DARCY-WEISBACH ROUGHNESS COEFFICIENTS FOR SELECTED CROPS

Total hydraulic resistance on an upland agricultural site nay be influ enced by several factors including standing vegetation. In this labora tory study, Darcy-Weisbach roughness coefficients were measured for co rn, cotton, sorghum, soybeans, sunflower, and wheat vegetation. Experi mental variables used in this investigation in addition to crop type i ncluded plant population, row spacing, row orientation, and flow rate. For some of the experimental tests, a single row of vegetation was or iented within a flume parallel to the principal flow direction. For th e remainder of the tests, rows of vegetation were placed perpendicular to the flow using row spacings and plant populations recommended by c rop management specialists. Measurements of discharge rate and flow ve locity were used to calculate roughness coefficients for Reynolds numb er values ranging from approximately 550 to 22,000. Regression equatio ns which relate roughness coefficients to plant population, row spacin g, and Reynolds number were developed from the laboratory data. With t he exception of wheat placed perpendicular to flow, roughness coeffici ents produced by standing vegetation were negligible. On upland agricu ltural areas, total hydraulic roughness will be influenced primarily b y frictional drag over the soil surface, and residue and ground cover.