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Multiscale Hydrologic Remote Sensing: Perspectives and Applications
5.1 INTRODUCTION
Where human populations exist, water and its proper management remain a critical
and important issue across diverse regions of the world. This is particularly acute in
semiarid regions. The proper allocation and management of limited water resources
in locations that are vast in spatial extent necessitate important surface and meteorological information in order to accurately estimate evapotranspiration (ET) at
relevant spatial and temporal resolutions. The need for an accurate accounting of
consumptive water use continues to dominate ET research, largely because in many
regions throughout the world, available water resources are insufficient to meet all
water use demands. Thus, an accurate accounting of consumptive water use (or ET)
through evaporation (soil and/or plant surfaces) is indispensible (Brutsaert 1982) and
is at the core of many hydrologic studies.
ET is a complicated and important component of the hydrologic water balance
or water cycle. The difficulty of estimating ET arises from the physical and chemical interactions that exist among the soil, vegetation, and surface–boundary layer
meteorological continuum. The diversity of soils, vegetation types in both native and
modern agricultural systems, and varying local meteorological conditions present
unique challenges to quantifying partitioning of incident thermal radiation into the
components of the surface energy balance (SEB) to ultimately estimate ET at spatial
and temporal scales that are commensurate with water management needs.
Remote sensing offers the opportunity to capture critical surface information
that can be processed into regional estimates of sensible heat from which ET can
then be computed as a residual of a regional SEB. Critical to supporting regionalscale attempts to estimate ET through remote sensing algorithms is the ability to
validate remotely based estimates of ET with sound physically based measurements. One component that is a complicating factor for semiarid regions is advection of saturation deficit over irrigated fields. The term “saturation deficit” means
warm dry air is advected from hot dry surfaces to cool wetter surfaces that are
under irrigation.
A direct approach over semiarid surfaces is problematic with eddy covariance
(EC) measurements under certain conditions. However, one approach that offers a
measure of self-consistency is to examine the energy balance closure values, defined
as the ratio of turbulence energy fluxes over available energy (Xiao et al. 2011). As
reported data continue to grow over a range of surfaces and conditions, it is clear
that there is a systematic bias in EC flux estimates and the range of energy balance
closure values can be large and variable at any given location (Xiao et al. 2011). The
implications for this bias are compelling for specific issues such as water and carbon
dioxide (CO 2 ) budgets. At present, there remains no general agreement as to the
causes of the bias, or what, if anything, to do in response.
EC estimates of heat and water evaporation are a standard for characterizing surface energy fluxes over diverse ecosystems. These measurements are often used in
conjunction with remote sensing experiments to serve as validation points for estimating heat fluxes and evaporation rates at varying spatial scales. This study focuses
on EC measurements in dry land and irrigated agriculture surfaces in a semiarid
region of Texas, USA, where extreme events of saturation deficit advection occur
Multiscale Hydrologic Remote Sensing: Perspectives and Applications
5.1 INTRODUCTION
Where human populations exist, water and its proper management remain a critical
and important issue across diverse regions of the world. This is particularly acute in
semiarid regions. The proper allocation and management of limited water resources
in locations that are vast in spatial extent necessitate important surface and meteorological information in order to accurately estimate evapotranspiration (ET) at
relevant spatial and temporal resolutions. The need for an accurate accounting of
consumptive water use continues to dominate ET research, largely because in many
regions throughout the world, available water resources are insufficient to meet all
water use demands. Thus, an accurate accounting of consumptive water use (or ET)
through evaporation (soil and/or plant surfaces) is indispensible (Brutsaert 1982) and
is at the core of many hydrologic studies.
ET is a complicated and important component of the hydrologic water balance
or water cycle. The difficulty of estimating ET arises from the physical and chemical interactions that exist among the soil, vegetation, and surface–boundary layer
meteorological continuum. The diversity of soils, vegetation types in both native and
modern agricultural systems, and varying local meteorological conditions present
unique challenges to quantifying partitioning of incident thermal radiation into the
components of the surface energy balance (SEB) to ultimately estimate ET at spatial
and temporal scales that are commensurate with water management needs.
Remote sensing offers the opportunity to capture critical surface information
that can be processed into regional estimates of sensible heat from which ET can
then be computed as a residual of a regional SEB. Critical to supporting regionalscale attempts to estimate ET through remote sensing algorithms is the ability to
validate remotely based estimates of ET with sound physically based measurements. One component that is a complicating factor for semiarid regions is advection of saturation deficit over irrigated fields. The term “saturation deficit” means
warm dry air is advected from hot dry surfaces to cool wetter surfaces that are
under irrigation.
A direct approach over semiarid surfaces is problematic with eddy covariance
(EC) measurements under certain conditions. However, one approach that offers a
measure of self-consistency is to examine the energy balance closure values, defined
as the ratio of turbulence energy fluxes over available energy (Xiao et al. 2011). As
reported data continue to grow over a range of surfaces and conditions, it is clear
that there is a systematic bias in EC flux estimates and the range of energy balance
closure values can be large and variable at any given location (Xiao et al. 2011). The
implications for this bias are compelling for specific issues such as water and carbon
dioxide (CO 2 ) budgets. At present, there remains no general agreement as to the
causes of the bias, or what, if anything, to do in response.
EC estimates of heat and water evaporation are a standard for characterizing surface energy fluxes over diverse ecosystems. These measurements are often used in
conjunction with remote sensing experiments to serve as validation points for estimating heat fluxes and evaporation rates at varying spatial scales. This study focuses
on EC measurements in dry land and irrigated agriculture surfaces in a semiarid
region of Texas, USA, where extreme events of saturation deficit advection occur