13. A BIOPHYSICAL PROCESS-BASED ESTIMATE OF GLOBAL
LAND SURFACE EVAPORATION USING SATELLITE AND
ANCILLARY DATA
121
5. Quantitative comparisons with micro-meteorological, catchment water
balance, soil moisture, and atmospheric water budget data have provided
a measure of uncertainty for the calculated results at a range of spatial
and temporal scales, which is not evident in previous estimates.
2. DESCRIPTION OF THE MODEL AND DATA
A brief description of the model and the data is presented here due to
space limitation; details can be found in Choudhury and DiGirolamo (1998).
Validation of the model against observations (micrometeorological and soil
moisture measurements, catchment water balance, and atmospheric water
budget analyses) is also given in Choudhury and DiGirolamo (1998).
Transpiration has been calculated using the Penman-Monteith equation,
with the rate of carbon assimilation determining the canopy stomatal
resistance. The rate of carbon assimilation by the canopy depends upon
interception of photosynthetically active radiation, the maximum rate of
assimilation by the leaves, and the efficiency of quantum absorption as
determined by temperature. The actual transpiration depends upon the
available soil moisture and the fractional duration when the foliage surface is
dry. Soil evaporation is considered to occur in two stages; the PriestleyTaylor equation adjusted for fractional vegetation cover was used for the
first stage (i.e., the energy-limited rate), while Philip’s equation is used for
the second stage (i.e., exfiltration-limited rate). The exfiltration-limited rate
is not allowed to exceed the energy-limited rate. Interception has been
calculated using the Horton’s equation, adjusted for fractional vegetation
cover. A daily water balance model, which includes surface runoff and
drainage, together with snow accumulation, evaporation and melt, was run at
0.25° × 0.25° spatial resolution over the global land surface using satellite
and ancillary data. Net radiation and sensible heat flux are obtained by
solving the energy balance equation.
Satellite observations are used to obtain the fractional vegetation cover,
albedo, photosynthetically active and solar radiation, air temperature and
vapor pressure. The friction velocity is derived from a four-dimensional data
assimilation procedure, while precipitation values are derived from gauge
and satellite measurements. The spatial resolution and sensors (or sources) of
these data are given in Table 1, where correspondence with the future
sensors is also noted. Choudhury (1997) has assessed the accuracy of some
of these data. The monthly total precipitation data has been disaggregated to
obtain the daily values. The spatial distribution of biophysical parameters of
LAND SURFACE EVAPORATION USING SATELLITE AND
ANCILLARY DATA
121
5. Quantitative comparisons with micro-meteorological, catchment water
balance, soil moisture, and atmospheric water budget data have provided
a measure of uncertainty for the calculated results at a range of spatial
and temporal scales, which is not evident in previous estimates.
2. DESCRIPTION OF THE MODEL AND DATA
A brief description of the model and the data is presented here due to
space limitation; details can be found in Choudhury and DiGirolamo (1998).
Validation of the model against observations (micrometeorological and soil
moisture measurements, catchment water balance, and atmospheric water
budget analyses) is also given in Choudhury and DiGirolamo (1998).
Transpiration has been calculated using the Penman-Monteith equation,
with the rate of carbon assimilation determining the canopy stomatal
resistance. The rate of carbon assimilation by the canopy depends upon
interception of photosynthetically active radiation, the maximum rate of
assimilation by the leaves, and the efficiency of quantum absorption as
determined by temperature. The actual transpiration depends upon the
available soil moisture and the fractional duration when the foliage surface is
dry. Soil evaporation is considered to occur in two stages; the PriestleyTaylor equation adjusted for fractional vegetation cover was used for the
first stage (i.e., the energy-limited rate), while Philip’s equation is used for
the second stage (i.e., exfiltration-limited rate). The exfiltration-limited rate
is not allowed to exceed the energy-limited rate. Interception has been
calculated using the Horton’s equation, adjusted for fractional vegetation
cover. A daily water balance model, which includes surface runoff and
drainage, together with snow accumulation, evaporation and melt, was run at
0.25° × 0.25° spatial resolution over the global land surface using satellite
and ancillary data. Net radiation and sensible heat flux are obtained by
solving the energy balance equation.
Satellite observations are used to obtain the fractional vegetation cover,
albedo, photosynthetically active and solar radiation, air temperature and
vapor pressure. The friction velocity is derived from a four-dimensional data
assimilation procedure, while precipitation values are derived from gauge
and satellite measurements. The spatial resolution and sensors (or sources) of
these data are given in Table 1, where correspondence with the future
sensors is also noted. Choudhury (1997) has assessed the accuracy of some
of these data. The monthly total precipitation data has been disaggregated to
obtain the daily values. The spatial distribution of biophysical parameters of