Abstract
Abstract. Elevation in atmospheric carbon dioxide concentration (eCO2) affects vegetation water use, with consequent impacts on terrestrial runoff (Q). However, the sign and magnitude of the eCO2 effect on Q are still contentious. This is partly due to eCO2-induced changes in vegetation water use having opposing responses at the leaf scale (i.e., water-saving effect caused by partially stomatal closure) and the canopy scale (i.e., water-consuming induced by foliage cover increase), leading to highly debated conclusions among existing studies. In addition, none of the existing studies explicitly account for eCO2-induced changes to plant rooting depth that is overwhelmingly found in experimental observations. Here we develop an analytical ecohydrological framework that includes the effects of eCO2 on plant leaf, canopy density, and rooting characteristics to attribute changes in Q and to detect the eCO2 signal on Q via vegetation feedbacks over 1982–2010. Globally, we detect a very small decrease of Q induced by eCO2 during 1982–2010 (−1.7 %). Locally, we find a small positive trend (p < 0.01) in the Q–eCO2 response along a resource availability (β) gradient. Specifically, the Q–eCO2 response is found to be negative (i.e., eCO2 reduces Q) in low-β regions (typically dry and/or cold) and gradually changes to a small positive response (i.e., eCO2 increases Q) in high-β areas (typically warm and humid). Our findings suggest a minor role of eCO2 on changes in global Q over 1982–2010, yet we highlight that a negative Q–eCO2 response in semiarid and arid regions may further reduce the limited water resource there.
Highlights
Runoff (Q) is the flow of water over the Earth’s surface, forming streamflow, and represents one of the most important water resources for irrigation, hydropower, and other human needs (Oki, 2006)
Results show that the BCP model, when considering elevated atmospheric CO2 concentration (eCO2), performed better in estimating Q trends than the BCP model without considering eCO2, as evidenced by an improvement of R2 by 0.02, a reduction of root-mean-squared error (RMSE) by 0.03 mm yr−2, and a decrease of mean bias by 0.11 mm yr−2, averaged over all 2268 catchments (Fig. 4d)
By developing an analytical attribution framework, we detected a very small response of global Q to eCO2-induced changes in vegetation structural and physiological functioning (Figs. 6–8), suggesting that the eCO2 vegetation feedback only exerts a minor impact on water resources for the range of eCO2 experienced over 1982–2010
Summary
Runoff (Q) is the flow of water over the Earth’s surface, forming streamflow, and represents one of the most important water resources for irrigation, hydropower, and other human needs (Oki, 2006). ECO2 increases vegetation foliage cover (Donohue et al, 2013; Zhu et al, 2016), leading to enhanced canopylevel transpiration and reductions of Q (Piao et al, 2007) These two opposing responses of vegetation water use to eCO2 complicate the landscape-scale net effect of eCO2 on Q, and existing modeling results are highly debated since they focus on different aspects (i.e., physiological functioning and/or structural change) of how eCO2 affects the plants and the water cycle (Fatichi et al, 2016; Gedney et al, 2006; Huntington, 2008; Piao et al, 2007; Yang et al, 2016a, b). Observational and evaluation studies of eCO2 effects on Q remain limited, at regional to global scales
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