Agricultural and Forest Meteorology 104 2000 233–249
Sourcesink distributions of heat, water vapour, carbon dioxide and methane in a rice canopy estimated using Lagrangian
dispersion analysis
R. Leuning
a,∗
, O.T. Denmead
a
, A. Miyata
b
, J. Kim
c
a
CSIRO Land and Water, P.O. Box 1666, Canberra, ACT 2601, Australia
b
National Institute of Agro-Environmental Research, Tsukuba, Ibaraki 305-8604, Japan
c
Department of Atmospheric Science, Yonsei University, Seoul 120-749, South Korea Received 5 January 1999; received in revised form 1 June 1999; accepted 20 March 2000
Abstract
Source distributions for heat, water vapour, CO
2
and CH
4
within a rice canopy were derived using measured concentration profiles, a prescribed turbulence field and an inverse Lagrangian analysis of turbulent dispersion of scalars in plant canopies.
Measurements were made during IREX96, an international rice experiment in Okayama, Japan. Results for the cumulative fluxes of heat, water vapour and CH
4
at the canopy top were satisfactory once their respective concentration profiles were smoothed using simple analytic functions. According to the inverse analysis, water vapour was emitted relatively uniformly by
each of five equi-spaced layers within the canopy, whereas sensible heat fluxes were small 100 W m
− 2
and of either sign. Methane fluxes were predicted to be emitted most strongly in the lower 50 of the canopy, as expected from the distribution
of micropores along leaves and leaf sheaths, the major pathway for CH
4
loss from the soil–crop system. No smoothing was required for CO
2
concentration profiles and the inverse analysis provided close correspondence between the turning point in the concentration profile is the changeover from respiration by the soilpaddy water and lower canopy to net photosynthesis by
the upper canopy. These results could only be obtained by including both the near- and far-field contributions of sources to the total concentration profile. Neglect of the near-field contribution in the inverse analysis led to spurious source distributions.
Excellent agreement was obtained between cumulative fluxes of heat, water vapour, CO
2
and CH
4
at the top of the canopy from the inverse analysis and direct eddy covariance measurements when the friction velocity u
∗
0.1 m s
− 1
, and atmospheric stability was approximately neutral. Nocturnal fluxes of CO
2
and CH
4
from the inverse method exceeded micrometeorological measurements above the canopy by a factor of 2–3 when u
∗
0.1 m s
− 1
and stable atmospheric conditions prevailed within and above the canopy. Neglect of these stability effects will lead to an underestimate of the dispersion coefficients dimension
of resistances in the transport model and hence an overestimate of the fluxes. Further work is required to establish the correct procedure for incorporating stability effects into the inverse analysis. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Lagrangian dispersion analysis; Plant canopy sourcesink distributions; Rice
∗
Corresponding author.
1. Introduction
Information on the partitioning of sources and sinks of quantities such as heat, water vapour, CO
2
and CH
4
within plant canopies and the underlying
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 5 8 - 1
234 R. Leuning et al. Agricultural and Forest Meteorology 104 2000 233–249
soil is required for many practical problems. For example: 1 researchers concerned with water use
efficiency of crops Condon et al., 1990 need to separate transpiration from soil evaporation when
assessing the effectiveness of various breeding strate- gies, such as selection of varieties for early canopy
closure; 2 sourcesink distributions of sensible heat, water vapour and CO
2
within the canopy and at the soil surface are required to validate multi-layer
soil–vegetation–atmosphere-transfer models
e.g. Wang and Jarvis, 1990; Baldocchi and Harley, 1995;
Leuning et al., 1995; 3 separate contributions of soil and plants to net ecosystem carbon exchanges are
needed because soils and plants respond differently to such factors as temperature, moisture and rising
atmospheric CO
2
concentrations Wang and Pol- glase, 1995, with strong implications for response of
ecosystems to climate change Schimel et al., 1994; and 4 rice cultivation is one of the major biogenic
sources of atmospheric CH
4
Houghton et al., 1995. Methane is produced anaerobically by bacteria in the
soil and the CH
4
is transported to the atmosphere by bubble formation, water–air–gas exchange, and
transport through the aerenchyma of the rice plants Nouchi, 1994. Rice cultivation is one of the few
biogenic sources where management of methane emissions is possible Sass, 1994, and accurate esti-
mates of the relative contributions of CH
4
fluxes from rice plants and from paddy water will contribute to
the design of effective emission control strategies. Various techniques exist for partitioning fluxes
between plants and soil. Most current estimates of CO
2
emission from soils and CH
4
emissions from soils and vegetation are based on chamber studies
Denmead, 1979; Neue et al., 1994. Advantages of chambers for trace gas fluxes include the ability to
measure very small fluxes and to examine emissions as a function of soil temperature, oxygen, nitrogen
and carbon levels and microbial activity. However, chambers alter the microclimate, possibly causing
systematic errors in CO
2
and CH
4
flux measurements Denmead, 1994. Chambers are totally unsuitable
for measuring soil evaporation because they alter the humidity deficit compared to undisturbed conditions.
In this case, alternatives such as miniature lysimeters may be used e.g. Leuning et al., 1994.
The distribution of sourcesink strengths within canopies can also be measured using micrometeoro-
logical methods such as eddy covariance and eddy accumulation see review by Denmead, 1994. While
these approaches are suitable for use within tall canopies such as corn Wilson et al., 1982 and forests
Denmead and Bradley, 1987, they have been used relatively infrequently within short, dense canopies
such as rice, because current instruments are too bulky. This may cause errors in flux measurements
due to loss of covariance resulting from path averag- ing and instrument separation Moore, 1986; Leuning
and Judd, 1996, and to vertical flux divergence across the space occupied by the instruments.
In addition to the eddy covariance and eddy ac- cumulation methods, fluxes above canopies can been
measured using flux-gradient techniques, but this approach is not applicable within the canopy where
there is no simple relationship between fluxes and the local scalar gradient Denmead and Bradley,
1985; Finnigan and Raupach, 1987; Raupach, 1987. This is unfortunate because concentration gradients
within canopies are often large and relatively easy to measure. Raupach 1987 developed a Lagrangian
‘Localised Near Field’ LNF theory of dispersion in plant canopies which showed that the scalar con-
centration at a point within the canopy is a result of contributions from both local and distant sources;
hence the non-local relationship between fluxes and gradients.
In its inverse form Raupach, 1989a,b, the theory does allow us to deduce sourcesink strengths from
measured concentration profiles and knowledge of the turbulence field within the canopy. The inverse theory
has been used to quantify the sources and sinks of water vapour and CO
2
in wheat and sugarcane crops Denmead and Raupach, 1993; Denmead, 1995 and
for CO
2
in a loblolly pine forest Katul et al., 1997, but before the new technique can be applied routinely
to a variety of ecosystems, its strengths and limitations should be carefully examined.
The aim of this study was to evaluate the perfor- mance of the inverse Lagrangian method for inferring
net fluxes and source strengths of heat, water vapour, CO
2
and CH
4
in a rice canopy. This was done by making measurements of the profiles of the various
scalar concentrations and vertical velocity statistics as required for the inverse analysis and comparing the
inferred cumulative fluxes at the top of the canopy with micrometeorological measurements of the fluxes
R. Leuning et al. Agricultural and Forest Meteorology 104 2000 233–249 235
above the canopy as reported in a companion paper by Miyata et al. 2000.
2. Theory
