3.2 Soil moisture response depth to rainfall
Different rainfall patterns with different rainfall inputs significantly impacted the soil moisture response depth across the five experimental sites. Total amounts larger than 15.2 mm, 22.2 mm, and 114.2 mm could trigger SM responses at depths of 10, 30, and 70 cm, respectively. For example, heavy rains (Group I) infiltrated to a soil depth of at least 70 cm, while intermediate rains were limited at the 30 cm depth (Fig. 3). In addition, rainfall duration and intensity had a similar impact on the soil wetting depth. When the rainfall duration was greater than 2.3 h, 13.0 h, and 22.0 h, the rainwater could percolate to soil depths greater than 10, 30, and 70 cm, respectively. An average rainfall intensity larger than 0.6 mm/h, 1.3 mm/h, and 6.2 mm/h could stimulate SM responses at depths of 10, 30, and 70 cm, respectively (Fig. 3).
Land cover also strongly influenced the soil wetting depth after precipitation. The average response depth was as follows: grass (57.5 cm) > forest (52.5 cm) > bare land (49.7 cm) > shrub (46.2 cm) > crop (45.4 cm). After converting crops to forests, shrubs, and grasses, the average soil wetting depth increased by 15.7%, 1.7%, and 26.7%, respectively (Supplemental Fig. S1). Two heavy rains (P1 and P3) are obvious, which permitted rainwater to enter deeper soil in forest and grass sites than in other sites (Fig. 3). In intermediate rains (P2 and P4), vegetation recovery prevented rainfall infiltration, leading to a lighter SM response or nonresponse after rainfall (Fig. 3a, c-e). However, the SM of planted shrubs and bare land responded more deeply than that of the other sites (Fig. 3b, e). For light rains and continuous rains, forest and grass sites had deeper soil wetting depths in some specific response processes, such as at P6 and P10.