Abstract
<strong class="journal-contentHeaderColor">Abstract.</strong> River water plays a critical role in riparian plant water use and riparian ecosystem restoration along losing rivers (rivers losing flow into underlying groundwater) under climate warming. How to quantify the contributions of river water to riparian plants under different water tables and the related responses of plant water use efficiency is a great challenge. In this study, experiments of stable isotopes (δ<sup>2</sup>H, δ<sup>18</sup>O and <sup>222</sup>Rn) in different waters and leaf δ<sup>13</sup>C were conducted for riparian deep-rooted <em>Salix babylonica</em> (L.) during dry year (2019) and wet year (2021) along the Chaobai River in Beijing, China. The MixSIAR model in combination with an iteration method were proposed to quantify the proportional river water contribution (RWC) to riparian <em>S. babylonica</em> and its correlations with the depth of water table (WTD) as well as leaf δ<sup>13</sup>C. Results showed that riparian <em>S. babylonica</em> took up deep water (in 80−170 cm soil layer and groundwater) by 56.5 ± 10.8 %. River water that recharged riparian deep water was an indirect water source and contributed 20.3 % of water to riparian trees nearby the losing river. Significantly increasing river water acquisitions (by 7.0 %) but decreasing leaf δ<sup>13</sup>C (by −2.0 ‰) of riparian trees were observed as the WTD changed from 2.7 m in 2019 to 1.7 m in 2021 (<em>p</em> < 0.05). A short residence time (no more than 0.28 days) of groundwater indicated that there was rapid and frequent river recharge to riparian groundwater in 2021. It was found that the RWC to riparian <em>S. babylonica</em> was negatively correlated with the WTD but positively related to the leaf δ<sup>13</sup>C in linear functions (<em>p</em> = 0.000). The rising water table would stimulate riparian trees to maximize transpiration water consumptions and show a profligate water use strategy with increasing water extraction from the losing river. This study provides critical insights into understanding the mechanism of water cycle in Groundwater-Soil-Plant-Atmosphere Continuum, managing water resources and riparian afforestation along losing rivers.
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