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

Abstract

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 (API 2 ). 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.

Tree-root control of shallow landslides

“…At the hillslope scale, the hydrological effects of vegetation are assumed to play a small role on slope stability compared to the contribution of root reinforcement (Sidle and Bogaard, 2016;Sidle and Ziegler, 2017). At the catchment scale, however, the regulation of water fluxes may have important implications for the stability of those slopes that drain large areas, particularly for short and intense rainfall events.…”

Strategies to improve the explanatory power of a dynamic slope stability model by enhancing land cover parameterisation and model complexity

“…Tree root systems provide mechanical stability even after forest removal but their reinforcement potential decrease after time, leading to a minimum of potential stabilisation between 3 and 8 years (O'Loughlin, 1974;Sidle et al, 1985;Sidle 1992). At the hillslope scale, hydrological effects are presumably rather inferior compared to the biomechanical effects (Sidle and Ziegler, 2017;Cohen and Schwarz, 2017). From a spatial perspective, the effects of forest cover are strongly dependent on the scale on which slope stability is assessed.…”

How deep can forest vegetation cover extend their hydrological reinforcing contribution?

“…Our observations clearly suggest that the differences observed in hydrological reinforcing effects between grass and hardwood tree covers are negligible, as potentially unstable conditions can be observed at a depth of 100 cm under both vegetation covers, with the F S values less than 1.0 for some dates, therefore supporting the findings of those numerical modelling studies (Chirico et al, ; Arnone et al, ), that differences in hydrological reinforcing effects from different vegetation covers are less evident during wet period. Another study on the interception and smoothing effect of tropical forest canopies showed that the effect was not great enough to mitigate the shallow slope failure initiation for monsoon rainfall conditions (Sidle & Ziegler, ). On the other hand, it was interestingly discovered that the additional hydrological reinforcing effects (2.7 and 2.8 kPa) as a result of induced matric suctions from softwood tree cover (compared with the hardwood tree and grass covers, respectively) contributed to an increase of 0.35 (49.2%) and 0.33 (45.2%) in F S values from 0.71 under hardwood tree cover and 0.73 under grass cover to a value of 1.06 under softwood tree cover during the wettest period.…”