Variations in saturated and unsaturated water movement through an upland floodplain wetland, mid-Wales, UK

Abstract

Summary Variations in inferred water movement are examined during a two-year study of an upland floodplain wetland at Afon Llwyd, in mid-Wales, UK in 2001 and 2002. Soil–water pressures ( ψ ) were monitored by six tensiometer nests, each comprising two sensors at depths of 30 and 60 cm below the surface. Detailed sedimentary sections were produced and changes in saturated and unsaturated hydraulic conductivity (and their relationship to ψ ) were estimated. Seasonal trends in ψ were identified and vertical hydraulic gradients within different tensiometer nests were derived and interpreted in the context of the local channel and floodplain sedimentology, channel bedform morphology and hydro-climatic conditions. Although there is considerable variation in ψ , the results display characteristic trends at several levels: (1) in relationship to precipitation; (2) reflecting changes in river stage (with downstream stage providing base-level control through a riffle-pool couplet); and (3) marked spatial variations in the direction and rate of water movement. These controls influence water redistribution through the wetland, which reflects the relative position of distinct sedimentary units within the floodplain. The results demonstrate that water movement is episodic and characterised by relatively rapid horizontal water fluxes during and immediately following individual rain events, followed by residual seepage to the river and water movement under a local soil moisture gradient. The implications of these results include elucidating the role of upland floodplains in buffering sub-surface drainage, and attenuating patterns of river flow under base flow conditions. This has the potential to advance the basis for future upland floodplain wetland and channel restoration schemes.