▪ Abstract The single-point statistics of turbulence in the ‘roughness sub-layer’ occupied by the plant canopy and the air layer just above it differ significantly from those in the surface layer. The mean velocity profile is inflected, second moments are strongly inhomogeneous with height, skewnesses are large, and second-moment budgets are far from local equilibrium. Velocity moments scale with single length and time scales throughout the layer rather than depending on height. Large coherent structures control turbulence dynamics. Sweeps rather than ejections dominate eddy fluxes and a typical large eddy consists of a pair of counter-rotating streamwise vortices, the downdraft between the vortex pair generating the sweep. Comparison with the statistics and instability modes of the plane mixing layer shows that the latter rather than the boundary layer is the appropriate model for canopy flow and that the dominant large eddies are the result of an inviscid instability of the inflected mean velocity profi...

Turbulence in plant canopies

This paper argues that the active turbulence and coherent motions near the top of a vegetation canopy are patterned on a plane mixing layer, because of instabilities associated with the characteristic strong inflection in the mean velocity profile. Mixing-layer turbulence, formed around the inflectional mean velocity profile which develops between two coflowing streams of different velocities, differs in several ways from turbulence in a surface layer. Through these differences, the mixing-layer analogy provides an explanation for many of the observed distinctive features of canopy turbulence. These include: (a) ratios between components of the Reynolds stress tensor; (b) the ratio K H /K M of the eddy diffusivities for heat and momentum; (c) the relative roles of ejections and sweeps; (d) the behaviour of the turbulent energy balance, particularly the major role of turbulent transport; and (e) the behaviour of the turbulent length scales of the active coherent motions (the dominant eddies responsible for vertical transfer near the top of the canopy). It is predicted that these length scales are controlled by the shear length scale L s = U(h)/U′(h) (where h is canopy height, U(z) is mean velocity as a function of height z, and U′ = dU/dz). In particular, the streamwise spacing of the dominant canopy eddies Λ x = mL s , with m = 8.1. These predictions are tested against many sets of field and wind-tunnel data. We propose a picture of canopy turbulence in which eddies associated with inflectional instabilities are modulated by larger-scale, inactive turbulence, which is quasi-horizontal on the scale of the canopy.

Coherent eddies and turbulence in vegetation canopies: The mixing-layer analogy

The ever-increasing demand and competition for the finite water resource worldwide call for more efficient use of water in all sectors, including firstly agricultural food production. One important consideration is the existence of a limit to the amount of biomass a crop can produce per unit of water consumed. This article analyzes the theoretical background and the experimental evidence for the conservative behavior of the efficiency in water use by crops to produce biomass, i.e., biomass water productivity (WPb), under variable environmental conditions. Particularly, WPb is approximately constant for a given crop species after normalization for evaporative demand of the atmosphere and air carbon dioxide concentration. A stepwise scaling up approach, from leaf to canopy, is undertaken to underline the processes involved at the different hierarchical levels of biological organization that lead to the conservative behavior of WPb. Starting at the leaf level, the basic gas exchange equations are outlined to demonstrate that the normalized photosynthetic WPb at the leaf scale is proportional to the ambient CO2 concentration. New experimental evidence in support of that conclusion is presented for several C3 and a C4 crops. Additional factors are introduced to assess photosynthetic WPb at the canopy scale, including the extent of radiation capture and the role of respiration. The composition of biomass was then considered in the analysis of WPb over a season. The paper highlights the need to normalize WPb for differences in climate, specifically, in evaporative demand of the atmosphere to extrapolate WPb values between climatic zones, and in atmospheric CO2 concentration to account for changes in CO2 with time, when looking at the past and into the future. Two procedures for normalization for differences in evaporative demand are presented, and a procedure for normalization for changes in CO2 concentration is derived for the leaf scale and shown to be applicable to canopy scale. Some knowledge gaps and research needs are pointed out and the potential offered by the near constancy of normalized WPb in crop simulation modeling is emphasized.

