Quantification of Nocturnal Water Use and Its Composition in a Eucalyptus urophylla x E. grandis Plantation on the Leizhou Peninsula, South China

Nocturnal water use (Q(night)) is an important component of the eucalyptus water budget, but it has always been under-appreciated and poorly understood. To improve the accuracy of water balance estimates and the understanding of the nocturnal water use process in eucalypts plantations, we conducted a 3-year study to investigate the characteristics of Q(night) and its components in an E. urophylla x E. grandis plantation in southern China. The results showed that the Q(night) of E. urophylla x E. grandis was substantial and the ratio of nocturnal to daily water use (R-night) was on average 12.35%, with higher R-night (14.97%) in the dry season than in the wet season (9.50%). The Q(night) includes two components, nocturnal transpiration (Tn) and nocturnal refilling (Re), which are driven by different factors. Nocturnal Re-Tn dynamics were controlled by a combination of nocturnal environmental factors that drive Tn and corresponding daytime environmental factors that drive daytime transpiration. Therefore, the compositional ratios of Tn and Re differed between weather conditions and months. We developed a novel method to distinguish between Re and Tn and quantified the dynamics of their ratios. We found that on a 3-year average, the Q(night) of E. urophylla x E. grandis was mainly used for Tn (58.63%). Our results highlight the non-ignorability of Q(night) and the high variability of the compositional ratios of Re and Tn, and suggest that Q(night) and its components should be accurately quantified and considered when studying the water balance in eucalyptus stands.