The step of demonstrating the water transfer scale based on dynamic trial feedback
Based on dynamic trial feedback, the method of demonstrating the scale of a water transfer project is as follows.
(1) Determine the transferable water volume of the external water source and preliminarily formulate the water transfer route. Then, calculate the net agricultural water demand, the available local surface water and the exploitable amount of groundwater in each irrigation area along the water transfer route (assuming 6 irrigation areas, as shown in Fig. 1).
(2) Convert the net agricultural water demand of the irrigation area into the gross surface water demand (which can be determined by the net agricultural water demand divided by the effective utilization coefficient of the surface irrigation water), and then allocate the local surface water among each irrigation area (note that the general availability of local surface water is very small; thus, water shortages in large parts of the irrigation area are still serious) to obtain the surplus gross water demand in each time interval. Simultaneously, convert the surplus water demand into the gross groundwater demand (the surplus water demand is first converted into the net water demand and then the gross groundwater demand, that is, the surplus gross water demand is first multiplied by the effective utilization coefficient of the surface irrigation water, which is then divided by the effective utilization coefficient of underground irrigation water).
(3) Starting from the last irrigation area along the water transfer route, the groundwater is adopted for despiking; that is, the groundwater is used only during periods of large surplus water demand in that irrigation area. During the process of allocation, the starting line of groundwater allocation can be established; that is, groundwater is used only above the starting line. All of the water demand is accumulated as the exploitable amount of groundwater (i.e., the available amount of groundwater, which equals the exploitable amount of groundwater multiplied by the exploitable coefficient), and the starting line of the groundwater allocation can be determined by a trial calculation; this process is discussed in detail hereafter.
In conventional groundwater allocation, the annual exploitable amount of groundwater is generally allocated to each period of time on average; the allocation process is relatively simple according to the demand during the given time period. However, when the surface water and groundwater are combined, this allocation method is not conducive to the scheduling of surface water, especially for an external water source. Therefore, this study proposes an optimal dispatching method for despiking the groundwater allocation, that is, the groundwater is used only during periods of large water demand, and a trial calculation method can be used to determine the starting line of the groundwater allocation for the specific allocation process. The groundwater is allocated only during periods in which the water demand is larger than the starting line value, and the allocation amount is the water demand minus the starting line value. In the end, the annual amount of allocated groundwater is equal to the annual exploitable amount of groundwater.
After despiking the groundwater allocation, a substantial water demand remains for the water allocation period, and thus, groundwater should not represent the last source of water employed for the allocation of water resources. The surplus water demand is relatively stable after despiking the groundwater allocation; this provides certain benefits for the allocation of water sources, especially external water sources, and it can increase the utilization of the canal. In addition, this method can effectively reduce the scale of water transfer when determining the external water transfer scale (as shown in Fig. 2).
(4) After a long period of groundwater allocation (according to relevant requirements, this period is generally not less than 30 years), the water shortage in the last irrigation area is obtained for the entire period. The water shortage is converted to the gross surface water shortage, and the new water shortage process is obtained by considering the losses in the canal system due to leakage and evaporation. The loss due to leakage is related to the length of the trunk canal, the construction material of the trunk canal, the permeability coefficient of the underground soil, and the depth of the groundwater level, among other factors. The loss due to the trunk canal can be determined through field observation experiments in combination with relevant standard specifications and empirical parameters, and it may be simplified by an empirical coefficient.
(5) According to the design guarantee rate of agricultural irrigation in an irrigation area (assumed to be 75%), the scale of the water diversion outlet in the last irrigation area along the water transfer route can be preliminarily determined. The design guarantee rate of agricultural irrigation is generally the annual guarantee rate calculated using an empirical frequency formula expressed as P = m/(n + 1)×100%, where n is the total number of years, and m is the number of years fully satisfied within each period of time. The number of years of water supply can be calculated by a trial calculation. First, a numerical value is selected from the water shortage process to represent the initial water transfer scale (to reduce the number of trial calculations, all water shortage processes can be sorted from smallest to largest, after which approximately 75% of the values are selected as the initial scale of the water diversion outlet). From the first year, the water shortage process and the initial scale of the water diversion outlet can be obtained. The initial scale of the water diversion outlet is compared with the water shortage process on a yearly basis. If the amount of water shortage in all time periods is less than or equal to the initial scale of the water diversion outlet, then m = 1; otherwise, m = 0 and is incremented as m = m + 1 and is compared over the entire period; finally, the m value is inserted into P = m/ (n + 1)×100%. If the P value is less than the design guarantee rate of agricultural irrigation, the initial scale of the water diversion outlet will be increased. If the P value is greater than the design guarantee rate of agricultural irrigation, the initial scale of the water diversion outlet is reduced, and the trial calculation is continued until the P value equals the design guarantee rate of agricultural irrigation or until the P value is slightly larger than the guarantee rate of the agricultural irrigation design; then, the trial calculation is considered complete, and the initial scale of the water diversion outlet is obtained.
(6) Replace the water shortage processes greater than the initial scale in the last irrigation area with the initial scale in each year, that is, with the initial water transfer process of the last water diversion outlet.
(7) Determine the coefficient of irrigation infiltration recharge in the last irrigation area through irrigation tests and relevant references. By multiplying the initial water transfer process by the irrigation infiltration recharge coefficient, the amount of groundwater recharged by irrigation water can be obtained, and the new amount of exploitable groundwater in the irrigation area can be obtained by adding the amount of groundwater recharged by irrigation water to the initial calculated amount of exploitable groundwater.
(8) Repeat steps (3) ~ (7) until the difference in the water transfer scale is sufficient small during the last two iterations to stop the trial calculation. The last initial water transfer scale is the final scale of the last irrigation area along the water transfer route.
(9) Determine the scale of the water diversion outlet in the second-to-last irrigation area of the water transfer project. The surplus water demand in the second-to-last irrigation area is superimposed onto the water transfer process of the last irrigation area along the route, after which the new surplus water demand is obtained and then converted into the gross groundwater demand.
(10) The despiking of the allocation of groundwater should also be applied to the second-to-last irrigation area. Note that the allocation of groundwater in each time interval should not be greater than the surplus gross water demand of the second-to-last irrigation area.
(11) The water shortage after despiking is converted into the gross surface water shortage. Furthermore, the loss of water transfer in the canal in this section is considered, and the net surface water shortage process of the second to last irrigation area with the loss is obtained.
(12) The water shortage process obtained in step (11) minus the water transfer process in the last irrigation area, which is the initial water transfer process in the second to last irrigation area.
(13) The same trial calculation will ultimately determine the water transfer scale and process of the second to last irrigation areas.
(14) The scale of water dividing outlet in other irrigation areas and the water transfer process in the irrigation area are determined to be similar to the second to last until the canal head of the water transfer project.
(15) Finally, the canal head scale of the water transfer project and the water dividing outlet scale along the irrigation area were obtained.
Based on dynamic trial feedback, the detailed block diagram of the water transfer scale demonstration is shown in Fig. 3.