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Mapping landscape dynamics in the highlands and lowlands of northern Thailand
This study is based on the experience of six years of mapping landscape dynamics (five maps published and two others in the process of being published) drawn to different scales, based on a chronological series of aerial photographs and Landsat images of Northern Thailand. This approach is particularly effective for the analysis of occupied landscape and their dynamics, demonstrating the impact of man upon the natural environment.
After having defined the notion of landscape, we attempt to show its cartographic translation at different scales (large, intermediate, and small) and its application in the study of the highlands and lowlands in Northern Thailand. Finally, we try to demonstrate the importance of such a mapmaking process for regional planning and for study of the land development problems of this region.
The Landscape and Its Dynamic in Northern Thailand
The term ''landscape'' is used here in a purely physiognomic sense. It is the visible, observable space in its entirety As the visible form of contact between the biosphere, lithosphere, and hydrosphere, landscape is characterized by natural vegetation that is more or less degraded, and by human use and occupation of the soil in a given geomorphological framework. The study of landscape, or of units of landscape,' requires a synthetic approach. beginning from the actual. considered as a whole. taking each of its elements not separately but in their interrelationships and combinations.
The aerial photograph and, more recently, the satellite image have given to this notion of landscape its extreme richness. They are the faithful reflection of landscape according to the various horizontal directions with real relative surfaces and at comparable scales. Geographic documents par excellence. they reveal landscapes in their completeness: natural settings that are more or less transformed by the human groups that occupy or have occupied them.
Each (mappable) landscape or landscape unit is a combination of features belonging to three different types of phenomena the physical-chemical setting (relief. climate, bodies of water), the biological setting (the living plant and animal communities), and the human use and occupation of these settings (production methods and socio-economic organization).'These different phenomena evolve according to very dissimilar chronological rhythms. Geological and geomorphological elements evolve over a very long period of time. Soil and plant cover are transformed in the course of shorter periods. As for the action of social groups, they play a role on an even shorter timespan and conform to a logic that is radically different from that of the first two orders of phenomena. Seen from the historic perspective of a period of twenty or thirty years, a view that one has when in possession of the available photographic documents of the space under consideration, it is the last order of phenomena (anthropic action) which is here analysed. In our study the dynamic of mapped landscapes concerns the periods 1953 1972 and 1976 1977. These periods are rich in socioeconomic transformations (passage from a precapitalistic economic system to a merchant economic system) which have brought about important changes in the landscapes themselves
The first aerial photographic coverage of Thailand dates from 1953 1954 (approximate scale of 1:50,000) and corresponds to a period when the methods of economic production were still largely traditional and when demographic growth had not as yet made its impact felt. The landscapes of that period are thus still strongly influenced by the old form of society and civilization. In 1968 1970 (second aerial photographic coverage of Thailand at a scale of about 1:60,000), and even more in 1972 (coverage of some regions at a scale of 1:20,000) and more recently (satellite data beginning in 1973), modernization, penetration of the merchant economic system, and demographic explosion mark and at times even disrupt the landscapes. The dynamic of landscapes thus partly reflects the dynamic of socio-economic developments.
Landscape can be divided into different units according to the scale at which it is observed: large scale (1:20,000), small scale (1:1,000,000), and intermediate scale (1 :250,000). The limits of each is the reflection of geographic separations proper to its particular scale. For each of them we have a sectioning of space into landscape units of a certain type which do not necessarily interlock with each other. The significance of each unit must be determined and the type of representation that is to be given by each of the scales must be ascertained in order to show how they can be combined so as to produce a deeper understanding of a whole region
Large-Scale Cartography of the Dynamic of Landscapes
For rural landscapes analysed here, the large scale is generally considered to be 1 :25,000, but it can go as far as 1 :50,000. The landscapes presented are either homogeneous physiognomic units belonging to a vegetation type or to its different forms of degradation, or homogeneous types of soil utilization (for example. paddy-fields).
In the colour maps produced from this scale of photography, the shades used conform to the principles of H. Gaussen for the natural vegetation, from indigo for the thick evergreen forests of the mountains to orange for the clear forest of the piedmont. and employs greens or yellow-greens for the mixed or semi-dense forests. Blue represents the paddyfields.
