Water Table Recession in Subsurface Drained Soils

Theoretical drainage equations are intensively tested in many parts of humid and arid regions and are commonly used in drainage design. However, this is still a great concern in Japan as the drainage design is exclusively based on local experiences and empirical basis. There is a need therefore to evaluate the theoretical drainage equations under Japanese field conditions to recommend equations for design of subsurface drainage systems. This was the main motivation for this study. While drainage requirements of different crops are difficult to define quantiatively, adequate drainage can probably be provided by designing for a given water table recession rate. The method used was to measure the rate of water table recession and drain outflow after cessation of a considerable amount of rainfall. Based on these measurements, soil parameters were determined and nine drainage equations were tested in simulating water table recession against actual field data of two drained experimental sites, located in Soja city west of Okayama prefecture, Japan, having drain spacing of 10 and 8.5 m for a period of two years. An experimental drainage equation was also derived and theoretical equations were recommended for design of subsurface drainage systems.