RESULTS OF THE MOUNTAIN HOME RAINFALL INTERCEPTION AND INFILTRATION PROJECT IN BLACK WATTLE, 1953/54

Abstract

Summary Interception of rainfall by a mature plantation canopy is estimated at 15–20 per cent over the whole year. It varies between 40 per cent with light rains and 13 per cent with heavy. Net rainfall in the plantation is due, over the whole year, 86 per cent to throughfall and 14 per cent to stemflow. During light rain and drizzle as opposed to storms, stemflow forms a much higher proportion (up to 23 per cent). Condensation from mist never led to a higher precipitation than recorded in the open. If it occurred in the plantation during light rain its presence was masked by the interception. Throughfall under grass cover was found to be much less than under the plantation. It averaged 67 per cent for the whole year with grass burnt in spring and was from 53 to 58 per cent under mature grass. Stemflow of grasses could not be accurately measured but was evidently of considerable magnitude. Interception by grass cover is thought to be very small. It is unlikely to exceed 10 per cent over a whole year and may well be much less. Six per cent of the total annual rainfall was lost by run-off in the veld and 7.7 per cent in the plantation. Run-off in the plantation is higher in spring due to resistance to wetting of the floor, but in summer and autumn does not materially exceed that in the veld. Wattle plantations may therefore be a less efficient watershed cover than ungrazed natural grassland from the point of view of net infiltration, since the higher interception and run-off reduce net rainfall reaching the soil. The difference is not however of any great magnitude and importance, and when the run-off and erosion losses from heavily grazed grasslands and agricultural croplands are considered, it fades into insignificance. It would appear that the difference could be to a large extent reduced or eliminated in plantations where slash-piles have been placed along the contour and left to rot down.