section 7 : windbreaks

Crop protection from very strong winds: recommendations from a Costa Rican agroforestry case study

C. J. Stigter

TTMI-Project, Wageningen Agricultural University 
Wageningen, The Netherlands

T. Darnhofer

International Council for Research in Agroforestry (ICRAF)
 P. O. Box 30677, Nairobi, Kenya

H. Herrera S.

Agrometeorological Programme
Instituto Meteorologico Nacional
San Jose, Costa Rica

Abstract

Introduction of irrigated agriculture in Guanacaste Region (Costa Rica) during the dry season encounters a major problem in the occurrence of very strong winds. The literature was scanned for possible recommendations with respect to growing crops between rows or strips of trees under such conditions. Recommendations were formulated in this case study made for WMO with respect to shape and composition, height, length, width, direction, number and permeability of rows or strips of trees and the distance between them and on measurements and observations to be taken in trials with this agroforestry system. Finally, general recommendations are formulated for running such trials with trees, in rows or strips or in wider belts or scattered throughout the landscape, for crop protection from strong winds.

Introduction

Wind protection is a long-standing indigenous practice in traditional agriculture of many regions (Stigter 1985). In attempts to improve or establish wind protection schemes with trees, it makes sense to study the link which can be observed between traditional and relatively recent but promising agroforestry practices (Budowski 1983). And it makes sense as well to try to quantify phenomena taking place in such traditional and such promising practices alike (Stigter 1986).

In the case study in Costa Rica reported here, the National Meteorological Institute requested WMO/UNDP to assist in providing the agrometeorological input into the set-up of experiments under conditions in farmers' fields, with a system of wind barriers with trees, in irrigated crops in Guanacaste Region. No crops are usually grown in this area in the dry season due to lack of water. Irrigation would solve that problem, but in that case the limiting factor to agricultural production becomes the occurrence of extremely strong winds (hourly average wind speeds over 70 km/hr, measured at 4 m above the ground). Protection with wind barrier systems appears most appropriate in the moderately high external input food and cash crop agricultural production system to be developed. Those in charge were aware of the fact that a shelterbelt system of trees, by its components and aims, has to be considered as an agroforestry land-use system (e.g., Darnhofer 1983). Indeed, the attempt to design a shelterbelt should only be the result of a problem-oriented diagnostic approach to the specific land-use systems, which requires inputs from a multi-disciplinary team of specialists including agrometeorologists (Darnhofer 1983). And there is no substitute for a thorough site-specific design effort, including a local cost/benefit analysis as an obligatory component (Rijks 1986).

Below we have summarized site-specific recommendations, made as the agrometeorological input for the design effort for the Guanacaste region experiments. They were thoroughly discussed with the local chief agrometeorologist —one of the coauthors of this paper — because they were meant to strengthen his input to the multi-disciplinary team established locally to design the system and the experiments.

The materials of which natural shelters are made preclude the use of wind tunnels for the direct design of an optimum shelter (Plate 1971). The effect of shelter shape and permeability on the degree of sheltering is incompletely understood. The same is true for such effects as ground roughness in front of the shelter and the effect of areal roughness that multiple shelters add to individual shelter effects. The physical forces required to cause a certain damage in a specific tree or plant are virtually unknown. Nevertheless, wind tunnel and field research have provided enough information over the past 50 years to formulate general guidelines and to prevent several serious mistakes. However, local site-specific research will certainly expand those guidelines.

To formulate our recommendations, we made use of established reviews of the extensive agronomical and (agro)meteorological research on the improvement of crops and soils by wind protection (van Eimern et al. 1964; van Eimern 1968a, 1968b; Plate 1971; Rosenberg 1975; Sturrock 1975; Radke and Hagstrom 1976; Grace 1977; Fuchs 1979). But we also used more recent reviews, including and extending new and changing emphases and insights in these fields of study (Rosenberg 1979; Hagen et al. 1981; ILACO 1981; Wenner 1983; Jensen 1985; MacKerron and Waister 1985; Pitcairn and Grace 1985; Grace 1986; McNaughton 1986).

Choices made

We conclude from recent local wind measurements in the region, using established measuring systems (van Eimern 1968c; WMO 1981), that by far the foremost problem to be expected is the mechanical impact of extremely strong winds. All other agrometeorological considerations, even those related to water use and shading, are secondary. These will come in only in cost/benefit ratio calculations once this main problem has been shown to be solvable. This reduces our case study to one in which only wind reduction effects under neutral atmospheric conditions should be considered. So we excluded considerations of water use efficiency, area occupied by the belts, shading of crops nearest the trees and damage other than from mechanical impact of air movement.

