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Harnessing soil, terrain and hydrological conditions in a geographic information system

THE NEED FOR NATURAL RESOURCES INFORMATION

International and national agricultural research and policy-making entities concerned with environmental management and environmental changes, have repeatedly expressed the need for baseline information on natural resources (see Box 1).

The intense and increasing pressure on soil and water resources, leading to degradation and pollution of those resources, calls for an integrated approach that strengthens the awareness of users of the resources on the dangers of inappropriate management, and at the same time strengthens the capability of national land resources institutions to deliver reliable, up-to-date information on land resources, in an accessible format to a wide audience. See Box 2.

The development of a georeferenced natural resources information system with internationally accepted standards will provide policy makers and resource managers with important and indispensable tools to reverse the current trends of soil degradation in developing and industrialized countries, and to implement a programme for soil conservation and sustainable use of the land. A general outline of such a system is presented in Box 3.

A GEOGRAPHIC INFORMATION SYSTEM FOR SOIL AND TERRAIN CONDITIONS

The World Soils and Terrain Digital Database (SOTER) is a geographically referenced system, capable of providing accurate, useful and timely information on soil and terrain resources. The objective of SOTER is to utilize emerging information technology to produce an internationally accepted, standardized database, containing digitized map unit boundaries in a GIS and their attribute database in a relational database management system (RDBMS). Box 4 summarizes the history of SOTER.

Characteristics of SOTER

SOTER is structured to provide a comprehensive framework for the storage and retrieval of uniform soil and terrain data that can be used for a wide range of applications at different scales. It will contain sufficient data to allow information extraction at a resolution of 1:1 million, both in the form of maps and as tables. SOTER will be compatible with global databases of other environmental resources. The system will be amenable to updating; "old" databases can be saved as files for monitoring purposes. SOTER will be accessible to a broad array of international, national, and regional environmental specialists through the provision of standardized resource maps, interpretative maps and tabular information essential for the development, management and conservation of environmental resources. Finally, the system will be transferable to and useful for national database development at larger scales from 1:1 million to 1:100 000 (Oliveira and Van den Berg, 1991).

Structure of the SOTER database

Two types of data are present in the SOTER database. The geometric component, indicating the location and topology of SOTER units, is stored in that part of the database that is handled by GIS software. The attribute component, describing the non-mappable SOTER unit characteristics is stored in a separate set of attribute files, handled by RDMS software. The two types of data components are linked by a label, unique for each SOTER unit (see Figure 1).

SOTER mapping approach

The concept of a georeferenced database implies the mapping or delineation of areas. Being an agro-ecological database, the basic approach in mapping is the delineation of areas of land with a distinctive pattern of landform, surface form, slope, parent material and soils. Differentiating criteria have to be developed. These are then applied in a stepwise manner, following a hierarchical structure.

At the highest level of differentiation, major land forms are identified and quantified (dominant slope gradient and relief intensity). Areas in a similar landform can be further segregated according to their lithology (or parent material). Differentiating criteria for dividing these terrain units into terrain components are surface form, meso relief, aspects of the parent material.

At this stage the complexity of terrain components does not always allow individual mapping at the 1:1 million reference scale. In such cases, the percentage of occurrence of these nonmappable terrain components in the mapped unit is indicated in the terrain component table, while the attributes of these non-mappable terrain components is stored in the terrain component data tables.

The final step in the differentiation of the terrain is the identification of soil components within the terrain component. Differentiating criteria are based on diagnostic horizons and properties as formulated by FAO (FAO, 1988). As with terrain components, these soil components can be mappable or non-mappable. Most likely these terrain components comprise (at the reference scale of 1:1 M) a number of soil associations or soil complexes. The percentage of occurrence within the mapped unit is indicated in the soil component table, while the attributes are stored in the soil profile and soil horizon data tables. Figure 2 shows a schematic representation of a mapped SOTER unit.

