Geography Reference
In-Depth Information
especially in the case of passive sensors, the spatial reso-
lution is very low (of the order of 50 km).
One widely used sensor for soil moisture is the
Advanced Microwave Scanning Radiometer (AMSR-E)
on NASA's Earth Observing System (EOS; hence the
E in AMSR-E). One of the most recently introduced pas-
sive sensors is the Soil Moisture and Ocean Salinity
(SMOS) satellite launched in November 2009 (Kerr
et al.,
2001
,
2010
). Another mission planned to start in
2014/15 is the Soil Moisture Active Passive (SMAP) mis-
sion initiated by NASA (Wagner et al.,
2007
). One of the
first active soil moisture data sets was derived from the
ERS scatterometer data for the period 1992
hydrological connectivity of uplands to streams (Chirico
et al.,
2005
; Jencso et al.,
2010
; Jencso and McGlynn,
2011
).
3.5.1 Topography
Some topographic data is available for most regions of the
world. The US Geological Survey (USGS) has built a 30
arc-second digital elevation model (DEM) of the world
called GTOPO30. Hydrologically relevant derivatives,
such as catchment boundaries, river networks, slope, flow
direction, aspect, topographic wetness index and flow
accumulation have been extracted from GTOPO30 and
are available in the USGS HYDRO-1K geographic data-
base at a resolution of 1 km. Recently, the Shuttle Radar
Topography Mission (SRTM) updated the global 30-arc
second DEM (Farr et al.,
2007
). SRTM topographic data
are available (
Figure 3.9
), although the accuracy is much
lower in mountainous terrain than in flat terrain (Ludwig
and Schneider,
2006
). At the national scale, many coun-
tries have elevation information at a very fine spatial
resolution that is in the range of a few metres, however,
it is not always freely available. An example of a freely
available DEM is the National Elevation Dataset (NED) at
10 m resolution for the conterminous USA, Alaska, Hawaii
and territorial islands.
Increasingly, airborne LIDAR data are becoming avail-
able. Most data are currently being obtained through dedi-
cated research projects for regions with small spatial
extent, but large-scale observation missions are becoming
feasible. A number of countries around the world are
currently creating a state-wide DEM based on airborne
LIDAR data with a resolution in the order of 1 m. This
form of high-resolution topography and vegetation height
and density data will become increasingly available in the
future. It will prove to be very valuable for inundation
modelling and thus offers new opportunities for connecting
processes and form at an ever-increasing range of scales,
e.g., explicit extraction of channel heads (Tarolli and Dalla
Fontana,
2009
). The full value of this very high resolution
topographic information still has to be exploited (Mallet
and Bretar,
2009
).
2000 (Wagner
et al.,
2003
). Its successor is the Advanced Scatterometer
(ASCAT), which uses a very similar measurement concept
while improving significantly on the spatial (25 km) and
temporal (1
-
-
2 days) resolution. ASCAT has thus very
comparable sampling characteristics to SMOS and the
SMAP radiometer (Wagner et al.,
2007
). Soil moisture
estimates at a higher spatial resolution are derived by the
Synthetic Aperture Radar (SAR) instruments on-board
ESA's ENVISAT, or ESA's European Remote Sensing
(ERS) satellites (Wagner et al.,
2008
; Doubková et al.,
2012
). While the spatial resolution of these instruments is
typically higher, applications of SAR soil moisture
retrievals are typically limited to small areas or specific
catchments (e.g., Pauwels et al.,
2001
; van Oevelen,
2000
).
3.5 Catchment characterisation
Basin and catchment characterisation is typically focused
on assessment and quantification of those aspects of phys-
ical and ecological structure that influence the storage,
movement and release of water to evaporation and runoff.
As such, topography, soil characteristics, geology, stream
network geometry, land cover and land use are of primary
interest for PUB. These variables are reflections of long-
term hydrological and geomorphic processes and act to
mediate contemporary hydrological processes such as run-
off generation and evaporation, and catchment storage.
Catchment characterisation can be accomplished via
remotely sensed data (e.g., topography and land cover
classification) and field assessment. As indicated in the
case studies at the end of this chapter, catchment charac-
teristics can inform relative and absolute, as well as quali-
tative and quantitative, assessment of likely catchment
response to climate forcing. For example, geological infor-
mation can provide insight into deeper groundwater con-
tributions to runoff, while distributions of vegetation cover
can inform runoff production mechanisms. Surface flow
path lengths, structure and accumulation can provide add-
itional insights into the patterns of water redistribution and
3.5.2 Land cover and land use
Several global land cover data sets have been compiled
from remote sensing imagery. One of the older ones,
often applied in large-scale modelling studies (e.g., Troy
et al.,
2008
; Nijssen et al.,
2001
) is a global land cover
classification system that was compiled by the University
of Maryland's Department of Geography. Fourteen land
cover classes are distinguished, based on AVHRR
imagery from the period 1989
-
94. Data are available at
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