Subsurface drainage systems have little impact on water tables, salinity of clay soils

Subsurface drainage systems are traditionally installed in agricultural fields to control waterlogging (high water tables) and excess salinity affecting the crop root zone. However, in many clay fields of the Imperial Valley underlain by shallow fine-sand aquifers, the drains may be ineffective and provide limited relief for the root zone. After extensive work considering soil-water flow paths in a particular field at the UC Desert Research and Extension Center (DREC), we plugged whole-field drainage systems, then evaluated the impact on water-table levels and soil salinity during a 3-year period. We found that the shallow fine-sand aquifer underlying the DREC, originally identified in the 1950s, combined with the Imperial Irrigation District deep-ditch system, provided sufficient drainage relief for several areas of the DREC to successfully grow a variety of crops. Given the ineffectiveness of subsurface drainage systems in three different fields that had soil characteristics similar to more than half the Imperial Valley fields we expect that in many parts of the valley, augmentation of subsurface drainage systems through “splitting” the lateral drainline spacing may not be warranted. Indeed, efforts to reduce the salinity of heavier soils on the DREC, or elsewhere In the valley, may be better directed at improving water penetration and leaching of soils through deep ripping or other cultivation/mulching methods, rather than expending resources on improving the subsurface drainage system.