A Model for Adoption of Microirrigation Methods in Unit Canal Command Area
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Date
2010
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MPUAT, Udaipur
Abstract
Surface irrigation methods are the most commonly adopted methods in the command area of irrigation projects in Maharashtra and other parts of India. The project efficiency of these projects is very low (30-50%). Microirrigation methods save about 30-50 % of water and increases yield to the extent of 10-20 %, over the conventional surface irrigation methods. However, the adoption of microirrigation methods is mainly limited to the well irrigated areas. The adoption of these methods in surface irrigation project would enable to enhance the project efficiency of these project and overall agricultural productivity. However, the irrigation rotation feature of the canals, where in the water is delivered by gravity with the long off periods between successive irrigations and the need for heavy investment, makes it necessary to check the technical and economical feasibility of the adoption of these systems to canal command area. Hence, the present investigation was proposed and performed with the aim to develop the methodology for adopting microirrigation methods in the command area of irrigation project under rotational water supply.
The methodology was developed in four stages; namely development of an optimum crop plan, design of intermediate storage, design of microirrigation system and the design of a pipe network with a pumping system.
The “optimal crop plan" stage allocates the land and water resources among the different crops using the linear programming technique to obtain the maximum net benefits from the allocated areas subject to the constraints of total available area, total available water, predetermined crop area restrictions and the overlap constraints. The areas to be irrigated under different crops are taken as the decision variables. Overlap constraints make it possible to reutilize the microirrigation system installed for a seasonal crop to the other seasonal crops during the remaining period of the year. The crops having crop season more than six months have only one decision variable per crop where as two decision variables are considered for the crops having a crop season duration of less than six months to implement overlap constraints. One for the area to be irrigated under the crop considering the cost of the microirrigation system and another without considering the cost of the microirrigation system. The water availability constraints under microirrigation methods are formulated such that the irrigation requirement of all crops during the particular period should be equal to or less than the summation of water available in the reservoir at the beginning of that period (this includes the surplus water from the previous period) and inflow during that period less the evaporation losses during that period. The lining of a reservoir is considered to avoid the seepage losses.
The stage “design of storage reservoirs” consists of estimation of the minimum storage capacity and the optimum dimensions of the reservoir. The reservoir storage volume-surface area functions are developed for estimation of the evaporation losses. A technique developed based on linear programming estimates the active storage capacity of a reservoir considering the evaporation losses. The linear programming has an objective function of minimizing the reservoir capacity; subject to a set of constraints namely storage continuity, sequential flow and a storage bound by capacity for each period. The dimensions to be decided for a trapezoidal shaped reservoir are bottom and top width and length, respectively, for a predetermined depth and the side slope situation.
The stage “design of microirrigation system” designs the system by working out the head and discharge requirement at an individual field. The criteria for designing of a microirrigation sub unit (consisting of laterals and submains) are based on 20 % pressure variation in the sub unit. Hazen-William’s equation is used to estimate the headloss in lateral, submain and main lines. The permissible velocity of 1.5 m/s is considered as selection criteria for a main line. The optimal arrangement of lateral and submain is determined for an individual field. The field dimensions are split into a different even number of equal sections and the available diameter of lateral, submain and main lines are tested to satisfy the hydraulic design criteria of head loss and velocity limits. The layout that yields minimum total cost (fixed cost and energy cost) is finally selected as the optimal layout.
The stage “design of pipe network and pumping system” consists of design of distribution system that includes a water supply reservoir, pumping unit and demand nodes for water, all connected by pipelines. The minimal spanning tree technique is used to select a branched network connecting the different nodes with a minimum pipe length. The selected branch network is then subjected to a pipe network optimization. The pipe net work optimization chooses right combination of various pipe sizes to minimize the cost of pipe lines and satisfy the demand for water usage and the supply pressure. The objective function considered is to find out the combination of pipe sizes that gives the minimum cost (cost of pipe network, energy cost, pumping cost etc.), subject to the constraints of flow continuity, head loss in pipes, minimum- maximum pressure limits etc.
The methodology consists of the option of having the common reservoir and pumping unit for all the fields in the minor command area and the reservoir and pumping unit for an individual field. In case of reservoir constructed at an individual field, the pumping requirement for operating the microirrigation system installed at the field is worked out on the basis of discharge and head requirement for an individual field.
Programs viz., Investment Decision Model (IDM), Optimal Crop Allocation Model (OCAM), Storage Reservoir Design Model (SRDM), Microirrigation Layout & Design Model (MILDM), Crop Evapotranspiration Estimation (CETE) and Pipe Network Design Model (PNDM) are prepared for the methodology developed in this study.
The developed methodology was applied to a direct minor no. 3 of Main Right Bank Canal of a Mula Irrigation Project, Maharashtra State, having the total cultural command area of 431.75 ha. The discharge of this minor at the head regulator is 0.283 cumec. The land holding in the command area ranges from marginal to medium size.
The application of developed methodology to the study area indicated 260 % higher net benefits due to adoption of microirrigation methods using field reservoir and pump for an individual field and 188 % higher net benefits due to microirrigation methods using single common reservoir and pumping unit as compared to the surface irrigation method. The net benefits are observed to be 25% higher for adoption of microirrigation system using the field reservoir and pump for an individual field of the minor under case study when compared to adoption of microirrigation system with common reservoir and pumping unit. The reutilization of the microirrigation systems among the seasonal crops having growth periods less than six months showed approximately 6% increase in the net benefits.
This indicates that replacing surface irrigation methods by microirrigation methods is beneficial in a command area of an irrigation project under the rotational water supply system.
Description
A Model for Adoption of Microirrigation Methods in Unit Canal Command Area
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Citation
Gadge and Seivastava, 2010