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Surface flow redistribution on flat ground from crusted bare soil to vegetated patches following intense rainfall events elevates plant available water above that provided by rainfall. The significance of this surface water redistribution to sustaining vegetation in arid and semiarid regions is undisputed. What is disputed is the quantity and spatial distribution of the redistributed water. In ecohydrological models, such nonuniform flows are described using the Saint-Venant equation (SVE) subject to a Manning roughness coefficient closure. To explore these assumptions in the most idealized setting, flume experiments were conducted using rigid cylinders representing rigid vegetation with varying density. Flow was induced along the streamwise x direction by adjusting the free water surface height H(x) between the upstream and downstream boundaries mimicking the nonuniformity encountered in nature. In natural settings, such H(x) variations arise due to contrasts in infiltration capacity and ponded depths during storms. The measured H(x) values in the flume were interpreted using the SVE augmented with progressively elaborate approximations to the roughness representation. The simplest approximation employs a friction factor derived from a drag coefficient (C d ) for isolated cylinders in a locally (but not globally) uniform flow and upscaled using the rod density that was varied across experiments. Comparison between measured and modeled H(x) suggested that such a ''naive'' approach overpredicts H(x). Blockage was then incorporated into the SVE model calculations but resulted in underestimation of H(x). Biases in modeled H(x) suggest that C d must be varying in x beyond what a local or bulk Reynolds number predicts. Inferred C d (x) from the flume experiments exhibited a near-parabolic shape most peaked in the densest canopy cases. The outcome of such C d (x) variations is then summarized in a bulk resistance formulation that may be beneficial to modeling runon-runoff processes on shallow slopes using SVE.

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Seagrass meadows can retain fine particles, improving water clarity and promoting carbon sequestration. Laboratory experiments were conducted to investigate the influence of velocity and meadow density on the retention of fine particles within a meadow. Vertical profiles of velocity and turbulent kinetic energy (TKE) were measured along a model meadow. The net deposition was measured using microscope slides positioned inside and outside the meadow. The deposition was correlated with the evolution of velocity along the meadow. At the leading edge, the net deposition decreased over a distance L r, relative to the bare bed, which was associated with a region of vertical updraft and elevated TKE. Net deposition increased with the distance from the leading edge, associated with a decrease in vertical velocity and TKE. In some cases, a distinct peak in the deposition was observed at distance L p from the leading edge, associated with a minimum in TKE. Both L r and L p decreased with increasing meadow density. Deposition in the fully developed region of the meadow decreased with decreasing stem density and increasing channel velocity, and for the lowest stem density and highest channel velocity the deposition in the meadow was less than that in the bare channel. Diminished deposition was linked to resuspension driven by stem-generated turbulence. A model for canopy-averaged TKE was validated and used to explore the range of field conditions for which TKE within a meadow would be reduced, relative to the bare bed, which would support the accumulation of fine organic material within the meadow.

No abstract