DISTRIBUTED MODELLING OF CONJUNCTIVE USE IN A CANAL COMMAND

Abstract

For the survival of human race it is necessary that the rapid population growth is matched by a corresponding expansion of agricultural production. This realization has led to the development of high yielding varieties of crops, increased reliance on chemical fertilizers and more intensive irrigation. All these measures have increased considerably the water requirement for irrigation. The water requirement is to be fulfilled by the available water resources which are limited in general. The optimum utilization of the surface water and groundwater is, therefore, necessary so as to maximize the net benefits from the cropping activity in an area or to minimize the cost of providing these resources. The conjunctive use of surface water and groundwater provides the optimal allocations of these resources in space and time. Regions of high water table should be allocated lesser quantity of surface water which would result in more pumping of groundwater to meet the requirement. On the other hand regions of deeper water table should be allocated more surface water and less pumping of groundwater. This process of allocations of water would not only minimize the cost of providing these resources but would also keep the water table within some limited range of depth to water table. The most important aspect is that the water table should not rise so as to result in waterlogging and also should not fall below a certain limit so as to cause drying of shallow wells. It is important that all the available water resources are considered in unison and the advantages of both the resources have to be taken into consideration for effective and optimum management of the available water resources. An extensive review of the available literature on conjunctive use reveals that there are variety of models available for coordinated operation of surface water and groundwater. For real field problems many (iii) of the models are not appreciated by the field engineers because of the complex nature of the modelling techniques specially countries like India. Therefore, it seems that the use of a simple but representative model could be more useful in management problems as it would provide decision makers with a quick but relevant solution to the management problem. With this idea an attempt has been made in the present work to develop a conjunctive use model for regional water resources planning. The problem in hand is a nonlinear in nature as the cost of pumping of groundwater introduces the nonlinearity in the objective function of a general conjunctive use model. Therefore, a solution technique based on iterative procedure was contemplated for the present work. To account for the nonlinearity of the pumping cost, it is proposed to develop a conjunctive use model by successive linearization of the pumping cost. The linear programming can thus be used as a solution technique and the final results can be obtained through iterative procedure. The aquifer response model cans be employed to determine quantitatively the change in water table elevations as a result of pumping or recharge. These models are almost, always distributed in time and may be lumped or distributed in space. A distributed model of groundwater system based on Tyson and Weber (1964) has been prepared for the study area and calibrated with the available data. The aquifer parameters have been presented in graphical form. The costs of providing surface water and groundwater are very important in any conjunctive use management model. Cost of surface water comprises capital cost of the distribution system and operation and maintenance cost. The capital cost of the distribution system is to be allocated region-wise depending upon its use for conveying water for that particular region and conveyance efficiency. Unit cost of surface water (cost per unit volume of water) based on the annual costs of capital investment and operation and maintenance is computed for each region or zone. In the present work the study area has been divided into six zones. Cost of providing groundwater plays very decisive role in (iv) finding out the optimal allocations in any conjunctive use planning and operational model. Cost analysis of groundwater, is carried out taking into account the actual capital investment as well as the operational cost. Cost analysis for groundwater has been for shallow tubewells, to arrive at functional relationship for cost function (cost per unit volume of water in terms of depth to water table), which subsequently should be incorporated in the conjunctive use model. In the present work the cost function for groundwater, groundwater model and the optimization model have been coupled for conjunctive use modelling. The applicability of the conjunctive use model to real problem has been demonstrated by various cases for regional water resources planning and management problems. For illustrating the methodology, Nehtaur branch canal command is selected which was entirely irrigated by groundwater till recently. Now only the surface water irrigation is being introduced. The allocations plans based on the total cost and 0/M costs of providing surface water and groundwater are also described. Average optimal allocation plan is analyzed with 0/M costs and compared with the yearly allocation plans. Prior to the introduction of the surface water irrigation in the study area, the depth to water table is continuously increasing as the groundwater withdrawals exceeds the total recharge. With the introduction of surface water irrigation the downward trend of water table would be checked. After the introduction of the canal. Alternate conjunctive use policies have been studied and the results upto the year 2007 have been obtained for different conditions i.e. considering total costs and 0/M costs of providing surface water and groundwater at the existing level of irrigation (80% of water requirements). Investigations with these considerations have also been made for 1007. level of irrigation. It has been observed that the allocations of water based on the 0/M costs of surface water and groundwater result in minimum variations of depths to water table at the end of the study period i.e. year 2007 within the study area. On the other hand, when the allocations are obtained by considering total costs the average depths to water table within the study area vary more from zone to zone at the end of (v) the study duration as compared to the case when allocations based on 0/M costs. Allocations based on the total costs are primarily governed by the location of the zone and depth to water table as the total costs depend on these factors. On the other hand allocations based on the 0/M costs are primarily governed by the depth of water table as 0/M costs of surface water do not vary significantly from zone to zone. The optimal allocation plan when compared with the equitable distribution practice alternative, it was noticed that the cost of providing water would be more in case of equitable distribution of water than that of the conjunctive use allocations plans. The comparison of different operation plans suggested that the allocation plan based on the 0/M costs should be adopted as the zone-wise variation of depth to water table within the study area are small. However, it is very difficult to change the allocations from year to year. Therefore, an alternate operation plan with 0/M costs is analyzed with an average optimal allocation of surface water and groundwater and compared with yearly allocation plan. It is found that the behavior of water table and the pumping cost with average optimal allocation plan are very near to those obtained from yearly optimal allocation allocations plan. It is, therefore, recommended the average optimal allocations considering the 0/M costs of providing surface water and groundwater be adopted as the variation of depths to water table within the study area is minimum (at the end of the study period). At 1007. level of irrigation, these allocation plan would the study area.