Partitioning soil water balance under deep-rooted fruit trees in the thick loess deposits

To explore the impacts of vegetation change on hydrological processes, it is crucial to partition components of soil water balance (SWB); however, SWB partitioning is challenging because of the difficulty in deep drainage estimation and evapotranspiration (ET) decomposing. Here, we employed stable and radioactive isotopes to systematically partition SWB under farmland and apple orchards with trees of 11, 23 and 28 years old, and further investigated the impacts of vegetation change on soil hydrological processes in the Xifeng loess tableland of China. After the conversion of farmland into apple orchards, the mean soil water content tended to decrease, especially for deep soils (>8 m), while the mean soil water stable isotopes tended to be depleted. SWB partitioning revealed that apple orchards significantly reduced soil water storage by 16-19 % and deep drainage by 100 % in comparison to farmland. The apple orchard increased evapotranspiration by 7 % to 13 %, and altered the ET components through decreasing E by 11 % to 30% (i.e. 12.9-33.6 mm) and increasing T by 13 % to 28 % (i.e. 57.0-116.4 mm). In addition, the isotopes of soil water sources were depleted with increase of depths and tree age. This indicates that soil water under apple orchards is more difficult to recharge, requiring heavier precipitation events (e.g. > 20 mm/day). The results of SWB partitioning reveals the mechanism by which deeprooted vegetation affects soil hydrological processes, i.e., fruit trees with deep-rooted absorb substantial quantities of soil water to meet their transpiration needs, and consequently terminates the recharge of deep soil water and groundwater. The methodologies adopted to SWB partition provide technical supports for similar research, and the results can help to deepen understanding of the hydrological processes in the thick loess deposits.