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and shallower depth means that flow velocities are usually significantly smaller than those
in the main channel. Typical river flows have velocities in the range 1-3 ms -1 , whilst
floodplain flows in all but the most extreme events have velocity of \1ms -1 . Floodplain
storage therefore alters wave propagation and has important consequences for many
physical processes. Hence, whilst floodplains and wetlands cover only approximately 4 %
of the Earth's land surface, they exert a critical influence on global biogeochemical cycles
(Richey et al. 2002 ; Frey and Smith 2005 ; Zhuang et al. 2009 ), terrestrial run-off to the
world's oceans (Richey et al. 1989 ), sediment and nutrient transport (Beighley et al. 2008 ),
basinwide flood response (Turner-Gillespie et al. 2003 ) and global biodiversity (Tockner
and Stanford 2002 ) as a result of the multitude of landscapes generated by floodplain
geomorphologic complexity (Mertes et al. 1996 ). Moreover, over longer timescales,
floodplains and wetlands form sedimentary basins where significant oil and natural gas
reserves are found. It follows from this that surface water processes occur within, and are
mediated by, the wider catchment hydrological system (Destouni et al. 2010 ; Cvetkovic
et al. 2012 ) which also includes significant human activity (Destouni et al. 2013 ), and these
interactions become especially important when considering the role of water fluxes as
drivers of biogeochemical and ecological processes (e.g., Lyon et al. 2010 ).
Extreme floods can also be a significant natural hazard. According to the World Health
Organization EM-DAT natural hazards database ( www.em-dat.be ), in 2011 floods and
related hydrological hazards (e.g., wet mass landslides) accounted for over half of all
reported disasters and affected *140 million people (Guha-Sapir et al. 2012 ). In the UK
alone, 5 million people (i.e. 1 in 12 of the population) in 2 million properties live on coastal
and fluvial floodplains, and over 200,000 of these properties have less than the standard of
protection mandated by the UK government (1 in 75-year recurrence interval). The pro-
portion of at-risk population is likely to be similar in many other developed countries and
perhaps worse in developing nations, where risk is often poorly understood due to a lack of
numerical modelling supported by suitable hydrometric and topographic data sets.
Moreover, when they do inevitably occur, floods cause major social disruption, civil unrest,
economic loss and insurance sector bankruptcies (e.g., the floods in Mozambique, 2000;
New Orleans, 2005; Thailand, 2007; UK, 2007).
Surface water floods therefore play an important role in the Earth system, yet despite a
number of groundbreaking studies (e.g., Alsdorf et al. 2000 ; Hamilton et al. 2002 ; Mertes
et al. 1995 ), their dynamics at global scales remain poorly quantified through either ground
observations, satellite observations or modelling. For example, current estimates of global
inundated area from ground and satellite instruments vary from 1 to 12 million square
kilometres (Zhuang et al. 2009 ) and do not capture seasonal variation adequately. As a
consequence, estimates of the magnitude of other processes driven by such dynamics, such
as methane emissions from flooded wetlands, which are a significant contributor to global
atmospheric methane, also cannot be well estimated.
Given the importance of surface water floods in the Earth system over the last decade,
an increasing volume of research has been undertaken to better observe and understand the
above phenomena. The aim of this paper is to review this progress and look forward to
future satellite missions, which may further add to our knowledge.
The paper is organized as follows. In Sect. 2 , we review the data sets currently available
to describe flood dynamics globally and the recent progress in combining these data to
further our understanding. At present, there is no satellite mission dedicated to making
these observations, and therefore, surface water scientists make use of a range of sensors
developed for other purposes that are distinctly sub-optimal for the task in hand. Never-
theless,
by
careful
combination
of
the
data
available
from
topographic
mapping,
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