Landscape transformations are indicated by stripes or white bands alternating with colour bands. These are. for example. the rai extracted by clearing the forest on the pioneer front (Map of Si Satchanalai) or the anthropic savannah (former opium rai) in the Doi Inthanon area. Recently developed paddy fields have horizontal or slanted bands.
In the Karen zone of the Doi Inthanon area it has been possible to distinguish. in addition to the swiddens of the year shown in yellow, three stages of recovery of the vegetation on fallow and (aerial photographs at a scale of 1 :20,000 taken in 1972): a recent fallow strip (1 to 2 years) in a state of low thickets, an old one (3 to 5 years) in the thicket state. and a much older fallow strip (6 to 10 years) in a pre-forest state. Each is represented with a range of colours extending from yellow-green to dark green. the green darkens as the strip approaches the forest state. It is therefore possible to perceive the cycle of the Karen fallow strips which cover a period of 5 to 10 years, depending upon the site.
On the transect of Phrom Phiram the colours used on the two preceding maps with an ecological meaning are replaced by hachures running in various directions. The horizontals correspond to the wettest environments. flooded the longest (swampy meadow). Rotation of the hachures in a counterclockwise direction indicates drier and drier settings (swampy thicket, swampy forest, semi dense forest). The more closely the 1968 physiognomy (aerial photographs taken to a 1 :20,000 scale) approximates a forest state, the darker the zone is. Increasingly pronounced human impact is depicted by progressively lighter tones. Intermediate stages (a discontinuous thicket in the course of development) are represented by bands that are alternately dark and light, the light band indicating the transformation of the setting (in 1968) with relation to its former state (in 1953)
Such principles enable one to procure a monochrome cartography of landscapes and of their transformations on a relatively large scale (from 1 :20,000 to 1 :50,000). This representation of landscape units facilitates a rather detailed analysis of ecological settings and of land use along particularly significant transects.
The cartography of the dynamic of landscapes drawn to a large scale, supplemented with maps of slopes. soils. and soil erosion. has a positive value for the study of agroforestry systems in the highlands and lowlands. The map of Doi Inthanon (1 :25,000), for example, enables one to analyse two of these systems: the Karen system, where shifting cultivation with fallow strips on the flanks and irrigated paddyfields in the valley bottoms increases towards intensive use but without upsetting the basic ecological balance; and the Hmong system. where the opium poppy and corn growing on the same parcels several yeses in succession hinder the reconstitution of a forest plant cover, which instead is replaced by a savannah.
The retreat of the dense or semi-dense forest of the mountains does not have the same significance in each of these systems. A transition zone (intermediate forest or wooded savannah) is found along the points of contact between these two systems. The distribution of population and inhabited dwellings shows that the Karen system can handle greater population densities than can the Hmong system. However, recent growth of the latter (development of paddy-fields and peach orchards) can orient it in the future towards an intensification of agricultural techniques, as in the Karen case if the forest zones to be cleared disappear and if the population stabilizes.
On the map of Sisatchanalai, two clearly contrasting zones can be seen. The lowlands, consisting of valley bottoms and rolling low hills, where a pioneer front of commercial crops is advancing very rapidly at the expense of the dense and semidense forest, stand out from the highlands, where abandonment of shifting cultivation or forest exploitation favours forest regeneration. Between 1953 and 1968 the farmed area had doubled. destroying the forest in over 13.5 per cent of the area mapped.
The transect of Phrom Phiram shows the progress of landdevelopment on the Mae Nan flood plain between 1953 and 1968 and facilitates the derivation of the dynamic from it without mapping the entire area.
