No abstract

Unsaturated fl ow phenomena, such as unstable we ng fronts and preferen al fl ow, cannot be inves gated using small-scale sampling. Dye tracer experiments can help visualize the dynamics of water fl ow but are destruc ve and therefore irreproducible. We inves gated the applicability of high-resolu on ground penetra ng radar (GPR) for nondestruc ve visualiza on of unsaturated fl ow pa erns arising from a forced infi ltra on experiment. Synthe c studies using a refl ec on GPR two-dimensional fi nite diff erence me domain modeling code indicated that diff erences in water content caused by preferen al fl ow and fi ngering could be resolved. Moisture content contrasts down to approximately 2.5% within the top 2 m were detectable, but with increasing degrees of heterogeneity in the subsurface it becomes diffi cult to dis nguish these moisture content changes. We conducted a fi eld experiment in which 100 mm (900 L) of Brilliant Blue dyed water was infi ltrated across a 3-by 3-m area in rela vely homogenous and undisturbed sandy alluvial sediments. Refl ec on GPR data were collected before and a er infi ltra on. Dye-staining pa erns, revealed by excava ng a 2-m-deep trench through the infi ltra on area, were compared with changes in the GPR data. Refl ec on amplitude changes as well as refl ec on delay revealed significant diff erences within the dye-stained area. The GPR data provided informa on about the unsaturated fl ow below the extent of the dye staining, and the results of the synthe c GPR modeling, as well as the observed changes in the real GPR data set, underline the poten al of refl ec on GPR as a nondestruc ve method to map unsaturated fl ow phenomena.Abbrevia ons: BB, Brilliant Blue; GPR, ground penetra ng radar; TDR, me domain refl ectometry.The quality and quan ty of groundwater resources depend highly on the fl ow and transport properties of the unsaturated zone. Traditionally, unsaturated hydraulic parameters are estimated using retention and hydraulic conductivity experiments performed on small soil samples in the laboratory. Th ese small-scale analyses are not able to describe preferential fl ow paths or unstable wetting fronts that are observed at the fi eld scale. Prediction and characterization of fl ow in the unsaturated zone was found to be diffi cult and highly complex in several studies using dye tracers for visualization of fl ow paths (Flury et al., 1994;Schmalz et al., 2002;Weiler and Flühler, 2004). Large-scale experiments on both homogeneous and heterogeneous soil columns have shown that fi ngering and preferential fl ow exist and dominate the fl ow fi eld for soil types ranging from clayey soils to unstructured sandy sediments (Flury et al., 1994;Wildenschild et al., 1994, Schmalz et al., 2002. Šimůnek et al. (2003) and Kung (1990) furthermore showed that the infi ltration of water seemed to be dependent on the dip of geologic layers and that the dipping layers could also induce fi ngered fl ow.Dye tracing infi ltration experiments are powerful techniques for visualizing the dynamics o...

We conducted a ield experiment at the agricultural ield site Voulund within the Danish hydrological observatory, HOBE, with the purpose of estimating recharge using geophysical methods. In September 2011, a saline tracer was added across a 142-m 2 area at the surface at an application rate mimicking natural iniltration. The movement of the saline tracer front was monitored using cross-borehole electrical resistivity tomography (ERT); data were collected on a daily to weekly basis and continued for 1 yr after tracer application. The ERT data were inverted and corrected for temperature changes in the subsurface, and spatial moment analysis was used to calculate the tracer mass, position of the center of mass, and thereby the downwardly recharging lux. The recovered mass was underestimated by the ERT data by up to 50%. Mass balance errors are widely recognized and are a result of variable resolution of the tomographic models and smoothing applied in the inversion routine. The results were nonetheless in very good agreement with pore water samples collected and analyzed from ive cores extracted within the tracer application area during the same period. Recharge during the 7.5 mo from September 2011 to the end of April 2012 was estimated to be about 500 mm using the ERT data. This value is in good accord with recharge estimates made based on drainage data from buried lysimeters located only meters away from the cross-borehole ERT array. This suggests that long-term automated ERT monitoring of a surface-applied tracer is a promising technique for estimating groundwater recharge.

The vadose zone plays an important role in the hydrologic cycle. Various geophysical methods can determine soil water content variations in time and space in volumes ranging from a few cubic centimeters to several cubic meters. In contrast to the established methods, time‐lapse gravity measurements of changes in soil water content do not rely on a petrophysical relationship between the measured quantity and the water content but give a direct measure of the mass change in the soil. Only recently has the vadose zone been systematically incorporated when ground‐based gravity data are used to infer hydrologic information. In this study, changes in the soil water content gave rise to a measurable signal in a forced infiltration experiment on a 107‐m2 grassland area. Time‐lapse gravity data were able to constrain the van Genuchten soil hydraulic parameters in both a synthetic example and a field experiment with forced infiltration. The most significant reduction in parameter uncertainty was achieved for the saturated water content, while gravity data had some ability to constrain the saturated hydraulic conductivity and the van Genuchten n Cross‐borehole ground penetrating radar data were used to support the interpretation and control of the gravity data.