Furrow Flow Velocity Effect on Hydraulic Roughness

The hydraulic roughness of irrigation furrows is generally assumed to be uniform throughout a field. However, water flow, which varies along furrows, reduces roughness by breaking down soil aggregates and depositing sediments on the bed. A wide range of flow rates was applied with a recirculating infiltrometer to furrow sections constructed on a wide range of slopes to determine the effect of flow depth and velocity on hydraulic resistance. Although no depth effect was measured, final (steady-state) roughness varied inversely with average flow velocity. Since furrow flow velocity varies with flow rate and slope, roughness will also vary spatially with these factors. Since roughness affects wetted perimeter and thus infiltration and surface storage and, thus stream advance and recession, these interactions influence water distribution along irrigation furrows. Including this interaction in hydraulic models will improve their ability to describe the physical system.