Rainfall partitioning within semiarid juniper communities: effects of event size and canopy cover

Abstract

Partitioning bulk rainfall into canopy interception, litter interception, stemflow and throughfall allows an estimate of the physical impact of trees on the local hydrologic budget. Despite recognition of the potentially large effect of interception and associated processes on the hydrologic budget, few, if any, studies have quantitatively evaluated the multiple components simultaneously in semiarid savannas, nor have the effects of rainfall intensity within storms been rigorously evaluated. We monitored interception and rainfall partitioning in individual Ashe juniper (Juniperus ashei Buchholz) canopies at ten sites over a 3-year period. Averaged over all ten sites for 2700 total rain events, about 35% of the bulk rainfall falling on juniper trees was intercepted by the tree canopy, 5% was intercepted by the coarse litter and duff beneath the tree, 55% reached the ground surface as direct and released throughfall, and 5% was redirected to the base of the tree as stemflow. Small amounts of rainfall (<2·5 mm) were entirely captured by the canopy and evaporated to the atmosphere, contributing nothing to soil water under juniper trees. Low intensity rainfall (e.g. 13 mm over a 19-h period) that could conceivably benefit the local plant community was largely intercepted by the tree canopy (>60% interception). High intensity rainfall was less influenced by juniper canopies. At high intensities (e.g. > 70 mm over a 15-h period) only 20% of the bulk precipitation was intercepted by the canopy and litter. The hourly pattern of rainfall within extended storms demonstrated periods with low intensity and periods with high intensity. The interception rates during these periods closely mimic the rates seen in similar intensity short duration storms. Canopy and litter interception effectively reduced the beneath-canopy precipitation from 600 to 360 mm in the western region and from 900 to 540 mm in the eastern region. Copyright © 2006 John Wiley & Sons, Ltd.