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

An experimental campaign was set up to quantify the contribution of evapotranspiration fluxes on hillslope hydrology and stability for different forest vegetation cover types. Three adjacent hillslopes, respectively, covered by hardwood, softwood, and grass were instrumented with nine access tubes each to monitor soil water dynamics at the three depths of 30, 60, and 100cm, using a PR2/6 profile probe (Delta-T Devices Ltd) for about 6months including wet periods. Soil was drier under softwood and wetter under grass at all the three depths during most of the monitoring period. Matric suction derived via the soil moisture measurements was more responsive to changes in the atmospheric conditions and also recovered faster at the 30cm depth. Results showed no significant differences between mean matric suction under hardwood (101.6kPa) with that under either softwood or grass cover. However, a significant difference was found between mean matric suction under softwood (137.5kPa) and grass (84.3kPa). Results revealed that, during the wettest period, the hydrological effects from all three vegetation covers were substantial at the 30cm depth, whereas the contribution from grass cover at 60cm (2.0kPa) and 100cm (1.1kPa) depths and from hardwood trees at 100cm depth (1.2kPa) was negligible. It is surmised that potential instability would have occurred at these larger depths along hillslopes where shallow hillslope failures are most likely to occur in the region. The hydrological effects from softwood trees, 8.1 and 3.9kPa, were significant as the corresponding factor of safety values showed stable conditions at both depths of 60 and 100cm, respectively. Therefore, the considerable hydrological reinforcing effects from softwood trees to the 100cm depth suggest that a hillslope stability analysis would show that hillslopes with softwood trees will be stable even during the wet season.