/pdf/on-the-conservative-behavior-of-biomass-water-productivity-4r39037xtw.pdf

On the conservative behavior of biomass water productivity

linear relations with measurements of solar irradiance (r 2 = 0.95, relative bias: 8%), gross primary productivity (r 2 = 0.86, relative bias: 5%) and evapotranspiration (r 2 = 0.86, relative bias: 15%) in data from 33 flux towers that cover seven plant functional types across arctic to tropical climatic zones. A sensitivity analysis revealed that the gross primary productivity and evapotranspiration computed in BESS were most sensitive to leaf area index and solar irradiance, respectively. We quantified the mean global terrestrial estimates of gross primary productivity and evapotranpiration between 2001 and 2003 as 118 � 26 PgC yr � 1 and 500 � 104 mm yr � 1 (equivalent to 63,000 � 13,100 km 3 yr � 1 ), respectively. BESS-derived gross primary productivity and evapotranspiration estimates were consistent with the estimates from independent machine-learning, data-driven products, but the process-oriented structure has the advantage of diagnosing sensitivity of mechanisms. The process-based BESS is able to offer gridded biophysical variables everywhere from local to the total global land scales with an 8-day interval over multiple years.

/pdf/integration-of-modis-land-and-atmosphere-products-with-a-1b757hea7j.pdf

Integration of MODIS land and atmosphere products with a coupled-process model to estimate gross primary productivity and evapotranspiration from 1 km to global scales

Within the past decade or so, the accuracy of evapotranspiration ( ET ) estimates has improved due to new and increasingly sophisticated methods. Yet despite a plethora of choices concerning methods, estimation of wetland ET remains insufficiently characterized due to the complexity of surface characteristics and the diversity of wetland types. In this review, we present models and micrometeorological methods that have been used to estimate wetland ET and discuss their suitability for particular wetland types. Hydrological, soil monitoring and lysimetric methods to determine ET are not discussed. 
Our review shows that, due to the variability and complexity of wetlands, there is no single approach that is the best for estimating wetland ET . Furthermore, there is no single foolproof method to obtain an accurate, independent measure of wetland ET . Because all of the methods reviewed, with the exception of eddy covariance and LIDAR, require measurements of net radiation ( R n ) and soil heat flux ( G ), highly accurate measurements of these energy components are key to improving measurements of wetland ET . 
Many of the major methods used to determine ET can be applied successfully to wetlands of uniform vegetation and adequate fetch, however, certain caveats apply. For example, with accurate R n and G data and small Bowen ratio (β) values, the Bowen ratio energy balance method can give accurate estimates of wetland ET . However, large errors in latent heat flux density can occur near sunrise and sunset when the Bowen ratio β≈-1·0. The eddy covariance method provides a direct measurement of latent heat flux density ( E ) and sensible heat flux density ( H ), yet this method requires considerable expertise and expensive instrumentation to implement. A clear advantage of using the eddy covariance method is that E can be compared with R n -G-H , thereby allowing for an independent test of accuracy. The surface renewal method is inexpensive to replicate and, therefore, shows particular promise for characterizing variability in ET as a result of spatial heterogeneity. LIDAR is another method that has special utility in a heterogeneous wetland environment, because it provides an integrated value for ET from a surface. The main drawback of LIDAR is the high cost of equipment and the need for an independent ET measure to assess accuracy. If R n and G are measured accurately, the Priestley-Taylor equation can be used successfully with site-specific calibration factors to estimate wetland ET . The 'crop' cover coefficient ( K c ) method can provide accurate wetland ET estimates if calibrated for the environmental and climatic characteristics of a particular area. More complicated equations such as the Penman and Penman-Monteith equations also can be used to estimate wetland ET , but surface variability and lack of information on aerodynamic and surface resistances make use of such equations somewhat questionable.

Weigang Gao

Papers

Applications of solutions to non-linear energy budget equations

Detection of temperature ramps and flow structures at a deciduous forest site