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transpiration measurements were not made in either the alfalfa or the rangeland experiments, the results from 
these comparisons do not validate but simply demonstrate the feasibility of our new concept Estimates of daily 
transpiration can be compared with measurements of évapotranspiration to determine if the results are reasonable. 
3.1 Alfalfa Experiment 
Alfalfa ( Medicago sativa, L.) was planted in 18 plots in an 80m X60m experimental field. After one year's 
growth, the 18 plots were flood irrigated at different intervals to vary levels of water stress and biomass. Four 
different irrigation regimes were applied to the alfalfa: the WET treatment received two irrigations between 
cuttings; the EARLY treatment was irrigated once, immediately after harvest; the LATE treatment received 
water midway between cuttings, and the DRY treatment received no supplementary water by irrigation from one 
harvest until the next. The growth interval between harvests ranged from 3.5 weeks in the summer to 9 weeks 
during winter. 
Crop canopy reflectances were measured using handhe ld radiometers (Exotech MSS and Bames MMR) 
with a 15" field-of-view. The radiometers had s imil ar spectral bands to those of the Landsat Multi spectral 
Scanner (MSS) and the Landsat Thematic Mapper, respectively. Multispectral observations were made several 
times a week at a mo rning time period. MMR data were acquired at 10:30 Mountain Standard Time to coincide 
with the time of the Landsat overpass. Canopy surface temperatures were measured with an infrared thermometer 
(Everest). Evapotranspiration rates were measured with lm x lm x 1.5m weighing lysimeters in three different 
plots. Throughout the experiment, the lysimeters were irrigated by hand at the same times and with the same 
amounts of water as the surrounding plots. A multitude of other meteorological parameters were monitored 
every 30 min during the experiment, including air temperature, wind speed, reflected and incoming solar 
radiation, canopy and soil temperatures, and vapor pressure. 
3.2 Rangeland Experiment 
The Walnut Gulch '92 (WG'92) experiment was conducted in a semi arid rangeland over an entire growing 
season from April to November 1992, with a variety of soil moisture conditions and vegetation densities. The 
size of the main study site was about 50 km , at the eight locations of which both general meteorological 
conditions and surface energy balance were measured. Data used here were from a grassland site (Kendall). The 
spectral data used here are based on ground-based measurements using yoke-mounted Exotech MSS and Bames 
MMR radiometers and infrared thermometers (Everest). On five line transects of 30 m each, vegetation 
parameters such as percent cover, average plant height, and volume by species, total LAI and biomass, litter 
standing and lying, and plant water content were measured. Evapotranspiration flux densities were measured 
with eddy-correlation apparatus. 
4. RESULTS AND DISCUSSION 
4.1 Alfalfa Experiment 
4.1.1 Single Harvest Period During a harvest penod from the day of year (DOY) 154 to 184, when clear-sky 
conditions persisted for 28 of 30 days and reflectance and temperature measurements were obtained nearly every 
day, two lysimeters were subjected to EARLY and WET irrigation treatments, respectively. Results presented 
here are for spectral and meteorological measurements centered cm 10:30 each day. 
The values of a in Eq. 7 was computed based on evaluation of the relation between SAVI at 10:30, Rs 
and ET, (measured using lysimeter) for full-cover and well-watered canopies on clear days. Under these 
circumstances, we can assume that Tr = ET,. The a value for alfalfa was computed to be 1.50. Based on this 
value of a and the CWSI calculated using surface temperature measurements, the values of Tr, were computed 
for all treatment plots. The values of Tr, for the EARLY and WET plots correspond as expected with the ET, 
values measured by lysimeters in each plot (Fig. 1). That is, Tr, is substantially lower than ET, when 
vegetation density is low (DOY 158-170), and Tr t is nearly equal to ET, for full-cover canopy. Furthermore, the 
trend of decreasing ET, with time in the EARLY treatment is reflected in the Tr, estimates. The extremely low 
ET, values observed in the EARLY plot for DOY 179-183 was caused presumably by some unusual condition of 
the lysimeter. 
Estimates of Tr, for the eight treatments (two replicates of WET, EARLY, LATE, and DRY) were also 
reasonable Fig.2 shows one of two replicates. That is, the Tr, in the WET ¡dots remained high throughout the 
harvest cycle; Tr, in the EARLY plots was similar to the WET plots early in the cycle and decreased late in the