Télédétection et ressources en eau/Remote sensing and water resources
323
EXPERIMENTAL DATA
Location and site description
The Monsoon '90 multidisciplinary field campaign was conducted over the U.S. Department of
Agriculture's Agricultural Research Service Walnut Gulch experimental watershed in
southeastern Arizona (31° 43'N, 110°W) during the summer of 1990 (see Kustas et al., 1991,
and Kustas et Goodrich., 1994). The main objective of the experiment was to investigate the
potential of using multispectral remotely sensed data in conjunction with hydrological models to
quantify spatial and temporal variability of surface energy and water fluxes in arid and semi-arid
regions. The study sites were located in an area comprising the upper 150 km
2 of the Walnut
Gulch drainage basin and situated about 1300 m above mean sea level..
The annual precipitation in this region ranged from 250 to 500 mm/yr, with approximately
two thirds of the rainfall occurring in the monsoon season (July-August). The surface soil texture
was mainly sandy loam with high fraction of rocks. The vegetation in the western half of the
watershed was shrubs dominated, while the eastern half is grass dominated. The analysis in this
paper is limited to the shrub dominated site (Lucky Hills), where spatial heterogeneity between
shrub clumps and adjacent bare soil was higher (Kustas and Goodrich, 1994). The shrubs was
about 0.6 m height and covering about 0.26 % of the surface (see Weltz et al., 1994). The
surface leaf area index was about 0.3-0.4 which implies that the clump leaf area index of the
shrubs ranged between 1.15 and 1.54.
Surface measurements
The four components of the energy balance equation used in this study were measured using a 2
m tall roving eddy correlation tripod : ROVEC. Net radiation was measured with a REBS Q6 net
radiometer at a height of 1.6-1.7 m. The soil heat flux was measured using a soil heat flux plates
(MELCOR, CP 1.4-71-06-L). Latent and Sensible heat flux were measured with the eddy
correlation method, using a single axis sonic anemometer, a krypton hygrometer, and a 12.7 mmdiameter thermocouple, all manufactured by Campbell Scientific, Inc. Measurements of wind,
humidity and temperature fluctuations at 2 m height were made at 10 Hz, and 10 mn means were
used to compute covariances.
In addition to those measurements, the four components of the radiation budget were taken
with Eppley Precision Spectral Pyranometers (PSP) and Precision Infrared Radiometers (PIR).
The PIRs were equipped with thermistors to measure dome and case temperatures so that the
measured long-wave radiation could be corrected for temperature gradients between the dome and
case. Vapour pressure gradients were determined by measuring dew-point temperature at two
heights using Campbell Scientific, Inc. single-cooled-mirror hygrometer. The measuring heights
were 1.25 m and 2.25 meters above the soil surface. Air is alternately drawn through intakes at
each height and routed to the cooled mirror; a single pump aspirates the system. Air temperature
was measured at the same two, heights, using 76 mm diameter unshielded non-aspirated chromelconstantan thermocouples. In addition to these measurements, soil surface temperature was
measured using an Everest Interscience (IR) sensor, model 4000 with a 15° filed of view,
mounted 2 m above the soil surface. At this height the sensor viewed an area of bare soil about
0.45 m in diameter. Canopy temperature was measured with an Everest Interscience IR sensor
model 110 or model 4000 with a 3° field of view. It was mounted about 30 cm above the top of
the canopy. Since no measurement of the shaded soil temperature was made, it was assumed for
this analysis that shaded soil temperature is the average of the temperature of the unshaded soil
323
EXPERIMENTAL DATA
Location and site description
The Monsoon '90 multidisciplinary field campaign was conducted over the U.S. Department of
Agriculture's Agricultural Research Service Walnut Gulch experimental watershed in
southeastern Arizona (31° 43'N, 110°W) during the summer of 1990 (see Kustas et al., 1991,
and Kustas et Goodrich., 1994). The main objective of the experiment was to investigate the
potential of using multispectral remotely sensed data in conjunction with hydrological models to
quantify spatial and temporal variability of surface energy and water fluxes in arid and semi-arid
regions. The study sites were located in an area comprising the upper 150 km
2 of the Walnut
Gulch drainage basin and situated about 1300 m above mean sea level..
The annual precipitation in this region ranged from 250 to 500 mm/yr, with approximately
two thirds of the rainfall occurring in the monsoon season (July-August). The surface soil texture
was mainly sandy loam with high fraction of rocks. The vegetation in the western half of the
watershed was shrubs dominated, while the eastern half is grass dominated. The analysis in this
paper is limited to the shrub dominated site (Lucky Hills), where spatial heterogeneity between
shrub clumps and adjacent bare soil was higher (Kustas and Goodrich, 1994). The shrubs was
about 0.6 m height and covering about 0.26 % of the surface (see Weltz et al., 1994). The
surface leaf area index was about 0.3-0.4 which implies that the clump leaf area index of the
shrubs ranged between 1.15 and 1.54.
Surface measurements
The four components of the energy balance equation used in this study were measured using a 2
m tall roving eddy correlation tripod : ROVEC. Net radiation was measured with a REBS Q6 net
radiometer at a height of 1.6-1.7 m. The soil heat flux was measured using a soil heat flux plates
(MELCOR, CP 1.4-71-06-L). Latent and Sensible heat flux were measured with the eddy
correlation method, using a single axis sonic anemometer, a krypton hygrometer, and a 12.7 mmdiameter thermocouple, all manufactured by Campbell Scientific, Inc. Measurements of wind,
humidity and temperature fluctuations at 2 m height were made at 10 Hz, and 10 mn means were
used to compute covariances.
In addition to those measurements, the four components of the radiation budget were taken
with Eppley Precision Spectral Pyranometers (PSP) and Precision Infrared Radiometers (PIR).
The PIRs were equipped with thermistors to measure dome and case temperatures so that the
measured long-wave radiation could be corrected for temperature gradients between the dome and
case. Vapour pressure gradients were determined by measuring dew-point temperature at two
heights using Campbell Scientific, Inc. single-cooled-mirror hygrometer. The measuring heights
were 1.25 m and 2.25 meters above the soil surface. Air is alternately drawn through intakes at
each height and routed to the cooled mirror; a single pump aspirates the system. Air temperature
was measured at the same two, heights, using 76 mm diameter unshielded non-aspirated chromelconstantan thermocouples. In addition to these measurements, soil surface temperature was
measured using an Everest Interscience (IR) sensor, model 4000 with a 15° filed of view,
mounted 2 m above the soil surface. At this height the sensor viewed an area of bare soil about
0.45 m in diameter. Canopy temperature was measured with an Everest Interscience IR sensor
model 110 or model 4000 with a 3° field of view. It was mounted about 30 cm above the top of
the canopy. Since no measurement of the shaded soil temperature was made, it was assumed for
this analysis that shaded soil temperature is the average of the temperature of the unshaded soil