Boundary Layer Climates.

Objective requirements are drawn up for experimental determination of the roughness of homogeneous terrain, particularly with respect to matching of the observation array to a terrain situation with given fetch, blending height and displacement length. Some fifty well-documented published experiments over various surfaces, ranging from mobile surfaces (sea, or moving sand or snow) to forests and towns, are shown to meet these criteria and are compiled. It is shown that most presently popular terrain reviews underestimate actual terrain roughness lengths by about a factor two.

Representative roughness parameters for homogeneous terrain

The evolution, ecological processes, aerodynamics, and morphology of incipient or new foredunes on the upper beach is reviewed. Four types of incipient foredunes may be distinguished, namely: type 1, those initiated by shadow dune formation within zones of discrete individual pioneer annuals (e.g. Cakile spp.), and perennials (e.g. Spinifex spp.; Ammophila spp.); type 2, those initiated by dunelet or hummock formation within discrete colonies of perennial grasses and herbs; type 3, those initiated by sand deposition within laterally extensive colonies of pioneer seedlings; and type 4, those initiated by sand deposition within a laterally extensive plant rhizome cover.Type 1 dunes are dominated by high, local flow deceleration and 3-D flow separation. Shadow dunes form within and downwind of the discrete plants. If colonised by annual plants, invasion by perennial plants is necessary for survival. Later lateral plant spread and accretion produces hummocky terraces and ridges. Type 2 dunes (dunelets) are characterised by local landward flow deceleration and marginal high velocity side flows. Sand deposition results in the formation of low, discrete, semi-circular convex mounds (seedlings), or more elongate hummocks and mounds (rhizomes). Types 3 and 4 dunes are characterised by laterally extensive, often homogeneous vegetation canopies. High canopies display more rapid landward flow deceleration than low canopies, thus narrow, asymmetric ridges and longer convex ridges result respectively. High plant densities result in rapid down-canopy flow deceleration, maximum traction load retardation and the formation of narrow asymmetric ridges. Dune height decreases and dune length increases as plant densities decrease. Maximum deposition zones occur for each wind velocity range. Whether ramp, terrace or ridge morphologies are formed depends on mode of beach colonisation, plant density and distribution, sand volume, wind speed, and plant species type and morphology.A range of variables which affect foredune morphology and evolution, including plant canopy density, height and distribution, wind velocity, and various ecological environmental processes are examined.

A review of biological and geomorphological processes involved in the initiation and development of incipient foredunes

Measurements of Zo and other aerodynamic qualities of three crops in Germany (pine forest, lucerne, potatoes) are combined with British records and with those of Tanner and Pelton, permitting the following generalizations. For pliable agricultural crops the roughness length decreases with increasing wind speed (to ½ for 1 to 3 m sec−1), and in this range, Zo ≅ crop height/10. Calculations of eddy conductivity (1/ra) give values 3 times that of open water for the farm crops, and 30 times that of open water for the trees. From measured components of the energy balance the surface resistances of the crops are calculated using Monteith's empirical relation. Values of rs for the trees-between 1.0 and 1.5 sec cm−1- are always 2 to 4 times as great as for the farm crops, indicating very effective stomatal control of transpiration by pine leaves. Including the two specific crop parameters in Penman's combination formula gives estimates of evaporation for open water, pine forest, potatoes, and lucerne that agree within 5–15% with measurements in southern England and in California.

Aerodynamic and surface factors in evaporation

Cocoa is an important global commodity. It is mostly grown on small farms by millions of cocoa farmers who depend on the crop for their livelihood. Although potential yields exceed 6000 kg/ha, average farm yields are often around 400 kg/ha. Among the production constraints met by farmers is nutrient limitation. In this review, we compile current knowledge on nutrient cycling in cocoa production systems, nutrient requirements of cocoa, and yield response to fertilizer application in relation to factors such as management, climatic, and soil conditions. Large amounts of nutrients are cycled within cocoa systems, mostly through 5–10 t/ha/yr litter fall. Still, harvesting and small nutrient losses such as leaching lead to nutrient exports causing gradual soil nutrient depletion. Exact nutrient requirements of cocoa are unknown. Leaf and soil test interpretation to identify additional nutrient needs remain ambiguous. Recommended nutrient application rates vary more than 10-fold. In several trials fertilizer application more than doubled cocoa productivity; in other cases response is minimal. Differences in response between regions, fields and even trees have yet to be explained. Interactions with agroecology and management (especially shade) are poorly understood. Without this fundamental knowledge, farm level recommendations have a weak scientific base. Different types of research are recommended to complement current knowledge. Existing data and trials can be exploited through additional analysis and more detailed measurements. Cocoa farms are highly diverse and on-farm trials offer opportunities for understanding variability in production and fertilizer response. Finally, multifactorial shade–fertilizer response trials will be essential to address some of the fundamental knowledge gaps.

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I. F. Long

Papers

The plant and its environment