A critical problem challenging mankind today is how to manage the intensifying competition for water between expanding urban centres, traditional agricultural activities and in-stream water uses dictated by environmental concerns. In the agricultural sector, the prospects of increasing the gross cultivated area are limited by the dwindling number of economically attractive sites for large-scale irrigation and drainage projects. Therefore the required increase in agricultural production will necessarily rely largely on the affordability to apply new technologies, a more accurate estimation of crop water requirements, and on major improvements in the construction, operation, management and performance of existing irrigation and drainage systems. The failings of present systems and the inability to sustainably exploit surface and groundwater resources can be attributed essentially to poor planning, design, system management and development. This is partly due to the inability of engineers, planners and managers to adequately quantify the effects of irrigation and drainage projects on water resource systems and to use these effects as guidelines for improving technology, design and management [1-4].