Conjunctive water management model for a multi crop irrigation command

Abstract

Water is an important input to agriculture and its judicious use is necessary to attain food security of the nation. Irrigation compensates the lack of soil moisture for crops due to spatial and temporal variability in rainfall. The use of surface water in conjunction with groundwater during summer months reduces the crop stress as well as stress on water resources and creates underground storage space for the upcoming rainy season. Conjunctive water use should be adopted in a planned manner to ensure sustainability of water resources. Mathematical models are good tools for planning conjunctive water use. Optimization, simulation, and simulation- optimization models are used for the planning process. Linear programming is a simple optimization technique used for conjunctive water use planning. Command area of Chalakudy River Diversion Scheme faces acute water scarcity during summer due to the irregularities in canal water supply. The use of groundwater for irrigation is relatively nil in the area. Hence, a conjunctive water management model was developed using linear programming optimization technique and a stable conjunctive water use policy for the area was derived by simulation runs of the optimization model. The conjunctive water management system has three distinct and interlinked components – surface water, irrigated area, and groundwater. These three components were modelled and analyzed separately and linked together by an optimization model for sustainable use of both surface and groundwater resources. Surface water availability and adequacy of canal water supply were studied using the water withdrawal data obtained from irrigation department and field measurements of canal water discharge rate and seepage loss. The irrigation requirement of the command area was computed using the FAO CROPWAT 8.0 software. Land use map of the area was prepared with ERDAS Imagine 2015 software. Data on cropping patterns and its areal extent in the CCA of the CRDS was extracted using the ArcGIS 10.3.1 software. Groundwater status of the command area was studied using Visual MODFLOW 2.8.1 software. Water level observation data from the Central Groundwater Board (CGWB) and pumping datafrom Kerala Water Authority were used for calibration of the model in Visual MODFLOW. The calibrated and validated model was used to estimate the net groundwater inflow/outflow of the command area. An optimization model was developed using the linear programming technique with an objective function to maximize the relative yield of all the crops in the command area. Canal water availability was the major constraint to achieve maximum objective value. The potential irrigation demand of each crop was fixed as the upper limit of water allocation from both sources. The proportion of canal water and groundwater in the total water allocation to each crop forms another constraint in the model. The model was solved using the software LINGO 18.0. For developing a stable conjunctive use policy for the command area, the optimization model was run for different combinations of surface water-groundwater proportions for a normal year. The groundwater balance of the study area was computed from the results and the proportion which produces only negligible change in the groundwater storage was identified as the stable policy. Temporal allocation of this stable policy over a normal year was done by simulation runs of the model for monthly time periods. Using the identified stable policy, the LP optimization model was run for past years to get the impact of the policy on aquifer storage response over the years. From the prepared land use map, it was observed that 80% of the command area is covered by the crops that require irrigation. The average irrigation requirement of each branch canal was computed and summed up to get the total irrigation requirement of the command area. It was found that the CRDS command area required 46.90 Mm 3 of net irrigation water annually. Field measurements of canal water discharge rate and seepage loss showed that the discharge rate decreases from head to tail end in the main canal as well as the branches. The seepage loss rate per km increases towards the tail end in the main canals. The high seepage loss caused by damaged lining, waste dumping and vegetation growth in canals reduced the conveyance efficiency of the canal system to 51 per cent. Performance indicators like Relative Water Supply and Adequacy Iindicator showed that the performance of the CRDS canal system falls in the class of ‘fair’.Predictions using calibrated and validated Visual MODFLOW model revealed that the groundwater status of the area is sufficient for conjunctive management of water for irrigation. Net groundwater inflow/outflow from the aquifer obtained from zone budget output of Visual MODFLOW was used to predict the change in groundwater storage due to conjunctive water use for the pre- determined surface water ratio. The solution of the optimization model from LINGO 18.0 software gives the maximum relative crop yield as objective value and the quantity of water to be allotted from both sources to attain this value. From the results obtained by running the linear programming optimization model for a normal year, a stable policy of 76:24 (surface water: groundwater) was identified as best for the command area. With the developed stable policy, a temporal allocation pattern of canal water and groundwater within a year was identified. The impact of the application of the developed stable conjunctive use policy was checked by running the model with this policy for past years from 2005 to 2012. An increase of 74.72 mm in groundwater storage would occur by the application of this stable policy over a period of 8 years. This implies a groundwater storage change of 9.34 mm/year. This change in storage would result in a groundwater level rise of 24.6 mm/year which was considered negligible. Thus, it could be recommended that, the use of a stable policy, 76:24 for conjunctive management of canal water and groundwater in the CRDS command area is capable of maximizing the relative yield of all crops in the command area without affecting the groundwater storage in the area.