Scaling Effects of Standing Crop Residues on the Wind Profile

velocities for soil erosion supports management guidelines (Hagen, 1996; Nielsen and Aiken, 1998; McMaster Standing senescent stems increase the aerodynamic roughness of et al., 2000). Effects of standing stems on eddy diffusion the surface, reducing wind energy available for momentum transfer at affect convective transport of heat, water vapor, and the soil surface, such as for wind erosion, and also the soil–atmosphere convective exchanges of heat, water vapor, and trace gases. We contrace gases. Near-surface conductance can regulate soil– ducted studies to determine the predictive accuracy of an algorithm atmosphere exchanges due to strong concentration graderived for plant canopies to scale effects of standing crop residues dients near this interface (Reicosky and Lindstrom, on the wind profile. We used this algorithm to calculate aerodynamic 1993; Nobel, 1983, p. 473). Standing crop residue effects properties (displacement height and roughness length) of standing on the wind profile alter threshold velocities for wind crop residues related to the log wind profile equation. We also calcuerosion, the near-surface biological environment, and lated apparent roughness length from wind profiles measured under soil–atmosphere exchange of heat, water vapor, and neutral stability conditions over stems of wheat (Triticum aestivum greenhouse gases. L.), corn (Zea mays L.), millet (Panicum miliaceum L.), and sunStanding stems alter convective exchanges and nearflower (Helianthus annuus L.) using calibrated single-needle and cup surface ( 0.05 m) wind velocities by absorbing kinetic anemometers. A least-squares fit of roughness length calculated by an algorithm derived for crop canopies indicated a systematic, positive energy and modifying aerodynamic roughness. These bias when it was applied to standing stems. After adjusting for bias, effects are readily quantified as a log-linear decrease in calculated windspeeds generally were contained in 80% confidence wind velocity relative to distance above the land surface. intervals for observations above and within the crop stubble. PreThe slope of this relationship reflects the friction velocdictive root mean square errors (RMSE) within profiles ranged from ity, while the intercept can be interpreted as the aerody0.6 to 4.6% of reference wind speed. The nonlinear forms of the scaling namic roughness of the surface, or roughness length. algorithms are consistent with theory and wind tunnel observations, Vertical stems tend to raise, or displace, the level of representing an advance over parameterization schemes assuming a near-zero wind velocity while increasing aerodynamic linear relation with residue height. This advance warrants evaluation roughness and altering friction velocity (Pereira and of the adjusted algorithm for simulation of microclimate in the soil– Shaw, 1980). Though displacement height and aerodyresidue–crop canopy regime. Application to momentum transfer problems requires further investigation of drag partitioning. namic roughness are phenomenological coefficients, they tend to scale with crop canopy characteristics including height (Campbell, 1973; Rosenberg et al., 1983) and leaf area (Choudhury and Monteith, 1988). AnaloS crop residues alter wind profiles and wind gous relationships exist between residue architecture velocity near the soil surface. These effects help (horizontal projected stem area) and threshold velociprotect soils from wind erosion by reducing soil water ties required to initiate soil erosion (Hagen, 1996). loss (Van Doren and Allmaras, 1978); absorbing the Our research objective was to derive a modified algoerosive force of wind (Lyles and Allison, 1976); and rithm, which quantifies effects of standing stems on wind shielding the soil from saltating particles (Hagen and profiles above and within sparse canopies and to conArmbrust, 1994). Standing residues also help reduce duct field measurements of wind profile and geometries water erosion by reducing the kinetic impact of rainof standing residues for wheat, corn, millet, and sundrops (Van Doren and Allmaras, 1978). Crop residues flower to validate the modified algorithm. alter the biological environment near the soil surface (Doran et al., 1984). They affect emergence and development of crops and their plant, insect, and microbial THEORY pests by modifying preplant soil warming (Bristow and Standing senescent stems increase the aerodynamic Abrecht, 1989); soil water recharge (Doran et al., 1984; roughness of the subcanopy substrate, reducing wind energy Nielsen, 1998); and the transpiration fraction of total available for momentum transfer at the soil surface (Hagen, evaporation, before canopy closure (Lascano et al., 1996) and also the soil–atmosphere convective exchanges of heat, water vapor, and trace gases (Thom, 1971). This effect 1994). appears to be proportional to silhouette area index (SAI), the Knowledge of impacts of surface crop residue on horizontal projected area of roughness elements per unit of surface-exchange processes can enhance evaluation of land area (Nielsen and Aiken, 1998). Plant geometry provides alternative land-management practices. Quantitative a useful basis for analysis of drag partitioning (Raupach, 1992), knowledge of standing residue effects on threshold wind soil erosion (Raupach et al., 1993; Van de Ven et al., 1989), evaporation (Choudhury and Monteith, 1988; Dolman and Wallace, 1991), and wind velocities within the roughness R.M. Aiken, Northwest Res. Ext. Center, 105 Experiment Farm Road, sublayer (Pereira and Shaw, 1980). Standing stems may differ Kansas State Univ., Colby, KS 67701; D.C. Nielsen, USDA-ARS, from growing plants in the relative significance of skin friction Central Great Plains Research Station; and L.R. Ahuja, USDA-ARS, Great Plains Systems Research. Received 3 June 2002. *Corresponding author ([email protected]). Abbreviations: LAI, leaf area index; RMSE, root mean square errors; SAI, silhouette area index. Published in Agron. J. 95:1041–1046 (2003).