Studies of interactions between wind and saltating particles (i.e., the wind-saltation interaction) are usually conducted without consideration of the downwind air pressure gradient. However, in a wind tunnel with limited size, this gradient is required to maintain the movement of the saltation cloud. Attempts are made to investigate the effects of the downwind air pressure gradient on the wind-saltation interaction in a saltation boundary layer based on the experimental results from a wind tunnel with a relatively small cross-sectional area. The wind-saltation interaction is characterized by airborne stress, grain-borne stress, and the force exerted on the wind by the saltation cloud. Basic equations were developed for wind-saltation interactions without and with a downwind air pressure gradient. The results reveal that unacceptable values of negative grain-borne stress and negative force exerted on the wind by the saltation cloud are obtained if the downwind air pressure gradient is ignored. When this air pressure gradient is defined using the measured wind velocity profiles in the presence of saltation and the downwind air pressure gradient is taken into account, reasonable values for grain-borne stress and the force exerted on the wind by the saltation cloud are obtained.These results suggest that attention must be paid to the effects of downwind air pressure gradients when studying the wind-saltation interaction in a wind tunnel. Consideration of the downwind air pressure gradient, inertial forces, and other unidentified variables will provide a more thorough understanding of the interactions within a saltation boundary layer.

Wind velocity profiles in the presence of saltation were measured in a wind tunnel with a relatively small cross-sectional area and were used to analyze the wind-saltation interaction in a saltation boundary layer. We developed basic equations that describe the wind-saltation interaction with and without a downwind air pressure gradient,and used these equations to calculate the grain-borne stress, force on the wind, and downwind air pressure gradient.Physically unrealistic negative grain-borne stresses and negative forces exerted on the wind by the saltation cloud are obtained if the effect of a downwind air pressure gradient is neglected, but these problems can be avoided if this gradient is taken into account. Our results thus suggest that it is necessary to consider the effect of the downwind air pressure gradient in future wind tunnel studies of the wind-saltation interaction. Similar phenomena may also occur in the field. When a saltating cloud passes over a complex surface, the air pressure gradient, inertial forces,and other unidentified factors may significantly affect the wind velocity profiles, as well as the variation in particle concentration as a function of height, particle velocity, and particle transport rate. Researchers have often observed that the flux profiles differ for the same sediments and wind velocity when the cross-sectional areas of the wind tunnel or the underlying ground surfaces differ. It is obvious that a thorough understanding of the wind-saltation interaction in a saltation boundary layer depends on an improved understanding of the factors that influence this interaction, including the new understanding of the importance of the downwind pressure gradient.