Analyzing Effects of Shrub Canopy on Throughfall and Phreatic Water Using Water Isotopes, Western China

Abstract

Alpine shrub Quercus aquifolioides was selected to study the effects of shrub canopy on throughfall and phreatic water by analyzing the isotopic time series of precipitation, canopy throughfall and phreatic water and examining correlations among these series in Wolong Nature Reserve, Western China. Based on analysis of precipitation data in 2003, the local meteoric water line during the rainy season was dD ¼ 8.and the primary precipitation moisture in this region originated from the Pacific Ocean in the summer. Stable isotope analysis showed that the main supply of throughfall and phreatic water was from precipitation, and the shrub canopy has an important effect on the processes of rainwater transmuted into throughfall and phreatic water. Moreover, the differences of dD and d 18 O values between rainwater and throughfall were relevant to rainfall. Due to interception of the shrub canopy, there had a response hysteresis of phreatic water to the various rainfall events, which was mostly 2 days, except that this hysteresis was 1 day when rainfall was >15 mm/day.

The role of stable isotopes in understanding rainfall interception processes: a review

“…In dual isotope space, several studies have shown that TF, SF, and OP δ w tend to fall along similar local MWLs with slopes near 8, although one study found the TF slope to be well below that of OP . Similar to TF–OP single‐isotope differences, the d differences vary among events and studies .…”

“…Correlations between event interception loss and TF–OP δ w differences have been weak despite correlation between interception loss and intensity and amount . Smaller storms tend to have a greater likelihood of TF that is isotopically lighter than OP, just as individual events means are more likely than study means to have TF with lighter δ w than OP (Figure (b)). Inter‐event carryover of intra‐canopy vapor or residual storage has greater effects on TF in small storms too.…”

“…Canopy δ w effects depend on storm and prestorm conditions. Larger TF-OP δ w differences have been observed in events with lower OP amount, 31,35,40 duration, 31 or intensity. 24,36 Correlations between event interception loss and TF-OP δ w differences have been weak 23,24,27,36,40 despite correlation between interception loss and intensity and amount.…”

“…Within single events, simultaneously positive and negative TF-OP δ w differences have been commonly observed between plots, [23][24][25]27,37 between TF and SF, 27,29,36 or among individual collectors. 23,27,35,36,40 The range of δ w between TF and SF collectors varied widely among studies (Figures 3 and 4). However, because variance is related to collector size, collection interval, and number of collectors, 40 comparison among studies is difficult.…”

“…The ambient vapor δ w , a residual signal from previous events or larger-scale processes, and its difference from OP δ w is a strong predictor of TF-OP differences and fractionation. 22 In dual isotope space, several studies have shown that TF, SF, and OP δ w tend to fall along similar local MWLs with slopes near 8, 23,28,29,35,38 although one study found the TF slope to be well below that of OP. 31 Similar to TF-OP single-isotope differences, the d differences vary among events and studies.…”