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.
Citation
ID:
209663
Ref Key:
sidle2017hydrologythe