Regulation of tree water use by biophysical factors: Insights based on multi-layer hierarchical residual analysis

• Daily total photosynthetically active radiation ( PAR ) primarily influenced daily stand transpiration, while nighttime length was the main factor affecting nocturnal water uptake . • Soil moisture affected the interannual sensitivity of daily stand transpiration to daily total PAR and that of nocturnal water uptake to nighttime length. • Clear seasonal patterns of transpiration and nocturnal water uptake varied among years based on annual mean soil moisture. • Empirical models estimated daily stand transpiration and nocturnal water uptake despite large interannual variation. The complex impact of biophysical factors on tree water use in changing environments has been an important topic in the ecohydrological studies. However, understanding how to model tree water use based on key biophysical factors requires an in-depth analysis. This study monitored the sap flow ( Q ) of Populus popularis in an arid area over four consecutive growing seasons from 2018 to 2021, and leaf area index ( LAI ), soil water content at 0–30 cm soil depth ( SWC 0–30 cm ), groundwater level, and meteorological variables (photosynthetically active radiation ( PAR ), solar radiation, air temperature, relative humidity, wind speed at 2 m height, and rainfall). Based on a three-layer hierarchical residual analysis, three novel statistical models representing the total effects of biophysical factors (meteorological variables, water supply conditions, and circadian rhythms factors) on normalized daily sap flow ( Q/LAI ), normalized nocturnal sap flow ( Q n /LAI ), and the ratio of nocturnal sap flow to daily sap flow ( Q n /Q ) were constructed on a daily scale, respectively. These models could estimate Q/LAI , Q n /LAI , and Q n /Q over four years . The results further indicated that daily total PAR and mean SWC 0–30 cm best explained the variation in Q/LAI , while nighttime length and mean SWC 0–30 cm jointly influenced Q n /LAI . Interannually, the seasonal patterns of daily Q/LAI and Q n /LAI were correlated to the mean growing season SWC 0–30 cm . The proposed models can quantify daily tree water use, nighttime water use, and its proportion in daily tree water use using biophysical factors, specifically daily total PAR , SWC 0–30 cm , nighttime length, Q , and day of year. Our findings contribute to a deeper understanding of the drivers of daily and nighttime tree water use in arid regions.