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 for achieving efficient water-saving cultivation in agriculture. Considering that SDI is a localized irrigation method, the formation of a wetting front in the soil created by water applied via drip emitters is crucial for effective crop water uptake. Selecting appropriate irrigation parameters based on soil texture, emitter discharge, soil moisture content, and other factors can help avoid over- or under-irrigation. However, previous studies have 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 — using drip emitters at a depth of 30 cm, with three flow rates: 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 the soil wetting front. The results indicated that different soil types exhibited varying responses to changes in moisture content under different drip emitter flow rates. Loam and sandy soils tended to form regular wetting patterns at the high flow rate, whereas clay, because of its water retention properties, exhibited obvious water retention characteristics at the low flow rate. The influence of soil texture on water movement also varied. In sandy soil, the wetting front exhibited the greatest horizontal and downward migration distances, while the upward migration distance was approximately 15−20 cm, which was smaller than that 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, although the duration of water movement was the longest. In loam, the horizontal and downward migration distances were intermediate, whereas the upward migration distance was the greatest, indicating that loam had the strongest upward water transmission capability. This suggested that loam was more suitable for SDI with a burial depth of approximately 30 cm than sandy or clay soils. In sandy soil, the horizontal and downward migration rates of the wetting front increased with increasing emitter flow rate; however, the upward migration rate initially decreased and later 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 the 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 a smaller change compared to other soil textures. This indicated that the wetting front migration in these two directions in the clay was less affected by the flow rate. However, the upward migration rate in clay initially decreased and then increased with the increase in emitter flow rate, with the decrease being much more pronounced than the increase. This suggested that lower flow rates were more conducive to upward water movement in the clay. As the drip irrigation flow rates increased, the ratio of the horizontal to upward migration distances of the wetting front increased accordingly. This indicated that increased flow rates were more favorable for enhancing the horizontal migration capacity than upward water movement. Additionally, the migration rates of the wetting front in both the horizontal and vertical directions followed a power function with infiltration time. In sandy and clay soils, the wetting front migrated vertically to 10.0 and 10.3 cm from the surface, respectively. In contrast, in the loam, the wetting front migrated vertically to the surface under all three flow conditions. In summary, for SDI, the emitter burial depth in sandy soil should be within 15 cm, with a flow rate of 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-buried and deep-rooted plants.