Identifying the mean residence time of soil water for different vegetation types in a water source area of the Yuanyang Terrace, southwestern China.

The mean residence time of soil water (MRTsw) 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 MRTsw 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 MRTsw values of the forestland and shrubland both increased with soil depth. However, differences in the MRTsw 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.