Darcy-Weisbach Roughness Coefficients for Selected Crops

Total hydraulic resistance on an upland agricultural site may be influenced by several factors including standing vegetation. In this laboratory study, Darcy-Weisbach roughness coefficients were measured for corn, cotton, sorghum, soybeans, sunflower, and wheat vegetation. Experimental variables used in this investigation in addition to crop type included plant population, row spacing, row orientation, and flow rate. For some of the experimental tests, a single row of vegetation was oriented within a flume parallel to the principal flow direction. For the remainder of the tests, rows of vegetation were placed perpendicular to the flow using row spacings and plant populations recommended by crop management specialists. Measurements of discharge rate and flow velocity were used to calculate roughness coefficients for Reynolds number values ranging from approximately 530 to 22,000. Regression equations which relate roughness coefficients to plant population, row spacing, and Reynolds number were developed from the laboratory data. With the exception of wheat placed perpendicular to flow, roughness coefficients produced by standing vegetation were negligible. On upland agricultural areas, total hydraulic roughness will be influenced primarily by frictional drag over the soil surface, and residue and ground cover