The interception and smoothing effect of forest canopies on pulses of incident rainfall and its delivery to the soil has been suggested as a factor in moderating peak pore water pressure development within soil mantles, thus reducing the risk of shallow landslides. Here we provide three years of rainfall and throughfall data in a tropical secondary dipterocarp forest characterized by few large trees in northern Thailand, along with selected soil moisture dynamics, to address this issue. Throughout the sampling period, throughfall was an estimated 88% of rainfall, varying from 86–90 % in individual years. Data from 167 events demonstrate that canopy interception was only weakly associated (via a non-linear relationship) with total event rainfall, but not significantly correlated with duration, mean intensity, or antecedent 2-day precipitation (API2). Mean interception during small events (≤ 35 mm) was 17 % (n = 135 events) compared with only 7% for large events (> 35 mm; n = 32). Examining small temporal intervals within the largest and highest intensity events that would potentially trigger landslides revealed complex patterns of interception. The tropical forest canopy had little or no smoothing effect on incident rainfall during the largest events. During events with high wind speeds and/or moderate-to-high pre-event wetting, measured throughfall was occasionally higher than rainfall during large event peaks, demonstrating limited buffering. However, in events with little wetting and low-to-moderate wind speed, early event rainfall peaks were buffered by the canopy. As rainfall continued during most large events there was little difference between rainfall and throughfall depths. Comparing both rainfall and throughfall depths to mean intensity–duration thresholds for landslide initiation, throughfall exceeded the threshold in 75 % of the events in which rainfall exceeded the threshold for both wet and dry conditions. Throughfall intensity for all the 11 largest events (rainfall = 65–116 mm) plotted near or above the intensity-duration threshold for landslide initiation during wet conditions; five of the events were near or above the threshold for dry conditions. Soil moisture responses during a range of rainfall conditions in large events were heavily and progressively buffered at depths of 1 to 2 m, indicating that the time-scale of short-term smoothing of peak rainfall inputs (i.e., ≤ 1 h) has little or no effect on peak pore water pressure at depths where landslides would initiate. Given these findings, we conclude that canopy interception would have little effect on mitigating shallow landslide initiation during the types of monsoon rainfall conditions in this and similar tropical secondary forest sites.
Hydrology and Earth System Sciences / Vol. 21, No. 1, pp.651-667