FIGURE 1. SOTER units, their terrain components (tc), attributes, and location

Content of the SOTER database

The attributes of the database are elements that can be quantified, either through visual observations in the field or by measuring in the laboratory. The general approach adopted by SOTER is to screen all existing soil and terrain data in a georeferenced area and to complement the terrain information with remote sensing data where necessary. The various attributes of the terrain, terrain component and soil component tables are listed in Table 1. Each attribute is described in detail in the SOTER Procedures Manual (Van Engelen and Wen Tin Tiang, 1993).

LAND AND WATER LINKAGES IN SOTER

Although SOTER primarily collects soil and terrain conditions in a spatial context, there are direct and indirect linkages between land and water throughout the database. The surface flow of water its course and volume - is related to the landform. Since SOTER describes major landforms by their morphology and not by their genetic origin or by processes responsible for their shape, landform attributes are important characteristics in describing surface hydrology.

FIGURE 2. Schematic representation of a SOTER unit, with terrain, terrain components and soils

The dominant slope is the most important differentiating criterion for the major landforms, followed by relief intensity. Landforms are further described by their regional slope, hypsometry and the degree of dissection. SOTER uses the drainage density as a qualitative measure of the degree of dissection. The drainage density is defined as the total length of drainage channels per unit area of land, expressed as km x km^-2. Three classes are defined as illustrated in Figure 3.

SOTER terrain units give information on the percentage of that unit that is largely (i.e. >90%, thus excluding small islands, etc.) permanently (i.e. more than 10 months/year) covered by water. Note that bodies of water large enough to be delineated on the map are not considered part of a SOTER unit.

Terrain components attributes include more detailed information on the surface form. Characteristics to assess water erosion vulnerability are slope gradient (%), dominant length of the slope (m), and form of slope (uniform, concave, convex, irregular). The lithology and the texture of the non-consolidated parent material are important characteristics to assess deep water percolation.

Characteristics of the terrain component in SOTER that are of immediate importance to harness hydrological conditions are the surface drainage; flooding characteristics (frequency, duration and starting date); and the depth of groundwater. The full descriptions of these hydrological attributes are given in Box 5.

For each identified soil component in a SOTER unit, visible signs of (accelerated) erosion are to be indicated according to type (e.g. sheet erosion, fill erosion, gully erosion, tunnel erosion, deposition by water) using FAO definitions (FAO, 1990b), area affected, using GLASOD definitions (ISRIC, 1988), and degree of erosion, using FAO definitions (FAO, 1990b). The sensitivity of the soil to capping, which is to be indicated by the degree in which the soil has a tendency to capping and sealing, (FAO, 1990b) is an important characteristic to assess the infiltration of surface water. Finally, the hydrological conditions of the soil are very much determined by its position within the terrain component.

TABLE 1. Non-spatial attributes of a SOTER unit

FIGURE 3 Examples of degrees of dissection as indicated by drainage density on 1:50 000 maps

Hydrological characteristics related to the soil profile are the present drainage of the soil component (after FAO, 1990b), and the basic infiltration rate in cm/hr, following the 7 categories defined by BAI, 1991. Soil profile characteristics, which are clearly linked to the hydrological conditions of the soil, are certain diagnostic properties: fluvic properties (fluviatile, lacustrine or marine sediments, which receive fresh materials at regular intervals); gleyic and stagnic properties, indicating soils, which are saturated with water at some period of the year, or throughout the year, in most years.

Numerous soil attributes, which are to be identified for each soil horizon have an influence on the hydrology of the soil, or are indications of the hydrological conditions of the soil. Among these are the grade, size and type of structure; the presence of coarse fragments (abundance and size); the total sand, silt and clay fraction; the moisture content at various tensions; the hydraulic conductivity; the bulk density; the infiltration rate; and the electrical conductivity of the saturation extract.

CONCLUDING REMARKS

The SOTER database approach provides important information to characterize hydrological conditions in a Geographic Information System. Elements however, which are directly related to the surface flow of water do not form part of the attribute database of SOTER. The geometric database (i.e. the location and extent of an object represented by a point, line or surface) holds, in addition to information on the delineations of SOTER units, also base map date; such as roads and towns, location of deep wells, the hydrological network, water bodies and administrative boundaries, in a Geographic Information System. Characteristics of deep wells (points), the hydrological network (lines) and of water bodies (surfaces) can be stored in a separate set of attribute files. The length, width and depth of primary, secondary, tertiary drainage patterns; the dominant slope gradient and length of slope of these drainage patterns; the volume and quality of the surface flow at the entrance and exit of various drainage patterns measured at several time intervals, depending on the climatic characteristics in the watershed. Similarly the volume and the quality of water from deep wells and water bodies can be stored as attributes in the hydrological database.

