Agricultural and Forest Meteorology 104 2000 171–183
Estimating sensible and latent heat flux densities from grapevine canopies using surface renewal
D. Spano
a,∗
, R.L. Snyder
b
, P. Duce
c
, K.T. Paw U
b
a
Universita’ della Basilicata, Dip. di Produzione Vegetale, Facolta’ di Agraria, I-85100 Potenza, Italy
b
University of California, Atmospheric Science, Davis, CA 95616, USA
c
Consiglio Nazionale delle Ricerche, IMAes, I-07100 Sassari, Italy Received 26 October 1999; received in revised form 23 May 2000; accepted 7 June 2000
Abstract
Fine-wire thermocouples were used to measure high-frequency temperature above and within canopies and structure functions were employed to determine temperature ramp characteristics, which were used in a fundamental conservation of
energy equation to estimate sensible heat flux density. Earlier experiments over dense, tall, and short canopies demonstrated that the surface renewal method works, but requires a correction for uneven heating e.g. α=0.5 for tall, and α=1.0 for short
canopies. For sparse canopies, the α calibration factor was unknown. Experiments were conducted in grape vineyards in California and Italy to determine whether the surface renewal method works in a sparse canopy and to determine if calibration
is necessary. Surface renewal data were collected at several heights in the canopies and these were compared with simultaneous 1-D sonic anemometer measurements. The results indicated that the surface renewal technique provides good estimates of
sensible heat flux density under all stability conditions without the need for calibration when the data are measured at about 90 of the canopy height. The values were generally within ca. 45 W m
− 2
of what was measured with a sonic anemometer. Separating the canopy into two layers provided even more accurate estimates of sensible heat flux density without the need
for calibration. The best results were obtained when the lower layer was below the bottom of the vegetation and the upper layer included the vegetation. When combined with energy balance measurements of net radiation and soil heat flux density,
using a thermocouple and the surface renewal technique offers an inexpensive alternative for estimating evapotranspiration with good accuracy. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Energy balance; Eddy-covariance; Temperature ramps; Coherent structures
1. Introduction
Energy balance measurements over full canopies are common; however, little information is avail-
able on the energy balance over sparse grapevine canopies. Grape vineyards consist of widely spaced
∗
Corresponding author. Tel.: +39-079-229336; fax: +39-097-229337.
E-mail address: spanossmain.uniss.it D. Spano.
plants that allow for deep penetration of sunlight and air turbulence into the canopy. As a result, the soil
contribution to the energy balance is considerable. Because the radiative energy balance at the bottom
of the canopy is greater than in canopies that are more closed, management of the vineyard floor can
have a major effect on sensible H and latent λE heat flux density. This makes estimating crop evapo-
transpiration more complicated than for more dense canopies. Therefore, it is desirable to have a robust
0168-192300 – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 1 9 2 3 0 0 0 0 1 6 7 - 2
172 D. Spano et al. Agricultural and Forest Meteorology 104 2000 171–183
method for measuring crop evapotranspiration in situ.
Several researchers have investigated the determi- nation of evapotranspiration λE of grapevines using
energy balance. Ham et al. 1991 and Ham and Heil- man 1991 used a sap flow heat balance technique to
measure the transpiration of vines and a Bowen ratio system to estimate λE in a grape vineyard to separate
crop evapotranspiration into transpiration and soil evaporation components. Oliver and Sene 1992 used
eddy-covariance to measure λE over grapevines and they concluded that soil and vines are independent
energy systems with little interaction between them. However, Heilman et al. 1994 showed that sensible
heat from the exposed soil surface is a major contrib- utor to the canopy energy balance and transpiration
of the vines. They concluded that the soil and the canopy are not independent and that the soil energy
balance does affect energy balance of the vines. In a later study Heilman et al. 1996 showed that the
canopy architecture has a substantial effect on soil and canopy energy balance, mainly by changing the
partitioning of vineyard net radiation into its soil and canopy components. Trambouze et al. 1998 used
eddy-covariance to measure λE from vineyards. They reported good λE estimates with very small time
scale information, but they recommended against us- ing eddy-covariance for estimating actual evapotran-
spiration of a vineyard because of the maintenance requirements. Because equipment for the Bowen ra-
tio and eddy-covariance methods is expensive and requires a fairly high level of expertise to operate, a
less expensive and more robust method is desirable for determining evapotranspiration of agricultural
crops.
Tillman 1972 first reported the use of high- frequency temperature variance data to estimate H.
He obtained good results during unstable atmospheric conditions when the data were corrected for stabil-
ity. Later, Weaver 1990 used temperature variance data, similarity theory, and calibration coefficients
that vary depending on the surface and energy bal- ance to determine H over semi-arid grass and brush.
When corrected for stability, Lloyd et al. 1991 and De Bruin et al. 1993 also observed good estimates
of H using temperature-variance data collected over sparse, dry land vegetation, stones, pebbles, etc. In
all of these experiments, the method worked only under unstable conditions, and a stability parameter
was needed for the calculations. Paw U et al. 1995 studied the use of the variance method during stable
conditions and reported that it was inaccurate for data taken within a meter or two of the canopy height for a
maize crop, a walnut orchard, and a mixed deciduous forest. Clearly, the temperature variance method has
little utility for measuring H over canopies with high evaporation rates that are likely to have near neutral
atmospheric conditions close to the surface. Also, a temperature-based method that requires complicated
measurements for determining a stability parameter has little practical value. In recent years, the surface
renewal SR method has shown promise to provide estimates of H from high-frequency temperature data
regardless of and without a measure of stability con- ditions.
The objective of this study was to evaluate: 1 the SR method for estimating H in grape vineyards; 2
the possibility of estimating H without the a factor α=1.0 by separating the canopy volume into layers;
and 3 the accuracy of the SR method for determining λE as a residual term of the energy balance.
2. Theory
