Climatological, pedological, and geomorphological processes in tropical mountain ecosystems

Bruno Messerli

Abstract

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).

Introduction

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.

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

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.

Long-term measurements

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.

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:

• location of areas for re-afforestation:
• evaluation of different soil conservation methods for endangered types of soil and slopes; and
• possible changes in cultivation cycles and land use practices by the indigenous population

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

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).

References

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