Such large-scale mapping done with chronological series of aerial photographs requires long and detailed work of interpretation, knowledge of design and of the terrain involved. It can only be considered for areas particularly representative of much larger entities. The main advantage of the transect approach is that a maximum number of different environments car, be covered while the mapped space is reduced to a minimum. These large-scale studies are necessary in order to intensify the analysis of the landscape dynamics and to understand their mechanisms by referring to surveys taken at the level of the rural community or of a few communities. They can also be used for a perimeter development which is at the same time a test zone (Huai Thung Choa. for example)
Intermediate - Scale Cartography
On the intermediate scale. Iandscape units are more complex. Each is defined by a combination of traits, some belonging to the natural environment. others to modifications and improvements brought about by human action. They have been delimited either in relation to one or two dominant traits or in relation to the combination of three or four traits. These landscapes-some simple and uniform in structure, others complex and mosaic-have their own dynamic, which is governed by human action. They often correspond to geomorphological units: valleys, pediments, river banks, karst, and so on.
The map ''Soil Utilization and Landscapes in the North of Thailand" gives a picture of the distribution of the large types of landscapes on mountains. hills, and on the alluvial plains of the Chiang Mai and Lampang basins in 1970. These landscapes evolved mainly in the mountains and on the terraces, but this evolution during the course of a period of fifteen years is translated better with a 1:20,000 or 1:50,000 scale map than with one of 1:250,000. The smaller scale does not lead to a very significant shifting of limits. We thus contented ourselves with a cartography at a specific date (the aerial photographs of 1968 to 1970). without directly translating the dynamic.
The natural or developed landscapes which have been little affected by recent transformations appear in flat tones corresponding to the ecological mapping principles of H. Gaussen (the wettest settings shown with the darkest colours. the driest with light shades). Human action in the form of rai clearance is shown with slanted bands: alternation of a colour band of the setting (type of vegetation) and of a white band the width of which is proportional to the degree of human action. The anthropic savannah of the highlands, which corresponds to the opium poppy growing zone, is indicated with green broken lines of the same colour as that of the mountain forest. which is the original formation. This map shows the distribution of the major ecological systems and the degree of human impact on the mountain environment the terraces, and pediments along the borders of the basins or valleys.
The representation of the dynamic on this scale poses special problems because of the great complexity of the landscape units. Sometimes the whole of the landscape has been transformed, at other times one or two units among others have changed during the 15-year period under consideration (map of the dynamic of landscapes of the Sukhothai plain drawn to a scale of 1:250,000). The dynamic is indicated with the absence or presence, the orientation, and spacing of bands. Flat tones correspond to landscapes which either have not changed or have changed very little. Width and spacing of the bands expresses the degree of change. Slanted bands refer to the clearings in the highlands for the cultivation of dry-weather crops, horizontal bands imply that there has been a land development programme for the introduction of paddy-fields between the two dates considered (1953 and 1968). We have been able to distinguish landscapes that have been slightly, partially, or totally, transformed during the course of the period studied. The number of bands joined for the same landscape unit is in relation to the degree of complexity of this unit. Complex landscapes assure the transition between two and at times three simple landscapes. These are the types of transition that unite the traits of several simple contiguous units.
The map of Sukhothai, with slightly complex figures indicating a wide diversity of situations, clearly shows the rapid progress of land development in the course of these fifteen years. This includes the occupation of zones subject to flooding and the older alluvions (uplands) between Mae Yom and Mae Nan and the development of the pediments or hills along the borders of the plain, especially for dry-weather crops or, less frequently. for paddy-fields
A comparison of the large-scale maps and those described above shows that the legend has been simplified by about a third (for the same zone, there are, on an intermediate scale map, three times fewer the number of units represented than on a large scale map). This intermediate-scale cartography enables the acquisition of a view of the whole province or small region, of the occupation of the land and of its recent dynamic. Over a wide space, it simplifies and generalizes the analytic data of a large-scale map while at the same time retaining a sufficient precision as regards the localization and contours of the landscape units.
Landsat Remote-Sensing Data and Small-Scale Cartography
On Landsat images, landscape units drawn to a small scale are more extended and more uniform. In general. a trait of plant cover or of land use dominates the unit and confers its homogeneity to it. There is often a connection between these landscapes and large morphological-structural units.
Landsat images are of great value for studies conducted at a small scale. Band 7 is useful for indicating the morphologicalstructural units. the wettest zones and bush-burnt areas as well as the swidden. Band 5 provides an overall view of plantcovered landscapes and of certain aspects of land use. Colour composite images, combining bands 4, 5 and 7, are richer and more detailed with better contrasts. For the mapping of landscapes in general, they are better than band 5 (twelve landscape units instead of nine on the map of Northern Thailand). but less clear than band 7 for swampy areas or wet surfaces.
