TR(BR)-15/99-2000 : Optimum water management in a command area

Abstract

Success of an irrigation system depends on efficient water management. Irrigation scheduling, which anticipates the temporal water need of crops, i.e., when to irrigate and how much water to apply, is an important management activity affecting the performance of irrigation projects. Efficient operation of irrigation systems adopting sophisticated techniques leading to real-time water management is an urgent need of the hour. The dynamics of water within the unsaturated zone of the soil is a complex phenomenon dependent on the properties of atmosphere, soil and vegetation. Mathematical models are developed to simulate the physical processes of crop-soil-water systems for providing the knowledge of the amount and timing of water needed.

Large amount of information about various processes is involved in irrigation management of a command area. This information needs to be continuously updated for realtime management. With the advent of satellite remote sensing, it has now become possible to update the information at regular intervals. Using a Geographic Information System (GIS), the spatial information can be efficiently stored, analysed and retrieved.

The objective of this study is to simulate the dynamics of soil moisture within the root zone. An analytical model of soil-water balance, distributed in space and time, is developed and applied to field data of the Lakhaoti command under the Madhya Ganga Canal system. The data inputs to the model include rainfall, potential evapo-transpiration, and various crop and soil characteristics. Three important inputs of the model, i.e. rainfall, soil type, and crop type have been considered to be spatially distributed. The command area is discretized into a finite number of rectangular grids and water balance computations are made for each grid. Type of crop in each grid is determined using multi-temporal remote sensing data. Based on the soil survey maps and laboratory analysis, various soil parameters have been determined.

Effective soil depth is assumed to be the root zone depth in a grid which keeps on increasing with time till it attains a maximum value. The model estimates the actual evapotranspiration and the soil moisture content at the end of each day/week using the available information on soil water availability, rainfall, potential evapo-transpiration and the plant water uptake. The output of the model is the actual evapo-transpiration during the day/week, the soil moisture content at the end of the week, supplementary water requirement, water stress condition in the crop, and the recharge. The model is effective in predicting spatial distribution of average soil moisture conditions in a command area. A computer program is developed to perform the water balance computations for each grid in the command area. The developed program will allow us to estimate the spatial and temporal distribution of crop water demand in the command area that can be usefully incorporated into larger computer based irrigation management models. The system also permits display of information on maps for easy handling. This visualisation allows users to more readily participate in decision making processes.