Analyses of Groundwater Flow and Plant Evapotranspiration in a Vegetated Soil Slope

Understanding seasonal hydrogeological responses of vegetated soil slopes is vital to slope stability because pore-water pressure (PWP) varies from positive values upon rainfall in wet seasons to negative values upon plant evapotranspiration (ET) in dry seasons. There are, however, few case histories that report seasonal performance of vegetated soil slopes. In this study, a vegetated slope situated in Hong Kong was instrumented to analyse (i) groundwater flow during rainfall in the wet season and (ii) effects of plant ET on PWP in the dry season. Twoand three-dimensional anisotropic transient seepage analyses are conducted to identify groundwater flow mechanism(s) during a heavy rainstorm. Through water and energy balance calculations, measured plant-induced suction is interpreted with plant characteristic and climatic data. During the rainstorm, substantial recharge of the groundwater table was recorded, likely due to preferential water flow along relict joints and three-dimensional cross-slope groundwater flow. During the dry season, the peak suction induced by plant ET is up to 200 kPa and the depth of influence is shallower than 200% of the root depth. For the range of suctionsmonitored, root-water uptake is revealed to have been restricted by suction not very significantly and was driven mainly by the climatic variation.

[1]  Harianto Rahardjo,et al.  DETERMINATION OF THE SHEAR STRENGTH PARAMETERS OF AN UNSATURATED SOIL USING THE DIRECT SHEAR TEST , 1988 .

[2]  R. Feddes,et al.  Simulation of field water use and crop yield , 1978 .

[3]  Yeou-Koung Tung,et al.  Three-dimensional numerical investigations of groundwater responses in an unsaturated slope subjected to various rainfall patterns , 2001 .

[4]  Leslie George Tham,et al.  Field-monitored variations of soil moisture and matric suction in a saprolite slope , 2005 .

[5]  D G Fredlund,et al.  Calibration of Thermal Conductivity Sensors for Measuring Soil Suction , 1989 .

[6]  C. Ng,et al.  A field study of stress-dependent soil–water characteristic curves and permeability of a saprolitic slope in Hong Kong , 2011 .

[7]  D. Fredlund,et al.  Effect of rainfall on matric suctions in a residual soil slope , 1996 .

[8]  William Powrie,et al.  Factors controlling the seasonal variation in soil water content and pore water pressures within a lightly vegetated clay slope , 2012 .

[9]  Leslie George Tham,et al.  Field monitoring of rainfall infiltration in a loess slope and analysis of failure mechanism of rainfall-induced landslides , 2009 .

[10]  C. Ng,et al.  A Novel Root System for Simulating Transpiration-induced Soil Suction in Centrifuge , 2014 .

[11]  A. Rosenfeld,et al.  Residential cooling loads and the urban heat island—the effects of albedo , 1988 .

[12]  J. Bear.  Dynamics of Fluids in Porous Media , 1975 .

[13]  J. Godt,et al.  Stability of infinite slopes under transient partially saturated seepage conditions , 2012 .

[14]  C. Ng,et al.  Measurements of drying and wetting permeability functions using a new stress-controllable soil column , 2012 .

[15]  Harianto Rahardjo,et al.  Effect of antecedent rainfall on pore‐water pressure distribution characteristics in residual soil slopes under tropical rainfall , 2008 .

[16]  William Powrie,et al.  Seasonal changes in pore water pressure in a grass covered cut slope in London clay , 2006 .

[17]  Charles Wang Wai Ng,et al.  Performance of an unsaturated expansive soil slope subjected to artificial rainfall infiltration , 2003 .

[18]  K. X. Woon,et al.  Experimental investigation of induced suction distribution in a grass-covered soil , 2013 .

[19]  Rachapudi B. N. Prasad,et al.  A linear root water uptake model , 1988 .

[20]  K. El-Askari,et al.  New software for crop water requirements and irrigation scheduling , 1998 .

[21]  Antonio Gens,et al.  Water permeability, water retention and microstructure of unsaturated compacted Boom clay , 1999 .

[22]  Natasha Pollen-Bankhead,et al.  Hydrologic and hydraulic effects of riparian root networks on streambank stability: is mechanical root-reinforcement the whole story? , 2010 .

[23]  T. T. Lee,et al.  Response of a residual soil slope to rainfall , 2005 .

[24]  B. Huat,et al.  Water infiltration characteristics of unsaturated soil slope and its effect on suction and stability , 2006 .

[25]  C. Ng,et al.  Seasonal movement and groundwater flow mechanism in an unsaturated saprolitic hillslope , 2013, Landslides.

[26]  Malcolm D. Bolton,et al.  Seasonal ratcheting and softening in clay slopes, leading to first-time failure , 2011 .

[27]  Bolton,et al.  Identification of seasonal slope behaviour mechanisms from centrifuge case studies , 2004 .

[28]  Harianto Rahardjo,et al.  PERMEABILITY FUNCTIONS FOR UNSATURATED SOILS , 1997 .

[29]  Charles Wang Wai Ng,et al.  Field monitoring of an unsaturated saprolitic hillslope , 2011 .

[30]  L. S. Pereira,et al.  Crop evapotranspiration : guidelines for computing crop water requirements , 1998 .

[31]  Charles Wang Wai Ng,et al.  Experimental investigations of the soil-water characteristics of a volcanic soil , 2000 .

[32]  D. Fredlund,et al.  Equations for the soil-water characteristic curve , 1994 .

[33]  C. Ng,et al.  COMPARATIVE STUDY OF RAINFALL INFILTRATION INTO A BARE AND A GRASSED UNSATURATED EXPANSIVE SOIL SLOPE , 2007 .

[34]  C. Ng,et al.  A novel technique to model water uptake by plants in geotechnical centrifuge , 2014 .