Articles | Volume 21, issue 1
Hydrol. Earth Syst. Sci., 21, 651–667, 2017
Hydrol. Earth Syst. Sci., 21, 651–667, 2017

Research article 31 Jan 2017

Research article | 31 Jan 2017

The canopy interception–landslide initiation conundrum: insight from a tropical secondary forest in northern Thailand

Roy C. Sidle1 and Alan D. Ziegler2 Roy C. Sidle and Alan D. Ziegler
  • 1Sustainability Research Centre, University of the Sunshine Coast, 90 Sippy Downs Drive, Sippy Downs, Queensland 4556, Australia
  • 2Department of Geography, National University of Singapore, AS2, #03-01, 1 Arts Link, Kent Vale, 117570, Singapore

Abstract. 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 in soil mantles, thus reducing the risk of shallow landslides. Here we provide 3 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. Throughfall was an estimated 88 % of rainfall, varying from 86 to 90 % in individual years. Data from 167 events demonstrate that canopy interception was only weakly associated (via a nonlinear 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 smoothing effect on incident rainfall during the largest events. During events with high peak intensities, 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. A comparison of both rainfall and throughfall depths to conservative mean intensity–duration thresholds for landslide initiation revealed that throughfall exceeded the threshold in 75 % of the events in which rainfall exceeded the threshold for both wet and dry conditions. Throughfall intensity for the 11 largest events (rainfall  =  65–116 mm) plotted near or above the intensity–duration threshold for landslide initiation during wet conditions; 5 of the events were near or above the threshold for dry conditions. Soil moisture responses during large events were heavily and progressively buffered at depths of 1 to 2 m, indicating that the timescale of any short-term smoothing of peak rainfall inputs (i.e.,  ≤  1 h) has little influence on peak pore water pressure at depths where landslides would initiate in this area. 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.

Short summary
Rainwater intercepted by forest canopies has been suggested to moderate peak pulses of rainfall and resultant pore pressure within soils, thus reducing the risk of shallow landslides. Three years of data in a tropical secondary forest in northern Thailand show that average canopy interception during large storms was only 7 % of rainfall. These peak rain inputs had little effect on soil moisture pulses below 1 m. Thus, canopy interception would have little effect in mitigating shallow landslides.