Abstract:Aquatic macrophytes are often the dominant factor influencing flow conditions within the channels they occupy. Existing knowledge of how stream plants affect the flow is outlined, and the different scales at which vegetation resistance operates are proposed. Resistance is shown to be a function of the size of the plants, their structural properties, location in the channel, and the local flow conditions. Current models to calculate this composite resistance effect are assessed in the light of theoretical considerations of the nature of vegetation resistance. New theory is also presented, which demonstrates the non-linear relationship between channel resistance and the proportion of the channel occupied by vegetation.

The proportion of a river channel containing vegetation (termed the blockage factor) has historically been determined in three ways: cross-sectional, surface area and volumetric. The first two versions are two-dimensional measures. Meanwhile, the threedimensional volumetric version is biased towards deeper sections of a reach. A fourth version of the blockage factor is proposed that does not have such limitations: the multi-cross-sectional blockage factor. Between five and nine cross-sections were sampled to determine the four blockage-factor versions for 35 river sites containing the clumped submergent macrophyte, Ranunculus subgenus Batrachium (water-crowfoot). The ability of these four measures to act as predictors of vegetative channel resistance was then assessed.The vegetated proportion of individual cross-sections was poorly related to the vegetation resistance of a channel reach, primarily due to the high spatial variability of patch-forming macrophytes. The weighted median of all cross-sectional blockage factors measured at each site produced the strongest relationship with vegetation resistance, though this was not significantly better than the volumetric or surface area versions. The resistance model using the surface-area blockage factor gave a very high residual between predicted and calculated resistance for a mat-forming macrophyte species, and this model is unlikely to hold for conditions other than baseflow. Likewise, the volumetric version is not expected to hold for sites that have more depth variability than those measured in this study.

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Flow velocity and turbulence patterns were measured in and around a common lotic macrophyte, Ranunculus penicillatus subspecies pseudofluitans (stream water-crowfoot), using a two-dimensional electromagnetic current meter (EMCM). Due to the high shooting density of this species, there was a sharp velocity gradient at the plant boundary, with velocities dropping to a constantly low value after no more than 5 cm into the plant, thus forcing most of the flow over and around the macrophyte. There was a deadwater zone immediately downstream of the plant, beyond which the current moved in from the sides to allow flow under the trailing shoots. High turbulence intensities were recorded for both downstream and cross-stream velocity components at the lateral margins and downstream of the plant. Meanwhile, pulses of water upstream of the plant produced turbulence in the downstream component, but not in the crossstream component.

[1] The U.K. Government's 1995 White Paper on Rural England [Her Majesty's Stationery Office, 1995] proposed a doubling of the area of woodland within England by the year 2045. Questions were later raised concerning the possible impacts on water resources of such a large change in land use. This paper presents results of field study investigations of the water use of grass, heath, oak, and pine vegetation at Clipstone Forest, Nottinghamshire, United Kingdom, which were used to calibrate the water use model HYLUC and derive predictions of the impact of different vegetation types on recharge in this locality. Average annual recharge plus runoff (millimeters) over a 32.5 year period and the uncertainties due to spatial sampling, calculated with the HYLUC model, were 136 ± 11 for the grass site, 122 ± 3 for the heath site, 76 ± 5 for the oak site, and 34 ± 8 and 38 ± 3 for two pine sites. In this region of Britain the long-term recharge beneath pine is approximately one quarter that under grass and essentially only occurs in years of above average rainfall. Oak woodland is also predicted to have a significant impact by reducing recharge plus runoff by almost one half as compared with grassland.