INFILTRATION FROM A SURFACE POINT-SOURCE AND DRIP IRRIGATION .1. THE MIDPOINT SOIL-WATER PRESSURE

Bresler [1978] proposed a procedure for drip irrigation design which i s focused on the midpoint soil water pressure h(c). We present a pract ical field test of this approach in order to evaluate the validity of the underlying assumptions. The simulated h(c) values were obtained fr om Raats' [1971] steady state theory for 32 points in the field where the hydraulic conductivity parameters K-s and alpha were measured. The h(c) values were measured at the same locations during microirrigatio n of a maize crop. Measured h(c) appear to be lower than the simulated ones, especially late in the season. The measured spatial variability in h(c) appeared to be higher than the simulated ones. This could wel l have been caused by root uptake activity, which is not considered in the analysis, as well as by the large but typical drippers spacing of d = 1.00 m. Thus the tensiometers could have been beyond the practica l limit of wetting. Consequences for design and management are importa nt. For design, even if a high h(c), value is chosen, there is no real guarantee that the wetting would be effective at the midpoint. For ir rigation management, tensiometer placement too far from the dripper wo uld lead to overirrigation, so for a large dripper spacing d, the midp oint placement is not judicious.