Faulty Design Parameters and Criteria of Farm Water Requirements Result in Poor Performance of Canal Irrigation Systems in Ilocos Norte, Philippines

摘要

Canal irrigation systems in the Philippines are characterized by their poor performance. For an insight on the reasons for this situation, 10 canal irrigation systems were randomly chosen to assess the soundness of the water balance parameters assumedin estimating their design crop and farm water requirements. Paddy percolation losses were measured and were compared with the assumed values during the project design stage. Farm ditch losses were also measured. Farm water requirements were computed onthe basis of the measured percolation and farm ditch losses and estimated net seepage, evapotranspiration and other losses. The ratio of the actual area served to the design service area was computed for each of the sample irrigation systems. Data on actual area irrigated were determined based on the records of the farming activities by season of the institutions or associations responsible for the operation and management of the irrigation systems. The discrepancies between design and measured farm water requirements were then related to the proportions of the actual area served to design irrigation service areas to help explain, in part, the reasons for the poor performance of the irrigation systems. On the average, only about 27% of the aggregate design service area of the sample irrigation systems was actually irrigated during the dry season. This was mainly due to underestimation of the assumed on-farm water losses during the planning stage of these systems. In fact, only one of the 10 systems studied had its value of the design farm water requirement within 100% of the values computed by using the measured percolation and farm ditch losses. Although the designs and operations of canal irrigation systems in the Philippines are carried out by two agencies that are both under the Department of Agriculture, there is very limited interaction between design and operation engineers. Such absence of a feedback mechanism could have resulted in continuous use of the same flawed designs. Design shortcomings have not been corrected and the same design faults can be found in most of the canal irrigations systems. Failure to properly identify and rectify the design shortcomings could have been the major reason for insignificant increases in rice croppingintensity even after massive rehabilitation efforts. It is, therefore, high time to give more emphasis on the formulation of appropriate irrigation design criteria.