Water conservation forests significantly contribute to the stability of mountain agricultural ecosystems in Hani Terrace. In this study, we analyzed the relationship between the stable isotopic composition of soil water and precipitation to determine the mechanisms of soil water movement in the small watershed of Quanfuzhuang. We observed significant seasonal variations in soil water sources: antecedent precipitation was the dominant supply during the dry season, and current precipitation dominated during the rainy season. The recharge ratio of precipitation to soil water in the grassland was significantly higher than that in the arbor land and shrubland. The influence of water infiltration, old and new soil water mixing, and soil evaporation on the soil water stable isotopes gradually decreased from the surface (0–20 cm) to the deep (60–80 cm) soil. We observed significant seasonal variability in average soil water δ¹⁸O in the upper 0–60 cm and lower variability at 60–100 cm. The average soil water δ¹⁸O was generally higher in the dry season than in the rainy season. The mixing of old and new water is a continuous and cumulative process that is impacted by soil structure, soil texture, and precipitation events. We therefore identified a significant time delay in soil water supply with increasing soil depth. Moreover, the piston flow of soil water co-occurred with preferential flow, and the latter was the dominant supply during the rainy season.

The mean residence time of soil water (MRT_sw) for forestland and shrubland in a water source area of Yuanyang Terrace, southwestern China, was estimated using stable isotope tracer tests and the sine-wave regression model. Stable isotope analyses from precipitation and soil water were performed in 2015. The δ²H/δ¹⁸O relationship of precipitation resulted in δ²H = 7.31δ¹⁸O + 1.49, which is nearly identical to the local meteoric water line in Kunming, southwestern, China. The MRT_sw was simulated at five depth ranges (0–20, 20–40, 40–60, 60–80, 80–100 cm) of the two vegetation types by precipitation δ¹⁸O input data and soil water δ¹⁸O output data. The results showed that the MRT_sw values of the forestland and shrubland both increased with soil depth. However, differences in the MRT_sw of the forestland (between day 53 and 94) and of the shrubland (between day 76 and 142) were discussed. Regarding the physical properties of the soil profiles from the sample plots, non-capillary porosity decreased with soil depth in the forestland (from 48.5 to 20.5 %), and was clearly higher than that in the shrubland (from 38.8 to 18.7 %). Therefore, non-capillary porosity (macropores) could be a factor that shortens the mean residence time of soil water.

Precipitation in the Hani Rice Terraces water source area is redistributed through the soil, providing a continuous water flow to the terraces downstream and keeping a healthy regional ecosystem. The dynamic characteristics of soil moisture and its relationship with precipitation need to be further investigated. In this study, we examined the dynamics of soil moisture content (SMC) in different site types (based on vegetation), at varying depths, and at varying time scales from 2012 to 2016, using time-series data. Fitting and fuzzy query methods were used to determine the greatest positive correlation between daily/accumulated precipitation and SMC. The results showed that: (1) the dynamic characteristics of soil moisture at depths between 0–100 cm substantially varied between three site types; (2) the relationship graphs of Pearson’s coefficient r between SMC and prior 0–360 day’s daily precipitation were sine curves in three site types; the daily precipitation in the half a year before the test day has a positive effect on SMC at 0–100 cm depth, and the 2–3 month’s daily precipitation has the greatest effect from the test day’s SMC; (3) the influence of daily precipitation on SMC is limited, while cumulative precipitation is the main factor affecting SMC; and (4) there is a linear relationship between accumulated precipitation and SMC. A linear equation was constructed using the strongest correlation and was used to predict SMC. These models are useful to assess the sustainability of the water supply to the terraces downstream, and also to provide a basic understanding of the effect of different sites (by vegetation type) on soil moisture. This study is of great significance for the protection and sustainable utilization of Hani Rice Terraces.