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Multiscale Hydrologic Remote Sensing: Perspectives and Applications
19.1 INTRODUCTION
Terrestrial water cycle is of critical importance to a wide array of Earth system
processes. It plays a central role in climate and meteorology, plant community
dynamics, and carbon and nutrient biogeochemistry (Vörösmarty et al. 1998). Evapo -
transpiration (ET) is an important component of the terrestrial water cycle. At the
global scale, it represents more than 60% of precipitation inputs (L’vovich and White
1990), thereby conveying an important constraint on water availability at the land
surface. Through links between stomatal conductance, carbon exchange, and water
use efficiency in plant canopies (e.g., Hari et al. 1986; Raich et al. 1991; Woodward
and Smith 1994; Sellers et al. 1996; Farquhar et al. 2002), ET serves as a regulator of
key ecosystem processes. This, in turn, controls the large areal distribution of plant
communities and net primary production of vegetation (e.g., Dang et al. 1997; Oren
et al. 1999; Misson et al. 2004; Zhao and Running 2010).
Runoff or blue water is usually the major accessible renewable water resource
for human uses. For a watershed, runoff is roughly the balance between precipitation received from the atmosphere and ET (green water) lost to the atmosphere.
ET consists of evaporation from soil, evaporation from intercepted precipitation by
plants, and transpiration via plant tissues. Over a relatively long period (i.e., a season or a year), the available water for humans and ecosystems in a given region
can be approximated by the difference between accumulated precipitation and ET
(Donohue et al. 2007). Demand for the world’s increasingly scarce water supply is
increasing rapidly, challenging its availability for food production and putting global
food security at risk. Agriculture, upon which a burgeoning population depends for
food, is competing with industrial, household, and environmental uses for this scarce
water supply (Vörösmarty et al. 2010; Rosegrant et al. 2003). Yet, all signs of sustainability suggest that water scarcity is getting worse and will continue to do so
(United Nations World Water Assessment Program 2003; Millennium Ecosystem
Assessment 2005). Improving water resource management can alleviate water crisis and reduce droughts, wildfire, dust storms, and flooding (Cleugh et al. 2007).
This requires reliable and timely quantification of water cycling at regional levels.
However, over the last decade in the last millennium, there was a widespread loss
of hydrological monitoring networks in both developed and developing countries,
which is of great concern to the scientific community for managing water resources
and detecting the impact of climate change on the hydrological cycle (Shiklomanov
et al. 2002). Hydrologic models are the centerpiece of terrestrial water cycle studies,
because modeling is the only method for understanding ecosystem processes in an
interactive manner, integrating the various scales of measurements and predicting
the impacts of geophysical and biogeochemical factors on the water cycle.
Remotely sensed data, especially those from polar-orbiting satellites, provide
us with temporally and spatially continuous information over vegetated surfaces
and are useful for accurately parameterizing surface biophysical variables, such as
albedo, biome type, and leaf area index (LAI) (Los et al. 2000). As a result, remote
sensing data can greatly reduce the uncertainties in ET estimates. The Moderate
Resolution Imaging Spectroradiometer (MODIS), onboard the NASA satellites Terra
and Aqua, may be the most complex instrument built on a spacecraft for civilian
Multiscale Hydrologic Remote Sensing: Perspectives and Applications
19.1 INTRODUCTION
Terrestrial water cycle is of critical importance to a wide array of Earth system
processes. It plays a central role in climate and meteorology, plant community
dynamics, and carbon and nutrient biogeochemistry (Vörösmarty et al. 1998). Evapo -
transpiration (ET) is an important component of the terrestrial water cycle. At the
global scale, it represents more than 60% of precipitation inputs (L’vovich and White
1990), thereby conveying an important constraint on water availability at the land
surface. Through links between stomatal conductance, carbon exchange, and water
use efficiency in plant canopies (e.g., Hari et al. 1986; Raich et al. 1991; Woodward
and Smith 1994; Sellers et al. 1996; Farquhar et al. 2002), ET serves as a regulator of
key ecosystem processes. This, in turn, controls the large areal distribution of plant
communities and net primary production of vegetation (e.g., Dang et al. 1997; Oren
et al. 1999; Misson et al. 2004; Zhao and Running 2010).
Runoff or blue water is usually the major accessible renewable water resource
for human uses. For a watershed, runoff is roughly the balance between precipitation received from the atmosphere and ET (green water) lost to the atmosphere.
ET consists of evaporation from soil, evaporation from intercepted precipitation by
plants, and transpiration via plant tissues. Over a relatively long period (i.e., a season or a year), the available water for humans and ecosystems in a given region
can be approximated by the difference between accumulated precipitation and ET
(Donohue et al. 2007). Demand for the world’s increasingly scarce water supply is
increasing rapidly, challenging its availability for food production and putting global
food security at risk. Agriculture, upon which a burgeoning population depends for
food, is competing with industrial, household, and environmental uses for this scarce
water supply (Vörösmarty et al. 2010; Rosegrant et al. 2003). Yet, all signs of sustainability suggest that water scarcity is getting worse and will continue to do so
(United Nations World Water Assessment Program 2003; Millennium Ecosystem
Assessment 2005). Improving water resource management can alleviate water crisis and reduce droughts, wildfire, dust storms, and flooding (Cleugh et al. 2007).
This requires reliable and timely quantification of water cycling at regional levels.
However, over the last decade in the last millennium, there was a widespread loss
of hydrological monitoring networks in both developed and developing countries,
which is of great concern to the scientific community for managing water resources
and detecting the impact of climate change on the hydrological cycle (Shiklomanov
et al. 2002). Hydrologic models are the centerpiece of terrestrial water cycle studies,
because modeling is the only method for understanding ecosystem processes in an
interactive manner, integrating the various scales of measurements and predicting
the impacts of geophysical and biogeochemical factors on the water cycle.
Remotely sensed data, especially those from polar-orbiting satellites, provide
us with temporally and spatially continuous information over vegetated surfaces
and are useful for accurately parameterizing surface biophysical variables, such as
albedo, biome type, and leaf area index (LAI) (Los et al. 2000). As a result, remote
sensing data can greatly reduce the uncertainties in ET estimates. The Moderate
Resolution Imaging Spectroradiometer (MODIS), onboard the NASA satellites Terra
and Aqua, may be the most complex instrument built on a spacecraft for civilian