Influence of soil texture and drip emitter flow rate on soil water movement under subsurface drip irrigation

Subsurface drip irrigation (SDI) has emerged as one of the most effective methods to achieving efficient water-saving cultivation in agriculture. Because SDI is a localized irrigation method, the formation of a wetting front in the soil through water applied via drip emitters is crucial for crop water uptake. Selecting appropriate irrigation parameters based on soil texture, emitter discharge, soil moisture content, and other factors can help avoid over- or insufficient-irrigation. Previous studies lacked systematic research on the combined effects of soil texture, emitter flow rate, and emitter depth. Therefore, this study focused on three soil textures: loam, sandy soil, and clay with 30 cm depth of drip emitters, and three flow rates of 0.39 L∙h⁻¹ (low flow rate), 0.90 L∙h⁻¹ (medium flow rate), and 1.38 L∙h⁻¹ (high flow rate), to observe changes in soil wetting front. The results indicated that different soil types exhibit varying responses to changes in moisture content under different drip emitter flow rates. Loam and sandy soils tended to form regular wetting patterns at high flow rate, whereas clay was affected by the water retention effect and exhibited obvious water retention characteristics at low flow rate. The influence of soil textures on water movement also varied. In sandy soil, the horizontal and downward migration distances of the wetting front were the greatest, but the upward migration distance was about 15~20 cm, smaller than in loam, indicating lower migration capacity. In clay, the resistance to water movement was the highest, resulting in the shortest migration distances in all directions, but the duration of change was the longest. In loam, the migration distances of water in both horizontal and downward directions were at an intermediate level, while the upward migration distance was the greatest, indicating that loam has the strongest upward water transmission capability. This suggests that loam is more suitable for subsurface drip irrigation with a burial depth of around 30 cm compared to sandy and clay soils. For the same texture, the horizontal and downward migration rates of the wetting front in sandy soil increased with the increase in emitter flow rate, but the upward migration rate first decreased and then increased. This indicated that at the flow rate of 1.38 L·h⁻¹, the impact of flow rate on the wetting front migration rate surpassed the influence of upward soil resistance and gravity. In loam, the migration rates of the wetting front in all three directions increased with the emitter flow rate. In clay, the horizontal and downward migration rates of the wetting front varied by 6%~10% with flow rate changes, showing smaller change compared to other soil textures. This indicated that the wetting front migration in these two directions in clay was less affected by flow rate. However, the upward migration rate in clay decreased initially and then increased with the increase in emitter flow rate, with the decrease being much greater than the increase. This suggests that lower flow rates are more conducive to upward water movement in clay. For all three soil textures and flow rates, the migration rates of the wetting front gradually slowed down with prolonged infiltration time. As drip irrigation flow rates increase, the ratio of the migration distances of the wetting front in horizontal to upward directions increased correspondingly. This indicates that increased flow rates are more favorable for enhancing horizontal migration capacity compared to upward water movement. Additionally, the migration rates of the wetting front in both horizontal and vertical directions followed a power function with infiltration time. The wetting front in sand and clay migrated vertically to 10.0 cm and 10.3 cm from the surface respectively, while it in loam migrated vertically to the surface under the three flow conditions. In summary, for subsurface drip irrigation in different soil textures, the emitter burial depth in sandy soil should be within 15 cm, with a flow rate around 0.39 L·h⁻¹ to ensure water availability during the seedling stage and reduce leakage. For loam, the emitter burial depth should be increased to 30 cm, and the flow rate should be determined based on crop emergence time, irrigation timing, and water-saving goals. Drip irrigation in clay is not suitable for deep burial and deep-rooted plants.