It should be noted that on the subject of mechanical damage to crops, new insights appeared about a decade ago (Grace 1977). Mechanical effects are now considered to be of more importance under many conditions (MacKerron and Waister 1985; Pitcairn and Grace 1985; McNaughton 1986).

The choice to use multiple tree breaks appears justified by the increase in roughness over a larger area in addition to separate wind break effects (Jensen 1985), and by the multipurpose use of trees and their products which is economically possible (Wenner 1983). In larger scale agriculture, where irrigated cash and food crops have to be protected against very strong winds, relatively narrow rows of trees are to be preferred above wider belts as an intercrop or scattered trees/bushes (see the table). The first mentioned kind of wind protection is traditionally well known in Europe and Japan, the latter two better known in Africa (Stigter 1985) but also in Costa Rica (Budowski 1983). However, strip cropping experiments (Radke and Hagstrom 1976) are worth trying on a somewhat smaller scale or in combination with tree systems. This may even include artificial fences whose permeability can be more easily manipulated. More experience will be gained in this way on actual agronomical wind damage risk in the area. At present this is lacking completely and relatively little can be learned from the literature for the extreme conditions concerned. Below we will mainly discuss the use of tree rows and come back to strip cropping at the end of this paper.

Factors to be considered in the design of a multiple shelterbelt system are: shape, composition, height, length, width and direction of a belt, and distance, numbers and permeability in the composite system. Subsequently one has to decide on measurements and observations to be made in the field trials. Also some other more general recommendations can be made (see the review table at the end).

Physical features

We will now consider briefly the physics of air movement in shelterbelts. Gustiness and turbulence complicate the determination of the threshold wind speeds that cause damage. In the adiabatic atmosphere, turbulent energy is derived from the kinetic energy of the mean wind; and in the atmospheric boundary layer, turbulence (eddy formation) is due to mechanical interference with the steady flow of air. The gustiness of the wind is therefore largely determined by upwind obstructions and their geometrical characteristics. The smaller scale turbulence is determined by the roughness of the upwind surface in general. The larger the eddy size and the more vigorous the eddy movement, the more kinetic energy eddies contain and the more horizontal momentum they exchange vertically. This is more damaging to objects meeting such eddies.

Windbreaks not only change the average wind speed by the friction (drag force) due to their obstruction, but they contribute to eddy formation and to breaking up eddies (and slowing down their motion) in the approach flow by the friction in their pores. The first can be harmful (van Eimern et al. 1964; McNaughton 1986) and the second will be beneficial in several respects (Rosenberg 1979). When permeability increases, the eddy formation contribution of a windbreak decreases in importance, until no flow separation occurs in front of the break. But then jetting through the windbreak takes over as the damaging factor for those cases in which eddy break up and air movement slow down have become insufficient. This simplified picture may assist in understanding design recommendations, including empirical ones.

In what is called primary wind injury in plant stress typology, damage to whole plants and trees (swaying, shaking, bending, lodging, breaking), and plant and tree parts (premature fruit and flower shedding, breakage, bruises, lesions, abrasion), is basically due to mechanical stress caused by asymmetrical air pressures acting on plant parts. However, secondary injuries to leave rubbing of adjacent leaves or by soil particles, are involved as well (Stigter 1985). Such damages influence photosynthetic capacities, evaporation and disease susceptibility. As to the kind of damage to be expected in the Guanacaste region, lodging (breaking) of the stems of unprotected crops and leaf surface damage are of prime importance. However, there is now sufficient proof that mechanical excitation of plants is likely to cause reductions in growth rate, coupled with a whole suite of anatomical responses, some of which will adversely influence yield quality (Grace 1986).

This stresses the absolute necessity of observations of phenological characteristics and yield parameters in wind protection trials, to which we will come back later on.

Shape, composition and height

There are many reasons for the selection of shelterbelt tree species (Raheja 1963). We limit ourselves here to agrometeorological aspects of rows of protecting trees. Then height H is a major factor, because protection length of the area behind a narrow tree belt is a function of H/Z_o, with Z_o a factor quantifying roughness of the surface under the approach flow (Hagen et al. 1981). From this point of view, the optimum height has to be a compromise between maximum possible height and resistance against mechanical wind damage of the tree species. Wind damage of the highest trees, as well as jetting through the trunks area, may be reduced by two to three storeys of trees, diminishing in height on the windward side. This influences the choice of composition and shape of the belt, and determines partly its permeability as well. It has the advantage of leading to a triangle shape with an almost vertical lee edge and an angled windward edge. This contributes to reducing the generation of damaging eddies at the windbreak top by deflecting the air; and to management of permeability (Raheja 1963; Gandemer 1981; Onyewuto 1983).