A comparison between images taken at the beginning (December. January) and at the end (March. April) of the dry season is possible and very instructive for understanding Northern Thailand. Evergreen forests of the mountains and anthropic savannahs can only be identified on the images of the beginning of the dry season. while forest and clear thicket cannot be easily distinguished from the semi-dense mixed forest except at the end of the dry season (the images of March or April). Delimitation of the zone of irrigated farming and of the stubble of dry paddy-fields is possible during both periods, but certain phenomena of the use of the environment by man (swiddens or dry crops in an advanced state of growth) appear only at one period (April) (Bruneau and Le Toan 1979). Rainy season images cannot be used because of the high cloud cover and uniformity of the plant cover (green over the entire area). They can, on the other hand, be useful for the study of floods and rice growing.
Very simple maps. plotted in black and white, were drawn after an interpretation of three Landsat images that cover a large part of Northern Thailand (Bruneau 1976). For this purpose. these images were enlarged to a scale of 1:500,000. Enlargement to 1:250.000 is also possible for cartographic purposes. But these Landsat images can only be interpreted correctly if based upon a good knowledge of the terrain and in conjunction with aerial photograph (1:20,000 to 1 :50,000) analysis of representative areas.
These images provide an overview of large areas far more effectively than that obtained from a mosaic of aerial photographs. It is then possible to delimit spatial units of the ''ecological regions" type which have a certain homogeneity with regards to morphologic structure. phytogeography. Iand utilization, and human occupation. Ecological variables that are preponderant on this scale, generally of a geomorphologic nature, provide a unity to a group of characters of a varied nature (vegetation. Iand use, soils, communication routes, dwellings. ethnic groups) and enables the perception of the broad integrity of the landscape (Bruneau 1976).
The repetition (theoretically every 18 days) of Landsat images and of their sequences (since 1973 with Landsats 1, 2, and 3) has the great advantage over aerial photographs of allowing a comparison between seasons and years, thus facilitating the study of the dynamic of phenomena. The increase in the data thus acquired will require more and more automatic processing and consequently a systematic recourse to digital data. But this problem is still in a study stage. The work of supervised classification and automated mapmaking can generally only be performed on a small portion of the image.
An attempt was made to produce a supervised classification for a part of the Doi Inthanon mountain region using four bands of the Landsat tape of 3 March 1975 (Bruneau and Le Toan 1979). The result is disappointing because of the shadow cast, which falsified the classification of the forests. Only the paddy fields in a stubble stage and the stretches of barren soil could be identified without any ambiguity To overcome this problem, ancillary data must be introduced through application of a more sophisticated software, as L.D Miller, K Nualchawee, and C. Tom (1978) have done for a small mountainous area in the vicinity of Chiang Dao. This technique is not yet operational for the cartography of larger areas
Results, however, are far better in an irrigated plain region (the Chiang Mai basin in the surroundings of San Patong). A supervised classification using bands 5 and 7 on two different dates (27 July and 3 March 1975) provides a precise cartography of the types of land use (6 in all) and of their changes between the two dates. These studies will be continued in 1979 and 1980 with the University of Chiang Mai, the objective being to produce an inventory of land use and of its changes, with the aid of three tapes of Landsat digital data (beginning and end of the dry season, five-year interval), for the whole of the Chiang Mai basin
Although they represent future solutions, these techniques cannot, for the moment. be applied efficiently except to restricted plains areas and thus are of use only for studies made with large scale maps where they can complement the analyses derived from aerial photographs.
Basic Documents for Rural Land Improvement and Development
The purpose of a cartography of the dynamic of landscapes is to locate and measure the degree and intensity of human impact on the environment. In Northern Thailand it brings into focus the transformations of the past 10 or 20 years These landscape transformations are related to the increase in population pressure and to penetration by the merchant economic system. which has caused an expansion of dry commercial crops resulting in degradation of natural vegetation types.
