MEMBER

S OIL erosion and other exchange processes involving heat and mass transfer are determined by wind distri-bution in and above roughness elements at the surface. Various structures of mean velocity and turbulence will oc-cur, depending on the characteristics of the roughness elements. Even if a sur-face has only one type of roughness ele-ment, the flow will depend not only on form and height, but also on the num-ber of roughness elements per unit area and the manner in which they are distri-buted. A knowledge of the effect of roughness on the flow field may suggest the best spacing of crops for erosion control, Chepil and Woodruff (1963) defined a critical surface-barrier ratio (CSBR) as the distance between nonerodible sur-face barriers, Lx, divided by the height of the barrier, H, that will prevent wind from moving the erodible particles. Ear-lier, Chepil (1950) called the reciprocal of this ratio a critical surface-roughness constant. On cultivated soil, L,/H has a value of 4 to 20, depending on the friction velocity of the wind and on the.thresh-old friction velocity of the erodible soil particles (Chepil 1958). However, the ratio reportedly remains constant for any proportion and size of nonerodible fractions present in the soil. Here we report on the effects of vari-ous arrays of cylinders and spheres in controlling sand movement in a wind-tunnel boundary layer. EXPERIMENTAL PROCEDURE The experimental variables are sum-marized in Table 1. The nonerodible cylinders were wood doweling and the Article was submitted for publication in February 1973: reviewed and approved for publication by.the Soil and water Div. o