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Payments for watershed management link upstream inhabitants whose actions affect water resources with downstream water users. This paper evaluates the effect of plausible shifts in watershed land use on hydrologic services on the Kona coast of Hawai'i Island by measuring vegetation effects on hydrologic fluxes, modeling land-use change impact on the water-supply aquifer, and evaluating the local water department's associated pumping expenses. Transitions between native and plantation forest will have a 25-40% greater impact on the aquifer than the transition from pasture to either forest type. However, for all transitions, the value to the water department is just 2-5% of the opportunity cost to landowners. To provide context for these findings, the effects of these land transitions on carbon storage, provision of native bird habitat, and land stewardship are assessed. Estimates show that delivery of other services does not always increase when water services increase and suggest that the value of impacts to these other services are greater than effects on water. In Kona, as in any watershed payment project, the particular physical and social context determines which services are valuable. [ABSTRACT FROM AUTHOR]

Modeling of hydrologic processes using multiphysics modeling packages shows significant promise in a number of applications. However, these packages have not yet developed a complete set of implementations for boundary conditions important in hydrologic modeling. Three such boundary conditions—rainfall infiltration, seepage faces, and evapotranspiration fluxes—are implemented using the generic boundary condition and internal sink routines provided in one multiphysics package, COMSOL multiphysics. Comparison with results from previous simulations using dedicated hydrologic models demonstrates that with care and creativity these boundary conditions can be implemented accurately and efficiently. Boundary condition implementation should not limit the applicability of multiphysics models to a broad set of problems of interest to the hydrologic community. [ABSTRACT FROM AUTHOR]

Soils are often layered at scales smaller than the block size used in numerical and conceptual models of variably saturated flow. Consequently, the small-scale variability in water content within each block must be homogenized (upscaled). Laboratory results have shown that a linear volume average (LVA) of water content at a uniform suction is a good approximation to measured water contents in heterogeneous cores. Here, we upscale water contents using van Genuchten's function for both the local and upscaled soil-water-retention characteristics. The van Genuchten (vG) function compares favorably with LVA results, laboratory experiments under hydrostatic conditions in 3-cm cores, and numerical simulations of large-scale gravity drainage. Our method yields upscaled vG parameter values by fitting the vG curve to the LVA of water contents at various suction values. In practice, it is more efficient to compute direct averages of the local vG parameter values. Nonlinear power averages quantify a feasible range of values for each upscaled vG shape parameter; upscaled values of N are consistently less than the harmonic means, reflecting broad pore-size distributions of the upscaled soils. The vG function is useful for modeling soil-water retention at large scales, and these results provide guidance for its application. [ABSTRACT FROM AUTHOR]