Whether it is in colour or monochrome, this cartography always has an ecological basis which shows the main vegetation types and distinguishes rice growing from dry crops, with the topography being expressed by contour lines. It is really a basic map, more concise than a map of vegetation. For more particular points, ecological conditions can be detailed in the insets: hypsometry, bioclimates. geomorphology and slopes, and types of vegetation. The main objective of the map-the human landscapes and their dynamic-is treated in greater detail. The major land-use categories have been delimited and situated chronologically in relation to each other; they often express the degree of agriculture which is associated with them.
Between the natural landscapes (principal vegetation types) and the human landscape (major land use), we have shown what can be called mixed landscapes in which the human element is only partial. It is in this zone where the changes that have occurred in the course of the past twenty years are most pronounced. One sees in them either forms of degradation: natural plant formations evolving towards their regrowth after abandonment, or more frequently, a progress of land clearing accompanied by land development and dissappearance of the natural vegetation.
Thanks to the topographical and ecological basis of these maps, a permanent correlation can be established visually between the natural environment and the use that man has made of it Thus an overview of the exploitation of ecological settings by human groups is provided The dynamic perspective of this representation of human landscapes is fundamental. The stages of the implementation of changes and the direction taken in the growth phases, including those that will appear in the near future. emerge from it.
The approach using different scales is necessary, especially for the small and large scales. For the first. Landsat imagery can be used profitably only by complementing it with a sampling of aerial photographs and with observations made in the field. The Landsat images are of interest both before undertaking a regional study and once it is completed. When a region is poorly known, they enable one to acquire more rapidly than from aerial photographs an overview of the terrain that is. however, still imprecise This approach also enables one to locate the areas that present problems, especially if the region is rather vast (more than 5,000 km²) One must then go to the large scale along a transect chosen as particularly representative, or to small areas on which a development action is exerted. The use of large scale aerial photographs and enquiries in the field are then required. The Landsat images can then be re-examined for generalization of the results thus obtained for a larger area.
Intermediate scales (between 1:60,000 and 1:250,000) are useful for the improvement of small regions exceeding the framework of a small watershed or of a small ecological or social unit (community or group of communities) which belong to the large scale (between 1:5,000 and 1:30.000)
Aerial photographs at scales close to 1:50,000, generally available for several periods (1954 and 1970), for they are employed to draw topographic maps, are easily used for this purpose The districts (Amphoe) of Mae Taeng, Chiang Dao, and Pail which form a group of highlands and lowlands in the vicinity of the perimeter of Huai Thung Chao, can thus be mapped at 1:100,000. This would enable the land improvement project under consideration to be situated in its immediate regional setting. At a smaller scale (1 :500,000), the watershed of the Mae Ping, to the North of Chiang Mai, and that of the Salween River basin in the same latitudes and extending as far as the Burmese border. would furnish. with the aid of Landsat images, a larger regional setting. This is considered necessary in order to provide an overview of the phenomena in all their complexity (savannahization of the highlands, main forestry ecosystems. highland-lowland interactive systems).
In a project of integrated rural development, the cartography of inhabited and man-created landscapes and of their dynamic is not a goal in itself but a necessary intermediate stage. Made at different scales, it furnishes basic documents that are at the same time analytic (large scales) and synthetic (small scales), which constitute a kind of study support. These documents play a role both at the beginning of such a research project, for clarifying objectives and for posing problems, and towards the end, as spatial reference in order to locate systems and structures, both ecological and socioeconomic. They contribute to establishing a link between natural and human phenomena.
Bruneau. M.1963. ''Dynamique des paysages et organisation de l'espace rural dans la plaine de Sukhothai (Thaïlande).'' L'Espace Geographique. 3: pp. 207-223. (With English summary.)
_________1976 ''Analyse des paysages et grands espaces en Thaïlande septentrionale a partir d'images du satellite Landsat 1.'' In Télédétection et Environnement Tropical, pp.125 - 149. Travaux et Documents de Geographie Tropicale, No. 25. Bordeaux: CEGET. (With English summary.)
