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.

A series of irrigation experiments was done at Rothamsted where the soil is a flinty silty clay loam over flinty clay. The results are compared with those obtained by Penman (1962, 1970, 1971) at Woburn where the soil is a loamy sand over sand. The limiting deficits, JD ( , above which irrigation increased yields, were about 2-6 times greater at Rothamsted than at Woburn; this ratio approximates to the ratio of the water-holding capacities of the soils (-0-1 to -15 bar) to a depth of 1 m. The limiting deficits at Rothamsted were 80 mm for spring-sown field beans, 84 mm for main-crop potatoes, 100 mm for spring barley and 140 mm for spring and winter wheat. The responses to irrigation were not determined accurately as there were few years with a large response for any crop. However, the evidence is that the maximum response that could be expected for potatoes was 0-19 t/ha/mm water, and for grain dry matter of beans 0-006 t/ha/mm. The figure for potatoes agrees with that obtained by Penman at Woburn; the response of beans was much smaller at Rothamsted, partly because of severe attacks of broad bean stain virus.

The effects of water deficit on growth of spring barley were analysed under five irrigation treatments. One crop was irrigated at weekly intervals from emergence throughout the growing season, and one was not irrigated at all after emergence. Soil water deficits in the other treatments were allowed to develop early, intermediate or late in the crop's development.Weekly irrigation produced a crop with a large leaf area index (maximum value 4) and maintained green leaf and awns throughout the grain-filling period. Early drought decreased leaf area index (maximum value 2) by slowing expansion of main-stem leaves and decreasing the number and growth of tiller leaves. Leaf senescence was also increased with drought. Drought late in the development of ears and leaves and during the grain-filling period caused leaves and awns to senesce so that the total photosynthetic areas decreased faster than with irrigation. Photosynthetic rate per unit leaf area was little affected by drought so total dry-matter production was most affected by differences in leaf area.Early drought gave fewer tillers (550/m 2 ) and fewer grains per ear (18) than did irrigation (760 tillers/m 2 and 21 grains per ear). Late irrigation after drought increased the number of grains per ear slightly but not the number of ears/m 2 . Thus at the start of the grain-filling period crops which had suffered drought early had fewer grains than irrigated (9-5 and 18-8 x 10 3 /m 2 respectively) or crops which suffered drought later in development (14 x 10 3 /m a ).During the first 2 weeks of filling, grains grew at almost the same rate in all treatments. Current assimilate supply was probably insufficient to provide this growth in crops which had suffered drought, and stem reserves were mobilized, as shown by the decrease in stem mass during the period. Grains filled for 8 days longer with irrigation and were heavier (36-38 mg) than without irrigation (29-30 mg). Drought throughout the grainfilling period after irrigation earlier in the season resulted in the smallest grains (29 mg).Grain yield depended on the number of ears, the number of grains per ear and mass per grain. Early drought decreased tillering and tiller ear production and the number of grains that filled in each ear. Late drought affected grain size via the effects on photosynthetic surface area.Drought decreased the concentrations of phosphorus, potassium and magnesium in the dry matter of crops, and irrigation after drought increased them. Concentration of nitrogen was little affected by treatment. Possible mechanisms by which water deficits and nutrient supply affect crop growth and yield are discussed.

In a field experiment on the effects of drought on spring barley the crop was protected from rain by automatic rain shelters. Various plots received irrigation at different times to give a range of drought treatments from full irrigation to no irrigation between emergence and harvest. The foliage area, light interception, stomatal resistance and leaf photosynthesis rate of five treatments were measured throughout the growing season, and a mathematical model has related the computed whole canopy photosynthesis to the measured total dry-matter yields at harvest. Hence, it was possible to estimate tha independent influences of drought on radiation interception, efficiency of use of intercepted radiation, and respiration. The analysis shows that for all treatments the decrease of intercepted radiation was the major factor in reducing yield, and it accounted for a loss of 30-40% for treatments that were stressed from the beginning of the season, and of 10-20 % for treatments that were stressed after mid-May. Stomatal closure caused a reduction of up to 11 % in daily photosynthesis, and the maximum effect was on plants that acquired a large leaf area before being stressed. However, the effect of stomatal closure integrated over the whole season was only 6 % or less. Our measurements of internal resistance to carbon dioxide transfer were not precise enough to show significant differences between treatments; but increases of internal resistance, caused by stress, may have contributed to loss of yield. AGS 93