Description of the LUE-NDWI model
The
NDWI,
strongly related to vegetation water content (Murphy et al., 2018;
)(50), can be a very good proxy for vegetation water stress. After
examining measurements from many flux towers, we found that the
following nonlinear function can be used to representWS , the regulation scalar of water stress on
GPP:
\begin{equation}
\mathbf{W}_{\mathbf{S}\_\mathbf{\text{NDWI}}}=\mathbf{a}*\left(-\mathbf{\text{NDWI}}+\mathbf{0}.\mathbf{5}\right)^{\mathbf{b}}+\mathbf{c}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (\mathbf{7})\nonumber \\
\end{equation}Using the NLS method of parameter optimization adjustment, the
coefficients were determined to be a (=0.35), b (=2.14)
and c (=0.086). The WS_NDWI values vary between
0 and 1, with values beyond the bounds set to 0 or 1, respectively. The
use of WS_NDWI can be very convenient for applications
from local to global scales as the NDWI fields can be directly derived
from satellite observations.