Effect of vegetation and slope orientation on water infiltration in a monitored embankment

Slope-mass wasting like shallow slides and surficial erosion are mostly triggered by climatic actions, where rainfall plays the most important role. The number of extreme weather events such as droughts and intense rainfalls has increased in the past decades due to climate change. Consequently, slope-mass wasting has become in recent years one of the most important environmental problems with many socio-economic repercussions. Slope mass-wasting may be the most dangerous geo-hazard in many mountainous regions and represents as well an important threat in artificial or man-made slopes like infrastructure and transportation embankments.

The erosion and stability of slopes depend on the soil-vegetation-atmosphere (SVA) interactions and the thermo-hydro-mechanical soil conditions. Therefore, understanding  the SVA interactions and the processes leading to slope-mass wasting is crucial to promote sustainable, low cost, and environmental-friendly mitigation strategies on potentially unstable slopes, such as the use of vegetation. Moreover, it is necessary for developing a correct land-use planning strategy. In this study, SVA interactions are assessed by a full-scale monitored embankment divided into four partitions with North and South-facing slopes and with bare and vegetated slope covers at each orientation. Monitoring is a fundamental task for understanding the physical mechanisms related to SVA interactions and for calibrating and validating models. The monitoring started in 2017 and includes 60 sensors recording 122 variables every 5 minutes. These devices provide accurate information on the thermo-hydraulic response of bare and vegetated slopes at both North and South orientations. In addition to hydraulic variables like suction and soil moisture, which are measured at several depths, thermal and atmospheric variables are monitored: soil heat flux, soil temperature at different depths, air temperature, rainfall, wind speed/direction, solar radiation, etc. 

The results show that vegetation has a strong effect on both thermal and hydraulic slope response. On one hand, vegetation increases rainfall infiltration and induces a faster saturation of the soil, which may reduce slope stability (this effect should be counterbalanced with other phenomena not considered in this work, like raindrop impact protection and root soil reinforcement, among others). Such an increase in turn suggests that vegetation decreases runoff and hence reduces slope surficial erosion. On the other hand, vegetation increases in-depth suction by plant transpiration, which may increase soil strength and stability on slopes. Regarding thermal aspects, vegetation strongly reduces the incidence of solar net radiation. As a result, soil heat flux, daily temperature fluctuations and evaporation decrease. In addition, this research shows that North-vegetated slopes develop dryer soil conditions when compared to South-bare slopes. This shows that the vegetation transpiration induces higher soil drying rates than the solar radiation effects on a bare surface with full solar exposition (i.e. southward orientation). Therefore, these results recommend the implementation and maintenance of vegetated slopes as a sustainable solution for preventing soil erosion especially in sparse vegetated or bare areas and in present and forthcoming semi-arid regions.