A Method to Estimate the Return Period of the Irrigation-less Dry Season and to Prepare Countermeasures against Water Shortage for Rice Double Cropping in Tropical Monsoon Area

A case study of Muda Irrigation Scheme of Malaysia

Abstract

In any irrigation project, the command area must be well-balanced with the available water resources.At the planning stage of the project, the data accumulated may not always be reliable enough for the examination of this balance.Therefore re-examining this balance some years after the starting of project implementation when enough data have been compiled would be meaningful.If the result of this re-examination in a rice double cropping project indicates that the available water resource is not adequate, the return period of the year in which water can not be supplied in the dry season will have to be estimated and appropriate countermeasures for meeting the water shortage problem formulated.
In the irrigation planning of reservoir-fed irrigation system of single cropping, the reservoir should be usually full on the first day of the season.A drought year that returns roughly once in every ‘N’ years may be defined as the ‘reference year for design’.Water must be supplied with certainty in this year.On the other hand, in a double cropping project, the reservoir may not be full on the first day of the season.The water volume in the reservoir usually recedes every dry season, and recovers in the rainy season and fallow period.The water storage in the reservoir repeats this up and down movement every year.The concept of the ‘reference year for design’ can not be applied to a double cropping project.Some new concept and planning methodology will have to be devised and proposed.
In this report, as a case study of Muda Irrigation Scheme in Malaysia, the author illustrates an example of the re-examination of the demand-supply balance in the reservoir, and proposes a simple method to estimate the return period of irrigation-less dry season.He also cites an example of a method to prepare countermeasures to meet this water-shortage problem.The author considers that these methods will be suggestive to projects in tropical monsoon areas under similar conditions.