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\begin{document}

\title{The Compensatory CO2 Fertilization and Stomatal Closure Effects on
Runoff Projection in the Western United States}



\author[1]{Xueyan Zhang}%
\author[2]{Jiming Jin}%
\author[3]{Xubin Zeng}%
\author[4]{Wuchao Yang}%
\author[2]{Charles P Hawkins}%
\author[3]{Antônio A M Neto}%
\author[3]{Guo-Yue Niu}%
\affil[1]{The university of Arizona}%
\affil[2]{Utah State University}%
\affil[3]{The University of Arizona}%
\affil[4]{Northwest A\&F University}%


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  \date{May 16, 2024}


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\begin{abstract}
Water availability in the dry Western United States (US) under a warming
climate and increasing water use demand has become a serious concern.
Previous studies have projected future runoff changes across the Western
US but ignored the impacts of ecosystem response to elevated CO2
concentration. Here, we aim to understand the impacts of elevated CO2 on
future runoff changes through ecosystem responses to both rising CO2 and
associated warming using the Noah-MP model with representations of
vegetation dynamics and plant hydraulics. We first validated Noah-MP
against observed runoff, LAI, and terrestrial water storage anomaly from
1980--2015. We then projected future runoff with Noah-MP under
downscaled climates from three climate models under RCP8.5. The
projected runoff declines variably from the Pacific Northwest by --11\%
to the Lower Colorado River basin by --92\% from 2016--2099. To discern
the exact causes, we conducted an attribution analysis of two additional
sensitivity experiments: one with constant CO2 and another with monthly
LAI climatology based on the Penman-Monteith equation. Results show that
surface ``greening'' (due to the CO2 fertilization effect) and the
stomatal closure effect are the second largest contributors to future
runoff change, following the warming effect. These two counteracting CO2
effects are roughly compensatory, leaving the warming effect to remain
the dominant contributor to the projected runoff declines at large river
basin scales. This study suggests that both surface ``greening'' and
stomatal closure effects are important factors and should be considered
together in water resource projections.%
\end{abstract}%



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