DRAFT - Author’s version.
unique ability to directly track changes in total water storage anomalies (TWSa),
according to the following equation:
TWSa = CANa + SWa + SMa + SWEa + GWa (A2.1.1)
where CANa is canopy water storage anomaly, SWa is the surface water anomaly, SMa
is the soil moisture anomaly, SWEa is the snow water equivalent anomaly, and GWa is
the groundwater storage anomaly.
By utilizing supplemental observations from other remote sensing platforms and land
surface models and rearranging Eq. A2.1.1, scientists have been able to resolve
changes in groundwater storage within major river basins around the planet (Famiglietti
et al. 2014). From Bangladesh (Purdy et al. 2019) and India (Rodell et al. 2009) to the
Middle East (Voss et al. 2013) and the American Southwest (Castle et al. 2014), the
problem of declining groundwater storage has emerged with varying levels of severity
(Richey et al. 2015). Along with many other regions around the world, California shares
an overreliance on groundwater (Famiglietti et al. 2011). This tutorial demonstrates the
analytical steps to resolve groundwater storage changes using GRACE for California’s
Central Valley.
Practicum
Section 1. Exploring the Study Area
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Evaluating changes in hydrologic storage requires examining change within a
hydrologically connected system. Watersheds and basins represent areas of land where
precipitation drains to a common point. We will use already-generated basins from the
Watershed Boundary Dataset (WBD) to delineate the drainage area for California’s
Central Valley. The WBD includes hydrologic unit codes (HUCs) to identify connected
basins within the United States.
In the following sections of code, we will load three basins by their unique four-digit
HUCs and merge the basins together. To accomplish this task, we will use the
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