Bruneau, M.. and G. Cabaussel. 1973. La dynamique des paysages en zone tropicale. Travaux et Documents de Geographie Tropicale, No. 9. Bordeaux: CEGET. 73 pages. (With English summary.)
_________1974. ''Paysages. dynamique et organisation de l'espace rural a Phrom Phiram (Thaïlande).'' Etudes Rurales, 53-56: pp. 169 - 191.
Bruneau, M., and T. Le Toan. 1979. ''An Interpretation of Northern Thailand Swiddening and Multiple Cropping Systems Using Multidate Landsat Images and Computer Compatible Tapes.'' In Proceedings of the 12th International Symposium on Remote Sensing of Environment, 20 26 April 1978, Manila, pp. 1883 1894. ERIM. Vok 3. Ann Arbor, Mich.. USA.
Miller. L.D., K. Nualchawee, and C. Tom. 1978. ''Analysis of the Dynamics of Shifting Cultivation in the Tropical Forests of Northern Thailand Using Landscape Modeling and Classification of Landsat Imagery'' Technical Memorandum 79545. NASA, Goddard Space Flight Center.
Climatological, pedological, and geomorphological processes in tropical mountain ecosystems
Soil erosion is a fundamental element in a natural resource programme. Especially for countries where there has not been sufficient basic research done and little time is available, adopting simple and efficient research methods is very important. For soil erosion we propose three approaches. First, damage mapping that does not require elaborate instruments and is realizable in a short time (months). Second, process study which requires some instruments and is realizable in a time span of years (more than one rainy season). Third, the long-term measurement for understanding the processes of natural hazards. This requires a significant number of stations and takes decades (more than ten years). One phase can overlap the other, but all are possible in isolated and remote areas for determining the appropriate measures required for soil conservation, Ianduse practices, and re-afforestation.
Soil erosion is only part of an integrated system. The analysis, qualification, and quantification of the elements and processes are just one part of a scientific undertaking, while the synthesis and the understanding of the mechanism of a whole system is the other and most important part, particularly in a resource conservation programme (Figs. 6 and 7).
Soil erosion is one of the most important processes in the tropical hill and mountain areas. contributing to serious disequilibrium in the existing man-made ecosystems. We have studied these processes in different countries under very different conditions and we have come to the conclusion that this problem cannot be solved by uniform and schematic data collection without taking the local and regional economic and social situation into consideration. We do not want to discuss the soil erosion forms and processes in detail (see Arnoldus 1977), but we will indicate different methods and their respective time frames which will give us first approximations and allow us to take first responsive measures. Especially for the humid tropics in the developing countries, where only limited climatological, pedological, and topographical data are available. the efficiency and simplicity of the methods adopted are of great importance. Finally we must understand that the measures are only part of an integrated economic-ecological systemic approach. For Northern Thailand Sabhasri ( 1 978) has provided some of the most valuable observations on soil fertility and soil losses under forest fallow cultivation so far available. These will form a useful beginning for the types of study proposed here.
Soil Erosion: A Man-made Process
Soil erosion is the accelerated process of soil displacement caused by human influence, which disturbs the natural relation between soil formation and natural slope processes. Soil erosion parameters are the intensity and duration of rain, the type of soil, the length and inclination of slopes, and the methods and type of land use. Very often we do not have sufficient and scientifically qualified basic data for all these elements. Therefore the question is: can we afford to spend a long time on research work. when the process of destruction is rapid and permanent? Therefore, we propose three research approaches, with different methods and with increasing accuracy from short- to long-term field work.
The damage mapping (Figs. 1 and 2)
This method has been applied and developed by Hurni (1978; 1979) in the Simien Mountains of Ethiopia, where the soil is removed, the yields diminished, and men are forced to seek arable land on ever steeper slopes and at increasingly greater altitudes. During field-work of only a few months, a very precise soil erosion map of more than 30 km² (one-third ploughed land and about two-thirds grassland and forest) has been completed. After our short field trip in the vicinity of the Huai Thung Choa field station, we are convinced that the same method could be applied there too.
