Water fluxes and energy partitioning over a Larix principis-rupprechtii plantation forest in a dryland mountain ecosystem, Northwest China

Plantations make up a sizable component of forest ecosystems in China's drylands and play an important role controlling the energy balance and water cycle. However, there is limited knowledge regarding the exchange of water and energy in dryland plantations, particularly those located within the intricate and unique environments of dryland mountain regions. The focus of this study was the investigation of a Larix principis-rupprechtii plantation within a dryland mountain ecosystem located in Northwest China. The eddy covariance method was employed to calculate the water and energy fluxes, while the thermal dissipation probe (TDP) method was utilized to measure forest overstory transpiration (T) from 2018 to 2021. The links between energy allocation and various components in water flux were investigated, as well as the variable dynamics of water and energy flux and their relationship with environmental conditions. The findings demonstrated that the annual mean cumulative evapotranspiration (ET) was about 510 mm, accounting for over 82.8 % of the precipitation, which was mainly from soil evaporation and understory transpiration, rather than overstory transpiration (T). The average value of T/ET was 0.25 for the growing season, and surface conductance (Gs) was always higher than canopy conductance (Gc) on both daily and monthly scales. The evaporative fraction (EF) and Priestley-Taylor model coefficient (alpha) in the growing season were significantly higher than those in the non-growing season, and alpha>1 from June to September, indicating that the ecosystem in the study area had sufficient water supply. Furthermore, during the growing season, the ratio of latent heat flux (LE) to net radiation (Rn) was larger than that in the non-growing season, while the proportion of sensible heat flux (H) was opposite, and soil heat flux (G) accounted for the smallest proportion of net radiation. This study provides valuable insights into the energy fluxes and water fluxes within the ecosystem of dryland mountain plantations, and it serves as a guide for choosing afforestation tree species in the future.