Vegetation Controls on the Spatio-Temporal Heterogeneity of Deep Moisture in the Unsaturated Zone: A Hydrogeophysical Evaluation

Abstract

Information on the spatio-temporal variability of soil moisture in the vadose zone is important to assess groundwater recharge and solute transport in unconsolidated substrate as influenced by biological processes. Time-lapse electrical resistivity imaging (ERI) was used to monitor soil moisture dynamics to a depth of 9 m in a grassland, a grassland encroached by a juniper species (eastern redcedar, Juniperus virginiana), a juniper woodland and an oak forest in the south-central Great Plains, Oklahoma, USA. A site-specific relationship between moisture content and electrical conductivity data was developed for the soil zone, and a perched water zone was monitored at two of the sites. Results showed that (a) change in soil moisture content was linearly correlated to change in electric conductivity in the soil zone; (b) vegetation cover type induced differences in vertical bulk electrical resistivity (ER) profiles and influenced the temporal evolution of soil moisture profiles; and (c) juniper encroachment lowered the water level in the perched groundwater aquifer. Our results suggest land use and vegetation cover type, as opposed to rock properties, controls deep water drainage for the vegetation transition zone. Methods used to measure hydrogeophysical changes, such as ERI, can be used for broader understanding of geological, physical, and biological processes and their links in Earth's critical zones.Understanding the existence and magnitude of deep drainage and water flowpaths in the vadose zone under contrasting vegetation types is critical to manage groundwater quantity and quality. Soil water content can be determined by using moisture probes or dielectric sensors, cosmic ray neutrons, gamma ray attenuation, distributed temperature sensing, and/or by using remote sensing methods 1 . However, these methods fail to provide adequate information on deep water content due to limits of coring depth 2 and calibration techniques 1 , especially where competent rock is shallow. Electrical resistivity imaging (ERI), a non-intrusive technique, has been used since the 1830's 3 to characterize and monitor water distribution, contaminant plumes, contaminations and remediation, fluid transport, groundwater flow and reactions, subsurface heterogeneity and anisotropy, to map soil texture and to monitor geo-hazards 4-8 , but it's use in monitoring vadose zone soil moisture dynamics and groundwater recharge is still limited 2, 9, 10 .Electrical resistivity imaging is a geophysical technique which uses surface electrodes for measurement and acquisition of apparent resistivity data 7,9,11,12 . Electrical resistivity (ER) data are influenced by soil particle size, form and distribution of voids, soil water content, fluid properties, and temperature 13,14 . Temporal changes in resistivity are derived by collecting apparent resistivity from same location at different time intervals 13 . The apparent resistivity data are converted to true resistivity using an inversion model and developed into two-dimensional and three-dimensional i...

Woody Plant Encroachment Impacts on Groundwater Recharge: A Review

“…Jobbagy and Jackson [51] reported that a Eucalyptus plantation reduced groundwater recharge, and decreased water table by 38 cm compared to adjacent grassland during a two-year period in the Argentine Pampas. Acharya et al [30] monitored water levels of perched aquifers under grassland and grassland encroached by Juniperus virginiana (Eastern redcedar) in the South-Central Great Plains, USA. They found that water level was higher under grassland than under Eastern redcedar woodland; this observation indicates that woody plants are likely to decrease water table in a perched aquifer.…”

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