The impact of design approximations on the operational performance of an irrigation scheme: A case study in Malaysia

A method is presented to analyse the impact of the selection of the size of irrigation gate on the operational performance at the Sungai Muda Irrigation Scheme in Malaysia. The method examines the discharge capacity of the water control gates at all levels in order to compare the specific water supply (the ratio of supply to command area) with the specific water demand which is the required hydromodule. The term hydromodule is the reciprocal of ‘water duty’ and thus has units of litres/second/hectare. The greater the deviation between the two, the greater the potential loss of control during the operation of the scheme. The method is relatively simple but is more complex in this particular example as two hydromodules are used for the irrigation of basin rice; one for the presaturation period and one for the normal supply period. The most common cause of loss of water control was found to be due to oversized turnout gates at the head of secondary and tertiary canals. Such design approximations enable more water to be used in those command areas leading to waste and shortages of water in other areas. It is suggested that during design and rehabilitation of irrigation schemes, the operational implications of design approximations should be examined more carefully.