The latter factor is more important in our case. At extremely high wind speeds, one should use the minimum permeability one can afford. At the same time one should allow for enough throughflow (assisted by underwood manipulation) at the bottom to prevent overtopping of too much air. This contributes to reducing the formation of dangerous eddies.

Length, width and direction

Too large a width complicates permeability problems, uses too much land and does not contribute to mitigation of eddy effects, which occur primarily when height is greater than width (Gandemer 1981). As to the configuration of belts, protection of crops at four sides or in circles might be preferable when strong winds occur from different directions, but this has drawbacks in mechanized agriculture. The choice is thus for long belts, preferably 100 m to 1 km long, as much as possible perpendicular to the strongest winds. In our case of persistent trade winds, this is a very acceptable solution. In trials where shorter windbreaks have to be used, lateral flow should be prevented. Establishment of porous cheeks at both ends and both sides, of the same composition as the main belt and each about the same length as the height of the highest trees in the belt, may solve the end effect problem and contribute to the prevention of lateral flow between the belts in a multiple system (Gandemer 1981).

Distance, number and permeability

Permeability of one narrow belt of trees should be considered separately from permeability of a system consisting of a number of such belts at distances between 10 and 20 H.

The resulting protection at and below the height of the protected crops is not identical or nearly so when the number of belts is very small or very large. For single belts, recommendable minimum permeability, even for extremely strong winds, is likely to be somewhat lower than hitherto accepted. It is on the order of 20% (Gandemer 1981; Hagen et al. 1981; Jensen 1985) at the top half or two-thirds of the break (depending on crop height) with preferably an increase of permeability towards the bottom half or third (Gandemer 1981; Onyewotu 1983).

It is evident from the measurements interpreted by Hagen et al. (1981), that in case of an overall 20% porosity, the quiet triangular standing eddy zone (Plate 1971) or recirculation zone (Hagen et al. 1981), which can be detected at lower permeabilities behind belts, will start closer to the belt and much closer again below that permeability. And the wake zone beyond it, which may become more harmful to crops than the original approach flow (van Eimern et al. 1964; McNaughton 1986), will start closer to the belt as well. Decreasing permeability with height permits some more throughflow at the lower wind speeds near the bottom, and therefore simultaneously contributes to higher safety in the wake flow closest to the belt (Gandemer 1981). The outer bound of the wake flow is dictated by the more energetic processes in the wake, and so its extent does not vary very much with permeability up to 50% (McNaughton 1986).

The fact that the quiet zone is triangular and therefore the wake flow, which is accelerated by the flow above break height, is situated above that zone, shows the advantage of a large height difference between protecting belt and protected crops.

It has been shown with much evidence earlier (van Eimern et al. 1964), and recently well summarized by Jensen (1985), that a small number of identical windbreaks slightly diminishes shelter effect behind the rearmost break of such systems, in the whole range of distances from less than 10 H up to 20 H. So the resulting permeability of the whole composite system has become higher. Between the tree rows the situation will depend on permeability of single rows and distances as well. It will at smaller distances be more protective for the permeability distributions advocated above. This applies especially to the wake flow zone, which is likely to be the more problematic (Gandemer 1981). This points to a choice of distances not much higher than 10 H in our case. To this will certainly be added an increasing decoupling of all protected areas from the main flow, so an ever increasing overall protection, when the number of rows becomes substantial, that is when it covers from one to many kilometers. An areal roughness has in this way been established from this composite system of narrow tree rows (Jensen 1985). The same effect of decoupling would be obtained with interspersed trees, bushes etc. higher than the crops to be protected and over a considerable area (Hagen and Skidmore 1974; ILACO 1981; Jensen 1985).

Measurements and observations

As mentioned earlier, where mechanical damage from strong winds is the primary limiting factor, the agronomist member of the team should pay primary attention to phenology, growth and yield parameters and visual or even microscopic observations of actual mechanical damage. This will make it possible to observe differences between unprotected crops and protected ones at different distances from belts. On the meteorological side, cup anemometer measurements should be made as an indication of what is going on, although errors exist (MacCready 1966; Busch and Kristensen 1976). It should be noted, however, that turbulence and gustiness are not quantifiable with cup anemometers. Determinations of average wind speeds, preferably as 10-minute to half-hourly averages at 20 cm above the crops, are therefore not more than indicative in simultaneous comparisons of protection at different places. Wind profile measurements will normally be useless because of the existing streamline deflections.

