Time‐lapse borehole radar for monitoring rainfall infiltration through podosol horizons in a sandy vadose zone

Abstract

The shallow aquifer on the Gnangara Mound, north of Perth, Western Australia, is recharged by winter rainfall. Water infiltrates through a sandy Podosol where cemented accumulation (B-) horizons are common. They are water retentive and may impede recharge. To observe wetting fronts and the influence of soil horizons on unsaturated flow, we deployed time-lapse borehole radar techniques sensitive to soil moisture variations during an annual recharge cycle. Zero-offset crosswell profiling (ZOP) and vertical radar profiling (VRP) measurements were performed at six sites on a monthly basis before, during, and after annual rainfall in 2011. Water content profiles are derived from ZOP logs acquired in closely spaced wells. Sites with small separation between wells present potential repeatability and accuracy difficulties. Such problems could be lessened by (i) ZOP saturated zone velocity matching of time-lapse curves, and (ii) matching of ZOP and VRP results. The moisture contents for the baseline condition and subsequent observations are computed using the Topp relationship. Time-lapse moisture curves reveal characteristic vadose zone infiltration regimes. Examples are (I) full recharge potential after 200 mm rainfall, (II) delayed wetting and impeded recharge, and (III) no recharge below 7 m depth. Seasonal infiltration trends derived from long-term time-lapse neutron logging at several sites are shown to be comparable with infiltration trends recovered from time-lapse crosswell radar measurements. However, radar measurements sample a larger volume of earth while being safer to deploy than the neutron method which employs a radioactive source. For the regime (III) site, where time-lapse radar indicates no net recharge or zero flux to the water table, a simple water balance provides an evapotranspiration value of 620 mm for the study period. This value compares favorably to previous studies at similar test sites in the region. Our six field examples demonstrate application of time-lapse borehole radar for characterizing rainfall infiltration.

Rapid Estimation of Volumetric Groundwater Recharge in the Vadose Zone via Ground Penetrating Radar

“…Local infiltration studies from Banksia woodland vegetation in the Gnangara mound suggest a value of 194 mm/year for approximately 800 mm of rainfall, i.e. 24% (Sharma, Farrington, & Fernie, 1983), which agree with geophysical infiltration studies by E. Strobach et al (2014).…”

“…This is considerably lower than both the historical average since 1993 (when these records began), and the preceding two years. E. Strobach, Harris, Dupuis, and Kepic (2014) suggests rainfall at several location in the Bassendean Sand of the Gnangara mound aquifer system may infiltrate at rates of close to 2 metres per 3 month period. It's possible that infiltration rates through the sands at the city beach site could be similar or faster than of the Bassendean Sands.…”

Monitoring Soil Water Content Using Time‐Lapse Horizontal Borehole GPR Data at the Field‐Plot Scale

“…For example, Looms et al (2008b) investigated unsaturated flow and transport processes using crosshole GPR and ERT in alluvial sandy sediments and estimated hydraulic patterns (Looms et al, 2008a). Strobach et al (2014) combined surface and borehole GPR measurements to monitor rainfall infiltration in the vadose zone and to investigate characteristic infiltration regimes. Allroggen et al (2015) measured and monitored flow processes in the near subsurface using three-dimensional surface GPR measurements during rainfall experiments and associated patterns of travel-time changes in GPR signals with SWC variations.…”

“…For example, Looms et al (2008b) investigated unsaturated flow and transport processes using crosshole GPR and ERT in alluvial sandy sediments and estimated hydraulic patterns (Looms et al, 2008a). Strobach et al (2014) combined surface and borehole GPR measurements to monitor rainfall infiltration in the vadose zone and to investigate characteristic infiltration regimes.…”

Influence of Land Use Change on Hydrological Cycle: Application of SWAT to Su-Mi-Huai Area in Beijing, China

“…The coefficient mainly depends on the lithology, structure and thickness of the vadose zone as well as the land surface slope [7][8][9]. The natural amplitude of phreatic water level, lysimeter observation, isotope analysis, equilibrium method, chloride tracer and numerical simulation are usually used to calculate α [10][11][12][13][14]. With the rapid economic and social development in recent years, a variety of human activities (such as the acceleration of urbanization process) have contributed to significant changes in land use and land cover, which directly affect the underlying surface of the basin [15], and then changes the rainfall infiltration process.…”