3.3 Soil moisture response time at the 10-cm soil depth
The SM response exhibited a hysteresis effect after rainfall, and the average response lag time (ART) was 5.6 h at a depth of 10 cm for all precipitation events. Group IV (continuous rains) showed the most obvious hysteresis effect (p<0.05) in which the ART reached 7.8 h, followed by Group III (light rains), Group I (heavy rains), and Group II (intermediate rains) in Table 3. In addition, the l0 cm SM response duration showed a similar trend after precipitation, followed by Group IV (21.9 h) > Group III (14.7 h) > Group II (13.4 h) > Group I (9.7 h). Therefore, a small rainfall amount with a lower intensity significantly delayed the surface SM response time and duration, while a higher rainfall intensity promoted a quick surface SM response.
Land-cover change significantly influenced the surface SM response time (p<0.05). Abandoned grass and planted forest showed a longer ART of 10 cm SM that surpassed 8 h, which was delayed 69.8% and 56.6%, respectively, compared to the crop type. While the shrub site was the exception and showed the earliest ART of surface SM, merely 3.4 h (Table 3 and Supplemental Fig. S2). The lasting response time of the 10-cm SM showed no significant difference among the five vegetation types (Table 3).
In addition, the minimum ARA necessary to trigger the SM response was approximately the same (5-5.9 mm) at all the monitoring sites (Fig. 4). The shrub site required the smallest ARA, approximately 5 mm, to trigger a 10-cm SM response, whereas the other sites required ~6 mm (the green dashed line in Fig. 4). The median value of the ARA needed to trigger the SM response was the lowest at the shrub site and highest at the forest site (the red line of the box plot in Fig. 4).