainder of the pumping capacity in some time interval. The water pumped and stored in advance is the guarantee of the normal water supply corresponding to the different kinds of the probabilities. It’s annual process may be represented by the pump-store curve, which can be calculated based on the inflow of the lake reservoir, the pumping capacity and the demand of the water users etc. When the pump-store curves are calculated and the simulation trials are made, the synthetic inflow series generated by the hydrologic time series model, together with the historical inflow data, are employed, because the historical hydrologic series are too short to satisfy the stochastical simulation. For the simulation trials, the feasible variables collection , which is the combination of Lake Weishan normal water level 32.5, 32.7, 33.0, 33.5 m and the water supply probabilities 50%, 75%, 95%, are determined to run the computer program. Figure 2 represents the pump-store curve of the probability 95%. Table 1 is the results of the simulation trial for Lake Weishan normal water level 33.5 m.
Fig. 2 Pump-store curves of the probability 95%(1present; 21995; 32000)
Table 1. Responses for System Operation
Lake Weishan normal water level h(m) / 33.5 / 33.5 / 33.5Probability p(%) / 50 / 75 / 95
Water shortage of Region 1,2,3 s’() / 0.52 / 0.44 / 0.20
Pumping water of Liushan PQ’() / 4.68 / 4.81 / 6.77
Water shortage of the system s () / 0.59 / 0.46 / 0.22
Total pumping water PQ() / 11.42 / 12.61 / 17.20
Evaporation loss of Lake Weishan () / 5.66 / 5.66 / 5.78
Evaporation loss of Lake Luoma () / 4.20 / 4.24 / 4.26
Spillage from Lake Weishan () / 6.56 / 6.54 / 6.71
Spillage from Lake Luoma () / 42.39 / 43.30 / 45.61
Multiobjective Decision
The weights in the response function can be discussed and determined by the decision makers, the experts, the water users and the system analysts, who are familiar with the water users in the system. According to the discussion, the range of
the weight ratio of is used to compute the system responses. For a weight ratio selected from the range, , the corresponding system responses can be computed according to the variables collection. Then the iso-response curves are drawn in the feasible variables region, and the deepest valley can be found which represents the optimal point of the system responses. A set of this kind of optimal points can be obtained while the weight ratio is changed successively within the range, and these points demonstrate the Pareto-solution collection. Based on the order of the priority of the system operation purposes, the planners together with the deciders and the water users assessed all of the alternatives. And the solutions (33.5, 95) and (33.5, 75) are considered as the acceptable operation policy.
OPERATION POLICY ANALYSIS
Raising the normal water level of Lake Weishan from the original 32.5 m to 33.5 m is very effective to improve the ability of Lake Weishan’s regulation and reduce the water shortage, pumping water and the spillage (overflow) of the system. For Lake Luoma, the water available for supplying Regions 1, 2 and 3 is 14.08* at present and will be 7.82* in the near future, while the probability of the runoff is 50%; no water is available for supplying the upstream three regions, while the probabilities of the runoff are 75% and 95%. Thus the normal water level of Lake Luoma should be also raised by 0.5m to increase the lake storage and improve the utilisation ratio of the system runoff. In the real-time operation, the curve corresponding to the probability of the water supply 95% is regarded as the high limitation of the operation level, and the curve corresponding to 75% as the low limitation of the operation level. The reasonable operation, in which the operation water level of the two lakes is within the high and low limitation, can achieve the purpose of minimising the water shortage and the pumping water.
CONCLUSIONS
The contradiction on the supply-demand of water resources is the remarkable problem at present and in the near future for the Lake Weishan-Lake Luoma water resources system. In arid region, making full use of runoff is an effective measure to improve the ecological environment, the condition of economic development and the industrial and agricultural production.
The utilisation of the rainwater resources is important as well as the improvement of the regulation of rivers for the development of the Lake Weishan-Lake Luoma water resources system. Because there are about 100 thousand ha. cultivated land in the hill region of this system whose irrigation condition is rather poor, maximising the use of the local rainwater resources is of great importance to solving this problem.
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