[1] There is increasing interest in the use of multiple measurement types, including indirect (geophysical) methods, to constrain hydrologic interpretations. To date, most examples integrating geophysical measurements in hydrology have followed a three-step, uncoupled inverse approach. This approach begins with independent geophysical inversion to infer the spatial and/or temporal distribution of a geophysical property (e.g., electrical conductivity). The geophysical property is then converted to a hydrologic property (e.g., water content) through a petrophysical relation. The inferred hydrologic property is then used either independently or together with direct hydrologic observations to constrain a hydrologic inversion. We present an alternative approach, coupled inversion, which relies on direct coupling of hydrologic models and geophysical models during inversion. We compare the abilities of coupled and uncoupled inversion using a synthetic example where surface-based electrical conductivity surveys are used to monitor onedimensional infiltration and redistribution. Through this illustrative example, we show that the coupled approach can provide significant reductions in uncertainty for hydrologic properties and associated predictions if the underlying model is a faithful representation of the hydrologic processes. However, if the hydrologic model exhibits structural errors, the coupled inversion may not improve the hydrologic interpretation. Despite this limitation, our results support the use of coupled hydrogeophysical inversion both for the direct benefits of reduced errors during inversion and because of the secondary benefits that accrue because of the extensive communication and sharing of data necessary to produce a coupled model, which will likely lead to more thoughtful use of geophysical data in hydrologic studies.

[1] Infiltration per unit area of the source region from discs, strips and furrows has previously been shown to be the sum of the one-dimensional infiltration and an edge effect term. Here we apply the same approach to examine infiltration under a constant head from boreholes (both lined and unlined). A critical empirical parameter (g) in the edge effect term is related to the radius of the borehole, soil hydraulic properties, boundary and initial conditions. For lined boreholes, g has a narrow range and for the examples investigated, a constant value of 1.06 introduces less than 5% error compared to using the case-specific g value. For unlined boreholes, g is larger, ranging between 1.02 and 3.16 for the examples investigated, and should be estimated for specific conditions.

There is increasing interest in the unique measurement capabilities of nuclear magnetic resonance (NMR) for hydrologic applications. In particular, the ability to quantify water content (both bound and free) and to infer the permeability distribution are critical to hydrologists. As the method has gained in acceptance, there has been growing interest in extending its range to near‐surface and vadose zone applications and to measurement in finer grained and magnetic soils. All of these applications require improved resolution of early‐time signals, which requires shorter measurement dead times. This paper analyses three physical/electrical processes that limit the minimum achievable measurement dead times in surface NMR applications: 1) inherent characteristics of electromechanical and semiconductor switching devices, 2) the effective bandwidth of the receiver and signal processing chain, 3) transient signals associated with induced eddy currents in the ground. We then describe two applications of surface NMR that rely on reduced measurement dead time: detection and characterization of fast decaying NMR signals in silt and clay and the detection of fast decaying NMR signals in magnetic geology.

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