This is the first of a series of three papers describing experiments on the dispersion of trace heat from elevated line and plane sources within a model plant canopy in a wind tunnel. Here we consider the wind field and turbulence structure. The model canopy consisted of bluff elements 60 mm high and 10 mm wide in a diamond array with frontal area index 0.23; streamwise and vertical velocity components were measured with a special three-hot-wire anemometer designed for optimum performance in flows of high turbulence intensity. We found that:(i) The momentum flux due to spatial correlations between time-averaged streamwise and vertical velocity components (the dispersive flux) was negligible, at heights near and above the top of the canopy.(ii) In the turbulent energy budget, turbulent transport was a major loss (of about one-third of local production) near the top ofthe canopy, and was the principal gain mechanism lower down. Wake production was greater than shear production throughout the canopy. Pressure transport just above the canopy, inferred by difference, appeared to be a gain in approximate balance with the turbulent transport loss.(iii) In the shear stress budget, wake production was negligible. The role of turbulent transport was equivalent to that in the turbulent energy budget, though smaller.(iv) Velocity spectra above and within the canopy showed the dominance of large eddies occupying much of the boundary layer and moving downstream with a height-independent convection velocity. Within the canopy, much of the vertical but relatively little of the streamwise variance occurred at frequencies characteristic of wake turbulence.(v) Quadrant analysis of the shear stress showed only a slight excess of sweeps over ejections near the top of the canopy, in contrast with previous studies. This is a result of improved measurement techniques; it suggests some reappraisal of inferences previously drawn from quadrant analysis.

The Langevin equation is used to derive the Markov equation for the vertical velocity of a fluid particle moving in turbulent flow. It is shown that if the Eulerian velocity variance u,,,~ is not constant with height, there is an associated vertical pressure gradient which appears as a force-like term in the Markov equation. The correct form of the Markov equation is: w(t + At) = aw(t) + bu,,i + (1 -a)Tr.d(&)/dz, where w(t) is the vertical velocity at time t, [ a random number from a Gaussian distribution with zero mean and unit variance, r, the Lagrangian integral time scale for vertical velocity, a = exp( -AI/T,), and b = (1 -a*)'/'. This equation can be used for inhomogeneous turbulence in which the mean wind speed, crWwE and T, vary with height. A two-dimensional numerical simulation shows that when this equation is used, an initially uniform distribution of tracer remains uniform.

Automatic mobile shelters were used to keep rain off a barley crop in a drought experiment. The treatments ranged from no water during the growing season to regular weekly irrigation. This paper reports the effect of drought on the harvest yield and its components, on water use and nutrient uptake.Drought caused large decreases in yield, and affected each component of the grain yield. The magnitude of each component varied by up to 25 % between treatments, and much of the variation could be accounted for by linear regression against the mean soil water deficit in one of three periods. For the number of grains per ear, the relevant period included tillering and ear formation; for the number of ears per unit ground area, the period included stem extension and tiller death; for grain mass, the period included grain filling.The harvest yields were linearly related to water use, with no indication of a critical period of drought sensitivity. The relation of grain yield to the maximum potential soil water deficit did show that a prolonged early drought had an exceptionally large effect on both yield and water use.Two unsheltered irrigation experiments, also on barley, were made in the same year on a nearby site. The effects of drought on yield in these experiments were in good agreement with the effects observed on the mobile shelter site.When fully irrigated, the small plots under the mobile shelters used water 11 % faster than larger areas of crop, because of advection. The maximum depth from which water was extracted "was unaffected toy the drought treatment. When 50 % of the available soil water had been used the uptake rate decreased, but the maximum depth of uptake continued to increase.Measurements of crop nutrients at harvest showed that nitrogen uptake was large, because of site history, and that phosphate uptake was decreased by drought to such an extent that phosphate shortage may have limited yield.

This paper describes a wind-tunnel experiment on the dispersion of trace heat from an effectively planar source within a model plant canopy, the source height being h, = 0.80 h,, where h, is the canopy height. A sensor assembly consisting of three coplanar hot wires and one cold wire was used to make simultaneous measurements ofthe temperature and the streamwise and vertical velocity components. It was found that:(i) The thermal layer consisted of two parts with different length scales, an inner sublayer (scaling with h, and h,) which quickly reached streamwise equilibrium downstream of the leading edge of the source, and an outer sublayer which was self-preserving with a length scale proportional to the depth of the thermal layer.(ii) Below 2h,, the vertical eddy diffusivity for heat from the plane source (KHP) was substantially less than the far-field limit of the corresponding diffusivity for heat from a lateral line source at the same height as the plane source. This shows that dispersion from plane or other distributed sources in canopies is influenced, near the canopy, by turbulence 'memory' and must be considered as a superposition of both near-field and far-field processes. Hence, one-dimensional models for scalar transport from distributed sources in canopies are wrong in principle, irrespective of the order of closure. (iii) In the budgets for temperature variance, and for the vertical and streamwise components of the turbulent heat flux, turbulent transport was a major loss between h, and h, and a principal gain mechanism below h,, as also observed in the budgets for turbulent energy and shear stress. (iv) Quadrant analysis of the vertical heat flux showed that sweeps and ejections contributed about equal amounts to the heat flux between h, and h,, though among the more intense events, sweeps were dominant. Below h,, almost all the heat was transported by sweeps.

Wind and tracer-concentration fluctuations, and hence the budgets for tracer variance, vertical flux and streamwise flux, have been measured in the dispersing plume from an elevated lateral line source in an equilibrium turbulent surface layer, using heat as a passive tracer. The results are analysed by testing closure assumptions for models of turbulent dispersion at first and second order. Except close to the source, a first-order (gradient-diffusion) model satisfactorily predicts both the vertical and streamwise tracer fluxes.The tracer-variance budget is essentially a balance between advection and dissipation, with production becoming significant as fetch increases. The vertical and streamwise heat-flux budgets have advection and turbulent-transport terms which are in balance (almost exactly for the vertical flux, only approximately for the streamwise flux), leaving balances between local production and pressure-gradient interaction. The turbulence-interaction component of the pressure term cannot be modelled as $-\overline{u^{\prime}_{i}\theta^{\prime}}/\tau, \overline{u^{\prime}_{i}\theta^{\prime}}$ being the flux vector and τ a scalar timescale.