Strategies to improve water management in Mexican irrigation districts : a case study in Sonora

Abstract

Few studies have been made regarding the efficiency of water use in irrigation systems in operation. Therefore, in this work the search for methodologies to evaluate the actual efficiency of water use in a Mexican irrigation district as well as methods which may improve this efficiency is emphasized. The work has been divided into four parts. In the first part, the analysis of the economic activities in the irrigation district by means of linear programming has permitted finding the marginal productivity of water which is used as a shadow price for economic evaluation of the operating rules. In this part a relationship between the marginal productivity of water and the conveyance efficiency has also been found. In the second part, using hydrological historical data and a pattern of monthly water demand for crops obtained in the first part, control curves of the reservoir levels which permit knowledge of the probability of risk of spill and shortage were computed. Utilizing these curves and a simple linear programming model for finding the optimal economic annual average release, operating rules for the reservoir have been deduced. These rules were tested by means of a simulation model and their economic effectiveness was estimated using a benefit-cost analysis, showing a significant improvement in benefits for the farmers as a consequence of using these rules. In the third part, the conveyance losses in the canal network are analyzed using a linear regression between releases and deliveries. The proposed model permits an estimation of the conveyance efficiency and its division into two component factors, the intrinsic efficiency directly related to the physical characteristics of the canals, and the operational efficiency related to water management in the canal network. Knowing these components, it is possible to classify the conveyance losses as well as deduce operating rules for improving the operational efficiency. In making decisions about the methods to be used for improving the conveyance efficiency, benefits and costs must be taken into account. Therefore, a practical relationship for evaluating the economic feasibility of lining canals, considering those benefits and costs, is found as well as a method for economic evaluation of the operating rules. Finally, in the last part of this work, a practical methodology for estimating the application efficiency at the farm level in an irrigation district and small divisions as well is shown. This method is useful in finding where the problems in water use are more important within the district and which could be their principal causes. Utilizing data from experiments, which are available in most of the Mexican irrigation districts, production functions relating yields of crops to the amount of water applied and the percentage of moisture depletion in the soil before and during the flowering season of the crop, have been found. These functions can be used not only for obtaining the optimal economic depth of water to be applied and irrigation timing but also for deriving the optimal water use efficiency, whi6h, when related to actual values of this water efficiency in each subdivision of the district, permits the deduction of a practical water use efficiency index which is useful for detecting where the problems in water use are and what are the potential returns for the farmers in each of the considered subdivisions.