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

机译:林冠截留-滑坡引发难题:泰国北部热带次生林的见解

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摘要

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.
机译:有人提出,森林冠层对降雨脉冲的拦截和平滑作用及其向土壤中的传递是缓和土壤幔中峰值孔隙水压力发展的因素,从而降低了浅层滑坡的风险。在这里,我们提供了热带次生龙虾林的三年降雨和降雨数据,该热带次生龙虾林以泰国北部的几棵大树为特征,并提供了选定的土壤水分动态,以解决这一问题。在整个采样期间,估计降雨量是降雨的88%,每年的降雨量在86%至90%之间。来自167个事件的数据表明,冠层截获仅与总事件降雨量(通过非线性关系)弱相关,而与持续时间,平均强度或之前的2天降水量(API2)没有显着相关。小事件(≤35 mm)中的平均拦截率为17%(n = 135个事件),而大事件(> 35 mm; n = 32)仅为7%。在可能引发滑坡的最大强度事件和最高强度事件中,检查较小的时间间隔会发现复杂的拦截模式。在最大的事件期间,热带森林冠层对入射降雨的影响很小或没有。在高风速和/或中高到事件前湿润的事件期间,在大事件高峰期间,测得的穿透量有时高于降雨,表明缓冲作用有限。但是,在湿润少,风速低至中度的事件中,树冠可以缓冲早期的降雨峰值。在大多数大型活动中,由于降雨持续,降雨和穿透深度之间几乎没有差异。将降雨和穿透深度与滑坡起始的平均强度-持续时间阈值进行比较,在降雨超过干湿条件阈值的事件中,有75%的穿透率超过阈值。在湿润条件下,在滑坡引发的强度持续时间阈值附近或之上绘制的所有11个最大事件(降雨= 65–116 mm)的穿透强度;其中五个事件接近或高于干燥条件的阈值。在大型降雨条件下,一系列降雨条件下的土壤水分响应在1至2 m的深度处被大量缓冲并逐渐缓冲,这表明峰值降雨输入的短期平滑(即≤1 h)的时间尺度很小或几乎没有。在发生滑坡的深度处对峰值孔隙水压力没有影响。根据这些发现,我们得出结论,在该季风和类似的热带次生林区,在季风降雨条件下,冠层截留对缓解浅层滑坡的启动几乎没有影响。

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