Measuring Soil Water Content with Ground Penetrating Radar: A Review

termines the diurnal evolution of the atmospheric boundary layer (Callies et al., 1998). Currently, there is a need We present a comprehensive review of methods to measure soil to establish and quantify the contribution of soil water water content with ground penetrating radar (GPR). We distinguish content–regulated land–atmosphere coupling to regional four methodologies: soil water content determined from reflected climate anomalies, such as continental droughts and wave velocity, soil water content determined from ground wave veloclarge-scale precipitation events (Entekhabi et al., 1999). ity, soil water content determined from transmitted wave velocity At the catchment scale, soil water content partly conbetween boreholes, and soil water content determined from the surtrols the separation of precipitation into infiltration, face reflection coefficient. For each of these four methodologies, we evaporation, and runoff and therefore has a large infludiscuss the basic principles, illustrate the quality of the data with field ence on soil erosion and river discharge. Recent studies examples, discuss the possibilities and limitations, and identify areas have shown that including soil water content heterogewhere future research is required. We hope that this review will further neity in spatially distributed hydrological models can stimulate the community to consider ground penetrating radar as one improve discharge predictions (Merz and Plate, 1997; of the possible tools to measure soil water content. Merz and Bardossy, 1998; Pauwels et al., 2001). However, there is no consensus on how to incorporate spatial soil water content heterogeneity in these models. Some W at the land surface is a vital resource for have suggested that it is sufficient to include the statistics both human needs and natural ecosystems. Soci(spatial mean and variance) of measured soil water conety’s fresh water needs for agriculture, sanitation, mutent structure (e.g., Pauwels et al., 2001), whereas others nicipal, and industrial supply are ever increasing. At have suggested that discharge predictions also improve the same time, natural hazards involving water, such as when a full description of soil water content variation floods, droughts and landslides are major natural threats is included (Merz and Bardossy, 1998). Furthermore, to society in many countries (Entekhabi et al., 1999). Merz and Plate (1997) argued that the improvement of The vadose zone, which may be defined as the transition discharge predictions by including soil water content heterogeneity will strongly depend on the event characzone between the atmosphere and groundwater reserteristics, including antecedent soil water conditions and voirs, is important for water resource management, berainfall intensities. cause it regulates the water availability for vegetation, At the field scale, information on the spatial distribuincluding crops, while at the same time provides a protion of soil water is important for precision agriculture tective buffer zone between land surface and groundwaprograms. With too much water, crop quality could deter against solutes and pollutants (Rubin, 2003). crease due to the adverse effects of waterlogged plant Hydrologists, soil scientists, ecologists, meteoroloroots (e.g., reduced root respiration due to depletion of gists, and agronomists all study the space and time variO2 and increased availability of toxic ions under reducability of water in the vadose zone, hereafter referred ing soil conditions). With too little water, crops can be to as soil water content, over a range of scales and for irreversibly damaged due to drought stress. In addition a variety of reasons. At the regional to continental scale, to the impact of water content on the quality and quanthe exchange of moisture and energy between soil, vegetity of crops, the outlay of resources and energy concomtation, and the atmosphere has an impact on near-suritant with crop irrigation is critical in water-scarce reface atmospheric moisture and temperature, which in gions, especially as competition for water resources between rural and agricultural land users increases. turn define the regional climate. For example, soil water Clearly there is a need for soil water content measurecontent determines to a large extent the relative magniments across a range of spatial scales. High-frequency tudes of sensible and latent heat fluxes and therefore deelectromagnetic techniques are the most promising category of soil water content sensors to fulfill this need J.A. Huisman, Center for Geo-Ecological Research (ICG), Institute because this category contains a range of techniques for Biodiversity and Ecosystem Dynamics (IBED)– Physical Geograthat measure the same soil water content proxy, namely phy, Universiteit van Amsterdam, The Netherlands (currently Dep. dielectric permittivity, at different spatial scales. Reof Landscape Ecology and Resources Management, Justus-Liebigmote sensing with either passive microwave radiometry University Giessen, Heinrich-Buff-Ring 26-32, 35372 Giessen, Geror active radar instruments is the most promising techmany); S.S. Hubbard, Lawrence Berkeley National Laboratory and University of California, Berkeley, CA; J.D. Redman and A.P. Annan, nique for measuring soil water content variations over Sensors and Software Inc., Mississauga, ON, Canada. Received 13 Mar. 2003. Special Section—Advances in Measurement and Monitoring MethAbbreviations: CMP, Common-MidPoint; CPT, cone penetrometer; ods. *Corresponding author ([email protected]). GPR, ground penetrating radar; MOP, multi-offset profile; SWC, soil water content; TDR, time domain reflectometry; VRP, vertical radar Published in Vadose Zone Journal 2:476–491 (2003).  Soil Science Society of America profiling; WARR, Wide Angle Reflection and Refraction; ZOP, zero offset profile. 677 S. Segoe Rd., Madison, WI 53711 USA