In fact the non-meteorological observations mentioned will be much more indicative of integrated consequences of mechanical stress than wind measurements can be at the moment. This situation will remain until more basic knowledge has led to increased understanding, which should make it possible to develop more relevant measuring methods. Indications of maximum speeds in gusts, with pressure anemometers, could already now be very helpful in risk assessments prior to introduction of agriculture and during the trials. As soon as mechanical-stress-induced yield (quality) reductions appear to remain under acceptable thresholds, water use efficiency determinations will become of the highest importance in irrigated agriculture or under advective conditions.

Permeability should be recognized as the most important single parameter describing narrow shelterbelts. A quantification of differences in permeability can be made by comparing relative minimum wind speeds just above crop height at the same distance from the same or different belts (Bean, Alperi and Federer 1975). In our low permeability cases this distance should be of the order of 2 H. This might well be the most fruitful use one can make of cup anemometers in our case. Determinations of absolute values for permeability are much more difficult and can physically only be made by measuring pressure drops across belts. Visual observations are only of limited value and photographs may assist but cannot indicate differences due to foliage flow characteristics, which are of high importance.

Final remarks

The main recommendations made above have been summarized and compared with wider belts and scattered trees and bushes in Table 1. We believe we have shown that the existing literature yields information and results from which sensible recommendations can be derived. This strengthens the agrometeorological input into the trials needed to determine the agricultural potential of the extremely windy dry season in the Guanacaste region.

Experiments with strip cropping should be included from the beginning in order to gain more insight into the consequences of wind stress. Recommended distances are here again 10 H and recommendations on measurements and observations made above apply to such experiments as well. The difficulties that Radke and Hagstrom (1976) had in interpreting some of the results they review on strip cropping arise from their lack of consideration of the influence of porosity of the strips as a function of height. Whether in strip cropping, in using narrow tree rows or in mixed experiments, cost/benefit ratio determinations are absolutely necessary to understand the proper gain from the multipurpose role of trees and from yield (quality) increases due to the trees/crops applied for protection from wind.

Acknowledgements

The authors thank the Director of the Instituto Meteorologico Nacional of Costa Rica and the Secretary General of WMO for permission to use the information they provided. In this paper we made use of material contained in Experimental studies of performance of wind barriers in irrigated crops in Guanacaste Region (Costa Rica), report to WMO on a consultancy mission to (Costa Rica) by C.J. Stigter, September 1986. All of the items cited in that report have been cited in the present paper.

Table 1 Recommendations on windbreaks for crop protection.

References

Bean, A., R.W. Alperi and C A. Federer. 1975. A method for categorizing shelterbelt porosity. Agric. Meteorol. 14: 417-429.

Budowski, G. 1983. An attempt to quantify some current agroforestry practices in Costa Rica. In P.A. Huxley (ed.), Plant research and agroforestry. Nairobi: International Council for Research in Agroforestry.

Busch, N.E. and L. Kristensen. 1976. Cup anemometer overspeeding. J. Appl. Meteorol. 15:1328-1332.

Darnhofer, T. 1983. Microclimatic effects and design considerations of shelterbelts. In: D.A. Hoekstra and P.M. Kuguru (editors), Agroforestry systems for small-scale farmers. Nairobi: International Council for Research in Agroforestry.

Fuchs, M. 1979. Atmospheric transport processes above arid-land vegetation. In R.A. Perry and D.W. Goodall (eds.), Arid land ecosystems: structure, functioning and management, 1: 393-408.

Gandemer, J. 1981. The aerodynamic characteristics of windbreaks, resulting in empirical design rules. J. Wind Engn. Ind. Aerodyn. 1:15-36.

Grace, J. 1977. Plant response to wind. London: Academic Press.

Grace, J. 1986. Plant response to wind. In: J.R. Brandle and D.L. Hintz (editors), Windbreak technology. In press.

Hagen, LJ. and E.L. Skidmore. 1974. Reducing turbulent transfer to increase water-use efficiency. Agric. Meteorol. 14:153-168.

Hagen, LJ., E.L. Skidmore, P.L. Miller and J.E. Kipp. 1981. Simulation of effects of wind barriers on airflow. Trans. Am. Soc. Agric. Eng. 24:1002-1008.

