Simulation of Conjunctive Operation in a Command Area

Abstract

Success of an irrigation system depends on efficient water management. Efforts to improve agricultural practice by making more efficient use of available water resources require mathematical models to simulate the dynamics of water distribution in an irrigation system. A number of computer-based models have been reported in the literature (such as SIMIS, CAMSIS, INCA, OMIS etc.) to help irrigation manager in real-time operation of a canal system. Such models analyze the system operation in terms of water demands and supply and optimize the water allocation to meet some performance-based criteria/objectives. Irrigation command areas may exhibit marked spatial heterogeneity in terms of cropping pattern, physiographic characteristics, irrigation practices, water availability and utilization etc. Groundwater availability in an irrigation command varies spatially as well as temporally depending on the depth of groundwater table below the land surface, and groundwater extraction facilities. Often, gross simplifying assumptions, such as areal average cropping pattern, uniform physiographic and agro-climatic characteristics and average groundwater availability etc. are made in planning and operation of canal irrigation projects. This may lead to glaring discrepancies with ground situation resulting in inefficient utilisation of water resources. The objective of this study is to develop a geo-simulation model that can integrate the spatial information on different variables related to water supply and water demand for realtime operation of a canal network. Broad aims of developing the scheme are: a) to integrate the spatial and temporal database for rational operation of an irrigation system, b) to integrate various processes of irrigation water management in the command area, and c) to depict the results of simulation model and performance parameters in form of maps for easy comprehension and decision-making. It is envisaged that such a model will help the irrigation manager for judicious operation of a canal network on the basis of current state of the system.The developed model operates at weekly time step and consists of two major distributed models [Soil Water Balance Model (SWBM) and Canal Network Simulation Model (CNSM)] and a number of sub-models for database generation and linking various models of the scheme. The purpose of SWBM is to simulate the moisture variation in root zone of crops for finding spatially distributed irrigation demands, groundwater recharge, water stress conditions in crops, and soil moisture content at the end of each week. SWBM is based on a book keeping procedure and incorporates spatial variability of crop, soil, rainfall, and topography in the dynamics of soil-water-plant interaction. The purpose of CNSM is to simulate the weekly operation of a canal network and allocate the available canal water and groundwater on the basis of irrigation demands (calculated by SWBM), system characteristics, and prevailing groundwater conditions in the area. For allocation of canal water under deficit conditions, five different water allocation policies have been proposed: a) Head-reach priority, b) Conjunctive utilisation of water, c) Proportionate supply, d) Tailreach priority, and e) Conjunctive use with minimum energy demand. For generating revised groundwater conditions corresponding to different canal operation scenarios, an existing groundwater simulation model (Visual MODFLOW) is linked to the modeling scheme.To analyze its performance and utilisation, the developed modeling scheme is applied to a branch canal command (with a gross area of about 1956 sq. km) under the Madhya Ganga Canal System in U.P. State, India. ILWIS GIS system is used for database development (soil map, Thiessen polygon map, digital elevation map, flow direction map, groundwater table map, irrigable command map etc.) and various spatial analysis. ERDAS IMAGINE system is used for processing of satellite data. Since the scheme provides a large area simulation, its calibration and validation is carried out using the analysis of groundwater behavior in the area. Application of the scheme is demonstrated for one crop season of the year 1998. Maps corresponding to irrigation demands, groundwater recharge, water stress conditions in crops, various canal operation details, such as discharge and run-time etc. can be prepared with the developed scheme. To summarize, the problem of integrated operation of a canal network considering real-time spatial information is analyzed in this study. A distributed simulation scheme is developed to study various operation scenarios for the canal system. Using remote sensing and GIS for database generation and management, representation of geographic characteristics of the command area has been made quite realistic. Using the simulation scheme iteratively, optimization is performed to find the canal run configuration for least requirement of pumping energy in the system. Using the geo-simulation scheme, the operation of a canal network can be planned, eco-system of a command area can be maintained, and energy demands for pumping groundwater can be optimized. The results of the scheme can be presented in pictorial form for easy understanding. The scheme can be used as a decision support tool for irrigation water management in command areas.