Estimating the dynamics of the groundwater in vegetated slopes based on the monitoring of streams

Abstract

Rainfall is commonly considered to be responsible for landslides, but strictly, it is the groundwater recharged by rainfall that triggers slope failures. The dynamics of the groundwater in slopes is meaningful for assessing landslide hazard. Groundwater monitoring methods through boreholes or wells are effective but invasive, slow and expensive. In addition, these approaches are unsuitable for vast alpine environment prone to frequent landslides. To find an easy and inexpensive approach of estimating the dynamics of the groundwater in vegetated slopes, the response behavior of the stream flow to rainfall in the Touzhai valley (Southwestern China) is investigated based on in-situ monitoring. Combined with Complete Water Quality Analysis (CWQA) and a range of experiments (leaching test, in-situ infiltration and dye tracer infiltration), the source of the stream and its response mechanism to rainfall are determined. Some results are obtained: (1) the monitored stream flow fluctuates sensitively with the variation of rainfall without obvious hysteresis whether it is in the high- or low-water period; (2) the CWQA suggests that the stream water is derived from the slope groundwater rather than the direct rainfall or overland flow; (3) what leads to the stream flow rapid response is the macropore system (comprising of root–soil interstices, rotten root channels, inter-aggregate pores, worm pores and earth–gravel interstices) developed in the unsaturated zone of the slopes, which offers preferential conduits for rainfall infiltration. Therefore, monitoring streamflow rates can be an inexpensive and practical method to estimate the dynamics of groundwater in vegetated slopes, which, ulteriorly, provides the basis for diagnosing the stability of remote headwater slopes subject to failures. More than that, the alternative monitoring sites could be placed in inhabited valley bottoms or alluvial fans to avoid the absence of electric power and communication in uninhabited headwater catchments.