158
M. Menenti
of Eagleson (1982) on land surface processes at spatial scales from local to global.
At the time (since the early 70's) there was already a widespread awareness of the
sensitivity of atmospheric circulation to water available at land surfaces, as well as
a growing body of work on detailed modeling of processes controlling water flow
in the soil-plant-atmospheric continuum (Fritschen, 1982). As a matter of fact today's concept of Soil-Vegetation-Atmosphere-Transfer (SV A T) models had seen
light already with Saxton et al. (1974) who described a Soil-Plant-AtmosphereWater (SPAW) model.
Advances have been significant since those early stages. There seem to remain
several outstanding issues, however. First and foremost the accuracy of both observations and models is moderate even under the well defined conditions of a
field experiment. Absolute error in direct measurements of the instantaneous latent
heat flux with eddy correlation devices is of the order of 30 Wm- 2 (Lagouarde et
aI., 1996), while the error on daily evaporation calculated with numerical SVAT
models is of the order of 1 mmd- ' ,or a mean latent heat flux of30 Wm- 2 (Ottle et
aI., 1996). Moreover, what is required for improved understanding of environmental processes and for practical applications, such as irrigation water management, is spatial patterns of evaporation rather than local values. Direct measurements of evaporation require complex and costly devices such as eddy correlation
systems or weighing lysimeters. Recent international experiments, such as EFEDA
(Bolle et aI., 1993) have seen an unprecedented deployment of eddy correlation
devices. This notwithstanding, determination of spatial patterns of evaporation was
only possible with additional remote sensing data (Pelgrum and Bastiaanssen,
1996). Numerical SV AT models have also been applied to produce spatial fields
of evaporation (Harding et aI., 1996) after calibration at five sites where eddy correlation measurements were available. No indication was given on the accuracy of
the spatial patterns.
8.1.2 Remote sensing of land evaporation
Evaporation cannot be measured directly by means of spectral radiometric observations. The latter provide, however, information on atmosphere and land surface
conditions useful to estimate evaporation, although concurrent ancillary data remain necessary. On the other hand, use of the spatial measurements of radiances,
obtained with instruments on board satellites and aircrafts, is attractive precisely
because of the need to determine spatial patterns of evaporation at heterogeneous
land surfaces. The last few years have witnessed a renewed interest in this specialised area of remote sensing. Estimates of areal heat fluxes and of the land surface
variables which control· fluxes provide unique proxy data to study theoretical and
modelling aspects of spatial variability (Beven and Fisher, 1996).
Two fundamental concepts need to be distinguished when dealing with evaporation of land surfaces (Fig. 8.1):
A. Potential evaporation Eo
B. Actual evaporation Ea
M. Menenti
of Eagleson (1982) on land surface processes at spatial scales from local to global.
At the time (since the early 70's) there was already a widespread awareness of the
sensitivity of atmospheric circulation to water available at land surfaces, as well as
a growing body of work on detailed modeling of processes controlling water flow
in the soil-plant-atmospheric continuum (Fritschen, 1982). As a matter of fact today's concept of Soil-Vegetation-Atmosphere-Transfer (SV A T) models had seen
light already with Saxton et al. (1974) who described a Soil-Plant-AtmosphereWater (SPAW) model.
Advances have been significant since those early stages. There seem to remain
several outstanding issues, however. First and foremost the accuracy of both observations and models is moderate even under the well defined conditions of a
field experiment. Absolute error in direct measurements of the instantaneous latent
heat flux with eddy correlation devices is of the order of 30 Wm- 2 (Lagouarde et
aI., 1996), while the error on daily evaporation calculated with numerical SVAT
models is of the order of 1 mmd- ' ,or a mean latent heat flux of30 Wm- 2 (Ottle et
aI., 1996). Moreover, what is required for improved understanding of environmental processes and for practical applications, such as irrigation water management, is spatial patterns of evaporation rather than local values. Direct measurements of evaporation require complex and costly devices such as eddy correlation
systems or weighing lysimeters. Recent international experiments, such as EFEDA
(Bolle et aI., 1993) have seen an unprecedented deployment of eddy correlation
devices. This notwithstanding, determination of spatial patterns of evaporation was
only possible with additional remote sensing data (Pelgrum and Bastiaanssen,
1996). Numerical SV AT models have also been applied to produce spatial fields
of evaporation (Harding et aI., 1996) after calibration at five sites where eddy correlation measurements were available. No indication was given on the accuracy of
the spatial patterns.
8.1.2 Remote sensing of land evaporation
Evaporation cannot be measured directly by means of spectral radiometric observations. The latter provide, however, information on atmosphere and land surface
conditions useful to estimate evaporation, although concurrent ancillary data remain necessary. On the other hand, use of the spatial measurements of radiances,
obtained with instruments on board satellites and aircrafts, is attractive precisely
because of the need to determine spatial patterns of evaporation at heterogeneous
land surfaces. The last few years have witnessed a renewed interest in this specialised area of remote sensing. Estimates of areal heat fluxes and of the land surface
variables which control· fluxes provide unique proxy data to study theoretical and
modelling aspects of spatial variability (Beven and Fisher, 1996).
Two fundamental concepts need to be distinguished when dealing with evaporation of land surfaces (Fig. 8.1):
A. Potential evaporation Eo
B. Actual evaporation Ea