ABSTRACT
The sustenance of viable agriculture depends strategically upon the provision of adequate subsurface drainage. The goals of providing adequate aeration and salinity control in irrigated lands have been translated into a number of criteria upon which drainage systems are designed. A brief review of these shows that most criteria for design can be reduced to that of an equivalent steady state condition. Many theories have been developed for the design of subsurface drainage systems. Some of these theories derive exact mathematical solutions while others use simplifying assumptions to derive approximate results that can be applied to field conditions. Most commonly used drainage theories are based on the extensive use of the experimental law of Darcy which postulates a linear relation between macroscopic velocity and hydraulic gradient despite the acknowledged fact that the law fails in flow situations of high Reynold's number. In this study, approximate analytical solutions are proposed for the water table profile between parallel drains, along with drain spacing formulae which generalize those conventionally used and based on the Darcy's linear law. The validity of these proposed equations is investigated experimentally using a sand tank seepage flow m under high velocity flow regimes. They are found to close agreements with experimental results as well as with similar studies reported in the literature, and provide for greater economy in drainage systems design.