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\begin{document}

\title{Understanding Interactions between Terrestrial Water and Carbon Cycles
Using Integrated SMAP Soil Moisture and OCO-2 SIF Observations and Land
Surface Models}



\author[1]{Zhijiong Cao}%
\author[2]{Yongkang Xue}%
\author[2]{Hara Prasad Nayak}%
\author[3]{Dennis P. Lettenmaier}%
\author[4]{Christian Frankenberg}%
\author[5]{Philipp Köhler}%
\author[2]{Ziwei Li}%
\affil[1]{University of California, Los Angeles}%
\affil[2]{University of California Los Angeles}%
\affil[3]{University of California, Los Angeles, California, USA}%
\affil[4]{California Institute of Technology}%
\affil[5]{EUMETSAT}%


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  \date{June 10, 2024}


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\begin{abstract}
Recently, more advanced synchronous global-scale satellite observations,
the Soil Moisture Active Passive enhanced Level 3 (SMAP L3) soil
moisture product and the Orbiting Carbon Observatory 2 (OCO-2)
solar-induced chlorophyll fluorescence (SIF) product, provide an
opportunity to improve the simulations of both water and carbon cycles
in land surface modeling. This study introduces a mechanistic
representation of SIF to the Simplified Simple Biosphere Model version 4
(SSiB4) coupled with the Top-down Representation of Interactive Foliage
and Flora Including Dynamics Model (TRIFFID). This newly developed model
with the observed satellite data indicates that introducing dynamic
processes can lead to substantial improvement in global carbon flux
simulation. In the SSiB4/TRIFFID/SIF, four critical soil and vegetation
parameters--B parameter, soil hydraulic conductivity at saturation (Ks),
wilting point, and maximum Rubisco carboxylation rate (Vmax)--were
identified through numerical sensitivity experiments. Among the four
parameters, the B parameter has the most significant effects on both
soil moisture and SIF simulations. With the optimized B parameter, both
soil moisture and SIF simulations were improved substantially, with
especially significant improvement for shrubs. The Ks and wilting point
also affect both soil moisture and SIF but with reduced magnitude. The
Vmax directly affects photosynthesis, and its modification can
substantially improve the SIF simulation of needleleaf trees and C3
grasses. With all four calibrated parameters based on SMAP L3 and OCO-2
data, the root-mean-squared error (RMSE) of soil moisture and SIF
simulations decreased from 0.076 to 0.063 m3/m3 and from 0.143 to 0.117
W/m2/\selectlanguage{greek}μ\selectlanguage{english}m/sr, respectively.%
\end{abstract}%



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