781 
(40 32' N, 119 49' W) is at an altitude of about 1191 m. Vegetation at the site is about 90% 
ereasewood; the remaining 10% consists of shadscale (At riplex confertifolia) and hopsage 
(Grayia spinosa), growing in an area with shallow ground water. Greasewood shrubs have an 
average crown height of about 0.5 m; the plant density was about 23% and the shrubs Leaf 
Area Index was about 3 (on a scale of 0 to 4) in 1991. Railroad Valley is located in east- 
central Nevada. The field study site (38 30' N,115 46’ W) is at an altitude of about 1454 m. 
Vegetation at the site is greasewood with minor amounts of shadescale. The shrubs had an 
average crown height of about 0.5 m; the plant density was estimated to be about 20% and 
the greasewood LAI was estimated to be about 2.5 to 3 in 1992. 
Data collected at each site include incident and reflected short-wave radiation, 
incident and emitted long-wave radiation, air temperature and vapor pressure at two heights, 
wind speed at two heights, soil heat flux and soil temperature, soil surface temperature, and 
canopy temperature. These data were used to solve the energy budget equation using the 
Bowen ratio method. Net radiation was determined by summing the four measured 
components of the radiation budget. These components were measured with Eppley spectral 
pyranometers (PSP) and infrared radiometers (PIR). The PIR’s 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. 
Corrections in net long-wave radiation typically are on the order of about -8 to -12 W/m- 
dunng midday hours and about -4 to +6 W/m- at night. Vapor 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. Numerous problems associated with 
systematic sensor error are avoided by this system. However, operating limitations of the 
cooled mirror under conditions of high temperature and low humidity can lead to invalid 
vapor pressure data. These conditions occur when ambient temperature approaches 35 C and 
humidity drops below 10 %. This is a common occurrence in central Nevada in July and 
August. Air temperature was measured at the same two, heights, using 76 pm diameter 
unshielded, non aspirated chromel-constant thermocouples. Soil flux was measured with two 
Radiation Energy Balance Systems heat flow transducers buried at a depth of 0.05 m. 
Changes in heat storage in the soil layer above each transducer was measured by four 
thermocouples wired in parallel so as to provide a spatial average soil temperature. Two 
thermocouples were place above each transducer, one at a depth of 0.01 m and the other at 
0.03 m. The change in soil temperature measured by the thermocouples together with 
periodically measured soil-water content and soil bulk density and an estimated value for the 
specific heat of dry soil were used to calculate changes in soil heat storage in the interval 
above the flux transducers. This heat storage was added to the flux value measured by the 
transducer. Vapor pressure and air temperature were sampled at 1 second intervals and 
averaged over 1 hour periods. Radiation, soil heat flux, and soil temperature were sampled at 
10 seconds intervals and averaged over 20 minutes periods. In addition to these 
measurements soil surface temperature was measured using an Everest interscience 1R 
sensor, model 4000 with a 15 degrees filed of view, mounted 2 m above the soil surface. At 
this height the sensor views an area of bare soil about 0.45 m in diameter; areas of bare soil 
at the study site typically are as much as 3 m by 3 m. Canopy temperature was measured with 
an Everest Interscience IR sensor model 110 or model 4000 with a 3 degree field of view, 
also mounted about 30 cm above the top of the canopy. Wind speed was measured at 1.25 
and 2.25 m above the soil surface using photo-chopper anemometers with a threshold of 0.2