Prediction of Salt Accumulation at Wet Soil Surface Under Drip Irrigation.

One of the drawback of drip irrigation is accumulation of salt to wet soil surface and it cannot be leached out by water from drip tube. To test validity of existing simulation model of water flow and solute transport considering vapor movement and effect of salt crust on evaporation, two experiments of drip irrigation with saline water under bare soil condition were carried out. Deep (40 cm heigh) and shallow (2 cm) soil tanks were used. A 3,000 ppm NaCl solution was applied to the edge of air dry loamy sand through porous pipe. Soil surface was placed at the bottom of wind tunnel and allowed to evaporate under constant meteorological condition except for radiation, which was automatically regulated so that temperature of soil surface became nearly uniform and constant. Numerical simulations with the Richards equation including vapor movement and the convection-dispersion equation considering crystallization were also performed. For both soil tank, salt was more dispersively distributed along the wet soil surface than numerical solution in which horizontal salt distribution had clear mode. When threshold value for crystallization was set at one-third of that of NaCl, horizontal distribution of salt became more smoothly, but still more concentrated at certain distance from the emitter. In upper dead-end of pore water, concentration due to evaporation and crystallyzation may proceed more promptly than simulated macroscopically.