Soil erosion damages can be studied in two ways: First, the diminishing of the uppermost soil horizon. For this purpose, analysis of soil profiles under natural conditions is required, for instance, in forests. where there has been no, or only little, human influence. The knowledge of the depth of the top humus horizon in undisturbed areas should be measured at different elevations, inclinations, and orientations. The results would then allow a comparison with conditions characteristic of the cultivated areas at different stages of the fallow cycle. In this way it will be possible to calculate the loss of soil not only under different natural conditions but also under different land use practices. In Fig. 1 the ideal soil profile is shown, and, since the top horizon is the most important for soil fertility and crop yield, the damage to that horizon in the ploughed land is indicated over the complete area of the map (which includes grassland and forest) (see Hurni 1978). Second, the damages accruing from sheet erosion and linear erosion (rills and gullies) have been mapped (cf. Fig. 2). Spots where the rock is completely denuded indicate the final phase of soil erosion. Places where soil horizons are mixed through ploughing must be marked as important for initial processes, especially if the uppermost horizon is completely eroded. An accumulation of soil in the foot zone of a slope or in a valley bottom is typical of damaged slopes.
FIG. 1. Soil Erosion Map from the Simien Mountains, Ethiopia. (Hurni 1978)
All the results must be mapped and generalized by area units, depending on the scale (Fig. 1; for the complete map see Hurni 1978). The map will permit an interpretation of the different processes or parameters influencing soil erosion (e.g.. Iength. inclination and form of slope, direction of precipitation, vegetation, duration of land use, type of fallow, and so on). The total loss of soil on any slope from the beginning of human cultivation can be estimated and certain threshold values for the future calculated. Finally this map is a first base for measures of soil conservation. re afforestation. or changes in land-use practices.
The process study (Fig. 3)
The objective of a detailed process study is to obtain quantified information for future conservation measures, especially on critical slope angles. Compared with the damage mapping approach it will require more time, certainly more than one rainy season and it will also need a laboratory and instruments for accurate analysis and measurement
The fundamental element in soil erosion is the effect of precipitation on the soil. The amount of soil erosion depends on the force of precipitation (erosivity), the soil type. and its resistance to erosion (erodibility). Together with other factors, the process of soil erosion under any local or regional conditions can be observed and measured We will not discuss all the details (see Arnoldus 1977; Hurni 1978,1979; Wischmeier and Smith 1962). but it must be understood that we will need precise information on climate (intensity, duration, distribution, and direction of precipitation), on pedology (type of soil with particle size distribution, insoluble aggregates, mineral components, organic material, and so on), on topography and geomorphology (length, inclination, and form of slope) and on land use or vegetation cover (from original forest to first- and second-year cropping, with different products such as rice, tea, beans, and maize in pure or mixed plantation. and finally after 10 or 20 years of fallow).
There is a universal soil-loss equation, which is well developed and described (see Arnoldus 1977; Hurni 1979; Wischmeier and Smith 1962). In summary. a precise knowledge of the climatological. pedological, and geomorphological elements and processes, is necessary for a quantitative understanding of this very important part of a humid tropical ecosystem
FIG.2. Soil Erosion Forms
Again we emphasize that such observation and measurement can be undertaken in remote areas where no technical assistance is available. As an example, we cite the Simien Mountains in Ethiopia, where sediment traps and runoff stations were constructed in a very simple but scientifically precise manner (Fig. 3). Only the climatological and hydrological instruments were imported. The amount of the eroded and transported material for unit areas has been weighed, but analysed in the laboratories of the home university.
While the damage mapping can be done without any expensive instrumentation and in a time period of months, the process study will need a certain level of expenditure for instrumentation and can only be completed in a matter of years, that is. more than one rainy season.
If we want to understand the long-term fluctuations of climatic elements, especially the frequency of heavy and catastrophic rains and the possibility of natural hazards, we will need measurements over a period of more than 10 years. In every country there are some long-term records available perhaps in a town far away from the investigated area. For a first approach it is possible to correlate these variations and calculate frequencies of catastrophic events. It seems to us very important that local observations and measurements for long periods are planned, at least at some significant stations. Only with long-term records will we be able to understand the real processes and take adequate measures in soil conservation, land-use practices, road construction, and so on.
