CONSIDERATION OF AIR ENTRAPMENT IN SURFACE IRRIGATION - A COMPUTER-SIMULATION STUDY

A numerical model, based on Richard's equation was used to simulate in filtration for continuous flow, surge flow, and surge flow with air en trapment on a Rubicon sandy loam soil. Air entrapment was described by reducing the rewet water content in the water retention function by 1 0% during the first redistribution. Surge simulations consisted of fou r cycles with 10 min on and off times followed by one last surge with 20 min on-time. For comparison purposes the opportunity time for the c ontinuous flow simulation was the same as for the surge simulations. R esults showed a 21% decrease in cumulative infiltration for the air en trapment simulations compared with that for continuous flow. A larger simulation time corresponding to an actual irrigation event would have increased the magnitude of the differences in cumulative infiltration between the continuous flow and air entrapment simulations. In summar y, it was demonstrated that air entrapment could significantly contrib ute to intake rate reduction under surge irrigation in sandy soils. Mo re experimental research is needed to accurately model air entrapment and conduct comprehensive simulation studies.