Wind speed above and within sunflower stalks varying in height and population

ABSTRACT: Concerns about wind erosion during the summer fallow period following sunflower harvest arise from the assumption that low residue amounts following harvest provide inadequate protection for the soil surface. The objectives of this study were to: 1) measure the effects of standing sunflower silhouette area index (stalk height × diameter × population) on wind speed within and above standing sunflower stalks, and 2) compare the resultant changes in friction velocity and erosion ratio for residue-covered compared with bare soil surfaces. Sunflower (He-lianthus annuus L.) stalks were either laid flat after harvest or left standing at one of two heights. Stalk densities were variable, from approximately 26,250 to 64,580 stalks/ha (10,620 to 26,140 stalks/a). Wind speed profiles within and above the standing stalks were measured with cup anemometers. Friction velocity increased linearly with wind speed and increased quadrati-cally with the silhouette area index. Increasing the silhouette area index increased the critical friction velocity ratio in a manner similar to previously reported wind tunnel results. Predicted reductions in the wind erosion ratio based on these field-measured wind speeds are similar to values predicted from wind tunnel studies. Standing sunflower stalks should reduce the erosion potential to 0 to 12% of that predicted for bare soils.