Again we must point out that as a first approach frequencies can be estimated by correlation of existing data and that only with the development of a project, can such long-term, regular and accurate measurement be undertaken.
FIG. 3. Soil Erosion Experiment Plots at Gich Camp (3,600 m), Simien Highlands, Ethiopia: Measurement of the Run-off and the Amount of Soil Eroded in Single Storms for the Evaluation of the Soil Erodibility Factor. (1 ) Test plot in continuous fallow; (2) test plot with natural grass cover; (3) wooden borders extending into the ground; (4) collecting channels made of flattened corrugated iron; (5) extension of 4 into the ground; (6) oil barrels with a capacity of approximately 35 litres (insufficient); (7, 8) drainage channels; (9) plastic cover over 4 and 6. (Hurni 1979)
Soil Erosion and Soil Conservation
It is essential that we develop methods so that very complicated processes can be identified with a certain degree of accuracy in a short time. without long-term foreign experts and without expensive instrumentation. In this sense we have shown several approaches which can overlap in time The main aim. however, is to determine as fast as possible the necessary measures:
All these measures must take into consideration the attitude of the population, particularly their attitude towards innovation. From our own experience we know that even conservative communities can be interested in innovations when problems are shown to a few members in an effective manner and when their soil conservation efforts are properly supported. In any case. the mental and cultural attitude of the inhabitants must be taken into consideration and, if possible, kept intact In the continuing attempt to strike a balance between development and conservation we must also refer to our own responsibility. When a soil which needs some thousands of years to form, can be destroyed in a few years, as we have seen in the mountains of Ethiopia, then we must accept the responsibility to collect the basic data to determine new methods of cultivation and to realize conservation measures even though conditions might be difficult.
Soil Erosion-Part of an Economic-Ecological System
FIG. 6. Model for Study of Closed and Open Systems in a Mountain Valley. (Modified from Moser 1975)
In Fig. 6 we tried to show a locally limited ecosystem with its natural conditions and socioeconomic structures both in a closed and in an open system. The closed system situation existed before the recent economic development and before any external influence developed. Even if we have in the mountains of Northern Thailand opium plantations for a world market, they will never provide an opening for an extended economic system because the products are sold at their point of origin to illicit traders who cannot influence the relatively closed and traditional system. Changes in the highland-lowland interactive system will only increase its complexity. Maintenance of traditional swidden agricultural system with its changes of fields and villages is becoming more and more difficult. Population pressure, school and education, production for a market. new farming techniques, political measures, and so on are precipitating a difficult transition with environmental deterioration, degradation. and loss of resources. Exactly at this time we have a special responsibility for the development of each element of the system to avoid catastrophes or destruction. In this sense, soil erosion is one part of an interactive and integral system
In Fig. 7 a regional economic-ecological system is demonstrated. Though it is developed for the Swiss Man and Biosphere (MAB) Programme of Unesco for the endangered areas of the Alps (Messerli and Messerli 1978). it can be used to indicate the fundamental processes in any particular test area where external factors are influencing a socioeconomic system (No. 1 in Fig. 7). This system can be derived from several subsystems depending on the local conditons (e.g., economic, political, demographic, cultural). All the activities going out from this socio-economic system influence the type and intensity of land use (No. 2 in Fig. 7). If this influence can be absorbed by the natural conditions, there will be a normal feed back from the land-use system to the socio-economic system (No. 3 in Fig. 7). If these changes lead to damage or destruction in the natural system, we must take into consideration serious feedback mechanisms to the land use and to the socioeconomic system (Nos. 4 and 5 in Fig. 7). As a whole this model can be and must be adapted to local conditions. But it helps us to understand the mechanisms and forecast the assessment of our responses. However, the analysis, qualification, and quantification of the elements is just one part of a scientific undertaking. and synthesis and the understanding of the mechanisms of a whole system is the other part, especially in a resource programme. ''It should lead to the refinement and local adaption of techniques which may be diffused in areas where problems of adaption to change are known to occur. and particularly in those areas of the humid tropics with fragile soils" (UNU 1977, p. 10).
FIG. 7. Schematic Representation of a Regional Economic-Ecological System
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