Hydraulic resistance relationships in irrigation borders

Abstract

Resistance to flow in borders may be due to the roughness of the ground surface or the retardance offered by vegetation. The flow conditions in irrigation borders differ from those in open channels in several ways. A review of the past work has revealed that the Manning n , calculated for uniform flow at a given depth and velocity, applies for all practical purposes, to non-uniform flow in a border strip and will adequately represent the composite value of the hydraulic resistance in a vegetated border strip. The resistance coefficient in non-vegetated borders may be described by the Darcy-Weisbach roughness coefficient f . The hydraulic resistance to flow in vegetated and non-vegetated border strips were evaluated by conducting a series of field experiments on specially prepared test border strips. The plot of n against discharge q on a rectilinear co-ordinate paper indicated a linear relationship between n and q . Statistical analysis of the data revealed that there was no significant difference in hydraulic resistance in 4 post-emergence irrigations. But the hydraulic resistance in the first irrigation was significantly different from other irrigations. This was true even though the absolute roughness offered by the stems and leaves of the plants varied over the irrigation period. Studies with non-vegetated borders showed that the values of the Reynolds number R n were above 5000 in all tests. The average value of f in pre-sowing irrigation was 0·025.