Abstract:
The energy budget of a pumice desert surface was analyzed under
clear skies during early, mid- and late summer periods. The pumice
site is in the semi-arid plateau region of Central Oregon at an elevation
of about 1500 meters. The flat pumice surface is approximately
250 hectares in extent, and is bordered by a sparse lodgepole pine
forest. Energy budget components of net radiation, soil heat flux,
sensible heat flux, and latent heat flux were evaluated for one clear
day in each of the three measurement periods.
The daily energy budget totals were (cal/cm2 day):
The most significant features of the pumice desert energy budget
were: 1) Radiant energy transformed by the pumice surface (net
17 July 1969 13 August 1969 4 September 1969
228 194 Net radiatiOn 258
Soil heat flux -7 -14 -2
Sensible heat flux -197 -197 -180
Latent heat flux -54 -17 -11
radiation) was approximately 60 percent of the amount measured over
a nearby forested surface; 2) Energy transfer into the soil amounted
to less than 3 percent of the energy supplied to the surface by net
radiation, while surface temperatures varied through a 50°C range each
day; 3) Sensible heat flux dissipated 85 percent of the net radiation
supplied to the surface; and, 4) Evaporation at the pumice site
averaged less than 0. 05 cm per day, although the pumice beneath the
dry surface layer remained moist.
A unique stability correction, φ , for the aerodynamic flux
analysis of sensible or latent heat was developed to extend over the
wide stability range found at the pumice site. The form of this correction
during unstable conditions is:
φ= (1-34Ri).55
where Ri is Richardson's stability parameter.
A method for estimating the uncertainty of the measurement
system and of the resultant flux analyses was developed and applied to
the results of this study. The average relative uncertainties of the
net radiation and soil heat flux analyses were estimated to be less
than 1 percent and 5 percent, respectively. The average uncertainty
of the sensible heat flux analyses was estimated to be 3 percent when
using an aerodynamic model, and 9 percent when using the Bowen
ratio model. The corresponding figures for latent heat flux are 25
percent with the aerodynamic model and 30 percent with the Bowen
ratio model. The larger percentage uncertainties associated with
latent heat are due in part to the small vapor pressure gradients near
the pumice surface, relative to the measurement capabilities, and in
part to the small values of the latent heat flux.
This study demonstrates the applicability of micrometeorological
theory in characterizing complex microclimatological relationships by
presenting them in a concise, comparable form through use of the
energy budget.