ILACO (with D.A. de Vries and C.J. Stigter). 1981. Agricultural compendium for rural development in the tropics and subtropics. Chapter I, Climate. Amsterdam: Elsevier.

Jensen, M. 1985. Sand dune stabilization, shelterbelts and afforestation in dry zones. In: FAO Conservation Guide Nr. 10, Chapter 11. Rome: United Nations Food and Agriculture Organization.

MacCready Jr., P.B. 1966. Mean wind speed measurements in turbulence.J. Appl. Meteorol. 5: 219-225.

MacKerron, D.K.L. and P.D. Waister. 1985. Wind and plant physiology a review. In J. Grace (ed.), Effects of shelter on the physiology of plants and animals. Progress in Biometeorology 2: 99-114. Lisse: Swets & Zeitlinger B.V.

McNaughton, K.G. 1986. Effects of windbreaks on turbulent transport and microclimate. In J.R. Brandle and D.L. Hintz (eds.), Windbreak technology.

Onyewotu, L.O.Z. 1983. Structural design and orientation of shelterbelts in northern Nigeria: suggested establishment considerations. Agric. Meteorol. 29: 27-38.

Pitcairn, C.E.R. and J. Grace. 1985. Wind and surface damage. In: J. Grace (ed..), Effects of shelter on the physiology of plants and animals. Progress in Biometeorology 2: 115-126. Lisse: Swets & Zeitlinger B.V.

Plate, EJ. 1971. The aerodynamics of shelter belts. Agric. Meteorol. 8: 203-222.

Radke, J.K. and R.T. Hagstrom. 1976. Strip intercropping for wind protection. In R.I. Papendick, P.A. Sanchez and G.B. Triplett (eds.), Multiple cropping. Special Publication 27. Madison, Wisconsin: Am. Soc. Agron.

Raheja, P.C. 1963. Shelter-belts in arid climates and special techniques for tree planting. Ann. Arid Zone, 2:1-13.

Rosenberg, NJ. 1975. Windbreak and shelter effects. In L.P. Smith (ed.), Progress in Biometeorology 1:108-134. Amsterdam: Swets & Zeitlinger.

Rosenberg, N.J. 1979. Windbreaks for reducing moisture stress. In B.J. Barfield and J.F. Gerber (eds.), Modification of the aerial environment of plants. St. Joseph, Missouri: American Society of Agricultural Engineers.

Rijks, D. 1986. Development of rainfed agriculture in arid and semi-arid conditions. The environment - assessing the problems. In Proceedings of the Sixth Agricultural Sector Symposium. Washington, DC: World Bank.

Stigter, C J. 1985. Wind protection in traditional microclimate management and manipulation: examples from East Africa. In J. Grace (ed.), Effects of shelter on the physiology of plants and animals. Progress in Biometeorology 2:145-154. Lisse: Swets & Zeitlinger.

Stigter, C J. 1986. Microclimate management and manipulation in traditional farming. CAgM Working Group report. Geneva: World Meteorological Organization.

Sturrock, J.W. 1975. The control of wind in crop production. In: L.P. Smith (editor), Progress in Biometeorology 1: 349-368. Amsterdam: Swets & Zeitlinger.

van Eimern, J., R. Karschon, L.A. Razumova and G.W. Robertson. 1964. Windbreaks and shelterbelts. WMO 147TD70 (Techn. Note 59). Geneva: World Meteorological Organization.

van Eimern, J. 1968a. Protection of soils, plants and animals against wind. In Agricultural Meteorology. Proceedings of the Melbourne WMO Seminar. Geneva: World Meteorological Organization.

van Eimern, J. 1968b. Problems of shelter planning. In: Agroclimatological methods. Proceedings of the Reading Symposium. UNESCO-Natural Resources Research Series 7: 157-166. Paris: United Nations Economic and Social Council

van Eimern, J. 1968c. Some experiences obtained in mapping the local wind pattern. In Proceedings of the regional WMO training seminar on agrometeorology. WMO and Agricultural University of Wageningen.

Wenner, C.G. 1983. Trees in erosion and soil conservation. In D.A. Hoekstra and F.M. Kuguru (eds.), Agroforestry systems for small-scale farmers, pp. 66-94. Nairobi: International Council for Research in Agroforestry.

WMO. 1981. Guide to agricultural meteorological practices. WMO No. 134. Geneva: World Meteorological Organization.