Though it is well known that vegetation affects the water balance of soils through canopy interception and evapotranspiration, its hydrological contribution to soil hydrology and stability is yet to be fully quantified. To improve understanding of this hydrological process, soil water dynamics have been monitored at three adjacent hillslopes with different vegetation covers (deciduous tree cover, coniferous tree cover, and grass cover), for nine months from December 2014 to September 2015. The monitored soil moisture values were translated into soil matric suction (SMS) values to facilitate the analysis of hillslope stability. Our observations showed significant seasonal variations in SMS for each vegetation cover condition. However, a significant difference between different vegetation covers was only evident during the winter season where the mean SMS under coniferous tree cover (83.6 kPa) was significantly greater than that under grass cover (41 kPa). The hydrological reinforcing contribution due to matric suction was highest for the hillslope with coniferous tree cover, while the hillslope with deciduous tree cover was second and the hillslope with grass cover was third. The greatest contributions for all cover types were during the summer season. During the winter season, the wettest period of the monitoring study, the additional hydrological reinforcing contributions provided by the deciduous tree cover (1.5 to 6.5 kPa) or the grass cover (0.9 to 5.4 kPa) were insufficient to avoid potential slope failure conditions. However, the additional hydrological reinforcing contribution from the coniferous tree cover (5.8 to 10.4 kPa) was sufficient to provide potentially stable hillslope conditions during the winter season. Our study clearly suggests that during the winter season the hydrological effects from both deciduous tree and grass covers are insufficient to promote slope stability, while the hydrological reinforcing effects from the coniferous tree cover are sufficient even during the winter season.
Bibliographical noteFunding Information:
This work was supported financially by University of Tehran (under grant number #3591874). The effort of J.L. Nieber on this project was partially supported by the USDA National Institute of Food and Agriculture, Hatch/Multistate Project 12-059. This work was also supported by the University of Minnesota during the data analysis and processing. The authors would also like to thank Mr Mohsen Gorgandi, Fatollah Ghomi, Rahmat Ghomi, Nour Ali Samarghandi and Ghodrat Daneshvar for their valuable assistance during experimental setup and data collection. We are grateful to Mr MT Tirgarsoltani and Mr Asadollahi for their support in the soil physics lab., and to Marzieh Esmaiili for her help in soil data analysis. Dr Ehsan Ghane's cooperation during statistical data analysis and Dr Diana Karwan's and Dr Andrew Simon's cooperative comments are much appreciated. The authors would like to thank the anonymous reviewers and the associate editor for their valuable comments and suggestions. The authors also declare that no conflicts of interest exist.
- Hyrcanian forest
- apparent soil cohesion
- hillslope hydrology
- soil suction
- soil water content