Variation in sprinkler irrigation droplet impact angle on the physical crusting properties of soils

The angle at which a droplet impacts determines its shearing and compaction effects on soil, which in turn affects the formation and properties of the physical soil crust. To investigate the effect of the droplet impact angle on soil physical crusting, this study considered droplet quantity, quality, and velocity and proposes an energy-weighted method for calculation of the equivalent droplet impact angle. Droplet information was acquired using a spraying platform with a controllable droplet impact angle and two-dimensional video disdrometer (2DVD). Six groups (38.10°, 49.43°, 58.77°, 68.88°, 76.81° and 84.10°) of droplet impact angles were established to obtain the parameters of the soil physical crust characteristics at different droplet impact angles by indoor soil tank tests. The results show that: 1) the stable values of crack length density (CLD), crust thickness (Ct), soil bulk density (BD), cohesion force (Cf), and internal friction angle (IfA) are strongly positively correlated with droplet impact angle; 2) the stable value of the crack area ratio (CAR) is strongly negatively correlated with the droplet impact angle, with maximum and minimum values of 6.94% and 5.57%, respectively; 3) the content of macro aggregates (MAA) tends to increase and then decrease with the increase in droplet impact angle, and the content of MAA has a maximum value of 50.2% at the impact angle of 68.88°. The content of micro aggregates (MIA) tends to decrease and then increase with the increase in the drop impact angle, with a minimum value of MIA of 20.4% at the impact angle of 58.77°; 4) the maximum soil bulk density of 1.99 g·cm−3 at an impact angle of 84.10° is significantly greater than the initial soil bulk density (1.35 g·cm−3); 5) the energy-weighted equivalent droplet impact angle takes into account the effects of both large and small droplets and characterises the overall impact angle of the sprayed droplet from an energy perspective. The results of this study provide new ideas for determining suitable sprinkler irrigation parameters to reduce soil erosion and promote the sustainable development of irrigated agriculture.