Tree rings: A key ecological indicator for reconstruction of groundwater depth in the lower Tarim River, Northwest China

Groundwater is becoming increasingly important in extremely arid areas due to climate change and human activities. Little is known, however, regarding the long-term dynamics of groundwater due to the lack of long-term historical records of groundwater depth data. In this study, we investigated the relationship between hydrometeorology and tree rings of Populus euphratica (P. euphratica) and the feasibility of reconstructing historical groundwater depths from these tree rings in the lower reaches of the Tarim River, an extremely arid area in China. Tree-ring growth of P. euphratica mainly reflected historical changes in regional water environments (including runoff and shallow groundwater), rather than climatic variables (including temperature, precipitation, relative humidity, and sunshine hours). Groundwater depth constituted the key environmental factor for directly controlling P. euphratica growth. Linear regression models using tree rings were developed to reconstruct historical changes of groundwater depths in the calendar year (January-December) and the growing season (April-August) from 1950 to 2015. Results tracked well with the main hydrological events in the study area. Our findings demonstrate that tree rings of P. euphratica are a key and effective ecological indicator for quantitatively reconstructing and elucidating historical changes of groundwater depth in the lower reaches of the Tarim River. Additionally, the suitable sampling range for establishing tree-ring chronologies of P. euphratica and reconstructing groundwater depth is 0-300 m from the river channel in the lower Tarim River. Taking into consideration the lagged effect between runoff of the Ecological Water Conveyance Project (EWCP), decrease in groundwater and tree growth, we recommend that the optimal implementation time of ecological water conveyance is between the previous September and November in the lower Tarim River. This application will effectively improve the groundwater environment in the current year and ensure the growth of riparian trees.