Research on the impact of drip emitter flow rate on moisture migration in biochar-amended expansive soil slopes

Current research suggests that excessive surface moisture loss in expansive soil slopes promotes cracks and may trigger slope instability. Appropriate drip irrigation replenishes near-surface water, minimizing or delaying crack formation. The objective of this study was to clarify the moisture migration mechanisms under a newly designed drip-irrigation scheme, to guide system optimization and water-use efficiency. Three slope-model tests were conducted with systematically varied emitter flow rates. Furthermore, the influence of biochar on moisture migration was further analysed. The test results demonstrate that drip emitter flow rate significantly affects soil moisture migration. Specifically, higher drip emitter flow rate lengthens the response time of volumetric water content at the same sensor location. At the same drip irrigation time, the wetting front migration distance increases with drip emitter flow rate. The horizontal-to-vertical infiltration ratio of the wetting front decreases with drip emitter flow rate. During redistribution, the wetting front displacement shows negative growth in the downslope horizontal direction, with its magnitude increasing as drip emitter flow rate increase, while other directions show positive growth. The drip emitter flow rate should not be excessively high or low; otherwise, it reduces the uniformity of soil moisture movement. This study identified 0.4 L/h as the optimal drip emitter flow rate. At 0.4 L/h, wetting-front migration in all directions decelerates with infiltration time. The profile shape of the wetted body and the slope-surface footprint are approximately elliptical and an elliptical function can therefore estimate wetted volume, thereby providing a theoretical foundation for slope drip-irrigation design.