Subsurface Drainage

Very saline, shallow water tables occur in many areas of Texas. Shallow water tables complicate salinity management since water may actually move upward into the root zone, carrying with it dissolved salts. Water is then extracted by crops and evaporation, leaving behind the salts.

Shallow water tables also contribute to the salinity problem by restricting the downward leaching of salts through the soil profile. Installation of a subsurface drainage system is about the only solution available for this situation. The original clay tiles have been replaced by plastic tubing. Modern drainage tubes are covered by a "sock" made of fabric to prevent clogging of the small openings in the plastic tubing.

A schematic of a subsurface drainage system is shown in Figure 2. The design parameters are the distance between drains (L) and the elevation of the drains (d) above the underlying impervious or restricting layer. Proper spacing and depth maintain the water level at an optimum level, shown here as the distance m above the drain tubes. The USDA Natural Resources Conservation Service (NRCS) has developed drainage design guidelines that are used throughout the United States. A drainage computer model developed by Wayne Skaggs at North Carolina State University, DRAINMOD, is also widely used throughout the world for subsurface drainage design.