also use soil dielectric properties to determine . These alternative sensors have received relatively little inde-Widespread interest in soil water content (, m 3 m Ϫ3 ) information pendent study; and critical practical issues related to for both management and research has led to the development of a calibration methodology and application have not been variety of soil water content sensors. In most cases, critical issues addressed. related to sensor calibration and accuracy have received little independent study. We investigated the performance of the Hydra Probe soil The Hydra Probe is an example of the alternative water sensor with the following objectives: (i) quantify the intersensors now available. 1 It is currently in widespread use sensor variability, (ii) evaluate the applicability of data from two (e.g., the Soil Climate Analysis Network of the Natural commonly used calibration methods, and (iii) develop and test two Resource Conservation Service) and has proven to be multi-soil calibration equations, one general, "default" calibration robust under a variety of field conditions. Previous reequation and a second calibration that incorporates the effects of soil search demonstrated that Hydra Probe measurements properties. The largest deviation in the real component of the relative are precise and accurate in fluids with known dielectric dielectric permittivity (ε r ) determined with the Hydra Probe using 30 properties and highly correlated with in soils, indicatsensors in ethanol corresponded to a water content deviation of about ing the potential of the instrument for quantitative mea-0.012 m 3 m Ϫ3 , indicating that a single calibration could be generally surement (Seyfried and Murdock, 2004). It was also applied. In layered (wet and dry) media, ε r determined with the Hydra Probe was different from that in uniform media with the same water found that the calibration relationship varied considercontent. In uniform media, was a linear function of ͌ε r . We used ably among soils and that the manufacturer-supplied this functional relationship to describe individual soil calibrations and calibrations were not accurate for some soils. Important the multi-soil calibrations. Individual soil calibrations varied indepenpractical considerations regarding the use of the Hydra dently of clay content but were correlated with dielectric loss. When Probe remain. These include: (i) the degree of variation applied to the 19-soil test data set, the general calibration outperin response among different sensors (i.e., the inter-senformed manufacturer-supplied calibrations. The average difference, sor variability), which determines if sensor specific calievaluated between ε r ϭ 4 and ε r ϭ 36, was 0.019 m 3 m Ϫ3 for the general brations are required, (ii) the optimal experimental equation and 0.013 m 3 m Ϫ3 for the loss-corrected equation.83712;

Abstract. The time and geographic sources of streamwater in low-relief watersheds are poorly understood. This is partly due to the difficult combination of low runoff coefficients and often damped streamwater isotopic signals precluding traditional hydrograph separation and convolution integral approaches. Here we present a dual-isotope approach involving 18 O and 2 H of water in a low-angle forested watershed to determine streamwater source components and then build a conceptual model of streamflow generation. We focus on three headwater lowland sub-catchments draining the Savannah River Site in South Carolina, USA. Our results for a 3-year sampling period show that the slopes of the meteoric water lines/evaporation water lines (MWLs/EWLs) of the catchment water sources can be used to extract information on runoff sources in ways not considered before. Our dualisotope approach was able to identify unique hillslope, riparian and deep groundwater, and streamflow compositions. The streams showed strong evaporative enrichment compared to the local meteoric water line (δ 2 H = 7.15 · δ 18 O +9.28 ‰) with slopes of 2.52, 2.84, and 2.86. Based on the unique and unambiguous slopes of the EWLs of the different water cycle components and the isotopic time series of the individual components, we were able to show how the riparian zone controls baseflow in this system and how the riparian zone "resets" the stable isotope composition of the observed streams in our low-angle, forested watersheds. Although this approach is limited in terms of quantifying mixing percentages between different end-members, our dual-isotope approach enabled the extraction of hydrologically useful information in a region with little change in individual isotope time series.

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Interactions among hydraulic conductivity distributions, subsurface topography, and lateral flow are poorly understood. We applied 407 mm of water and a suite of tracers over 51 h to a 12 by 16.5 m forested hillslope segment to determine interflow thresholds, preferential pathway pore velocities, largescale conductivities, the time series of event water fractions, and the fate of dissolved nutrients. The 12% hillslope featured loamy sand A and E horizons overlying a sandy clay loam Bt at 1.25 m average depth. Interflow measured from two drains within an interception trench commenced after 131 and 208 mm of irrigation. Cumulative interflow equaled 49% of applied water. Conservative tracer differences between the collection drains indicated differences in flow paths and storages within the plot. Event water fractions rose steadily throughout irrigation, peaking at 50% sixteen h after irrigation ceased. Data implied that tightly held water exchanged with event water throughout the experiment and a substantial portion of preevent water was released from the argillic layer. Surface-applied dye tracers bypassed the matrix, with peak concentrations measured shortly after flow commencement, indicating preferential network conductivities of 864-2240 mm/h, yet no macropore flow was observed. Near steady-state flow conditions indicated average conductivities of 460 mm/h and 2.5 mm/h for topsoils and the Bt horizon, respectively. Low ammonium and phosphorus concentrations in the interflow suggested rapid uptake or sorption, while higher nitrate concentrations suggested more conservative transport. These results reveal how hydraulic conductivity variation and subsurface topographic complexity explain otherwise paradoxical solute and flow behaviors. Key Points:Interflow thresholds ranged from 131 to 208 mm, and flow paths were state-dependent Event-water fractions increased nearly linearly as irrigation progressed, reaching a maximum of 50% after 407 mm of irrigation Hillslope fluxes integrate spatiotemporal-varying processes controlled by conductivity, soil depth, and subsurface detention distributionsCorrespondence to: (2016), Interactions among hydraulic conductivity distributions, subsurface topography, and transport thresholds revealed by a multitracer hillslope irrigation experiment, Water Resour. Res., 52, 6186-6206,