THE FREE-WATER POND UNDER A TRICKLE SOURCE - A FIELD-TEST OF EXISTINGTHEORIES

The size of the free-water ponds under drippers is an important field characteristic of trickle irrigation. Here we review four existing ana lytical theories that purport to describe 3-D infiltration. We show ho w they can be used to predict the extent of the pond from basic soil p arameters, and the dripper discharge rate. Via a numerical example it is shown that the different theories give significantly different free -water pond radii, especially for high drip discharge rates when the f low is dominated by gravity. We then describe field experiments that w ere carried out to permit a comparison of the measured and predicted v alues using independently-measured soil parameters. This test of the d ifferent theories was conducted on a loamy soil. Raats' (1971) solutio n gave the best estimates of the free-water pond radius, across the ra nge of discharge rates. The free-water pond radius, a determinant of t he lateral spread away from the dripper, was found to vary only weakly with drip discharge in the range of practical applications. Consequen ces for management are discussed.