BOX 5

Hydrological attributes of the SOTER terrain component (Van Engelen and Wen Tin Tiang, 1993)

28 Surface drainage

Surface drainage of the terrain component

29 Depth of groundwater

The depth in metres of the mean groundwater level over a number of years as experience in the terrain component.

FLOODING

Flooding is characterized by the items 30-32:

30 Frequency

Frequency of the natural flooding of the terrain component in classes

31 Duration

Duration of the flooding of the terrain component in classes

32. Start

Give the month (indicated by a figure) during which flooding of the terrain component normally starts. Three entries are possible.

There is a need for better definition of groundwater resources. SOTER has only one entry related to groundwater; the depth in metres of the mean groundwater level over a number of years as experienced in the terrain component. Additional information on the fluctuations of the groundwater level (e.g. in the dry versus the wet season), and information to indicate the quality of the groundwater is needed. As indicated in the background paper for this Workshop, water easily catches and carries anything soluble, bringing these elements along from the atmosphere and from human additions and from the soil complex via the root zone to the groundwater. The environmental link between soils, food crops and water supplies means that if soils are polluted there is every likelihood that food chains and drinking water will also be affected.

In arid and semi-arid regions, groundwater is often used for irrigation. The quality of the groundwater, in particular its electrical conductivity and its sodium and magnesium content are essential elements to assess salinization hazards of the soil. High salinity of the groundwater may also cause a threat to food crops and human beings.

REFERENCES

BAI. 1991. Booker Tropical Soil Manual. Booker/Tate, London.

Christoffersen, L., 1992. Chairman's Report on Workshop on Dryland Management Policies and Information Needs. Oslo, 9 p.

DGIS. 1990. Een wereld van verschil: mieuwe kaders voor ontwikkelingssamenwerking in de jaren negentig. Ministry for Development Cooperation, The Hague.

FAO. 1988. Soil map of the world, Revised legend. World Soil Resources Report 60, FAO, Rome.

FAO. 1990a. The Conservation and Rehabilitation of African Lands: An International Scheme. ARC/90/4, Rome, 38 p.

FAO. 1990b. Guidelines for Soil Profile Description. 3rd edition (revised). FAO, Rome.

FAO. 1992. Some thoughts of the new Director, AGL, Sombroek, W.G. In: Land and Water No. 36, FAO, Rome. 24 p.

IGBP. 1992. Global Change and Terrestrial Ecosystems: the Operational Plan. ed. by Steffen, B.L., Walker, B.H., Ingram, J.S.I. and Koch, G.W., Rep. 21, Stockholm.

ISNAR. 1992. Summary of agricultural research policy. International quantitative perspectives. ISNAR, The Hague, 24 p.

ISRIC. 1988. Guidelines for general assessment of the status of human-induced soil degradation. Oldeman, L.R. (ed), Working Paper & Preprint 88/4. ISRIC, Wageningen (in English and French).

ISSS. 1986. Proceedings of an international workshop on the structure of a digital international soil resources map annex database. Ed. by Baumgardner, M.F. and Oldeman, L.R. SOTER report 6. ISSS, Wageningen.

Oliveira, J.B. and van de Berg, M. 1992. Application of the SOTER methodology for a semi-detailed survey (1:100 000) in the Piracicaba region (Sao Paulo, Brazil). SOTER report 6. ISSS, Wageningen.

Sombroek, W.G., 1984. Towards a global soil resource inventory at scale 1:1 M. Working paper 84/4. ISRIC, Wageningen.

Stoops. G. and Cheverry, C., 1992. New Challenges for Soil Research in Developing Countries: A Holistic Approach. Proceedings of the Workshops, funded by the European Community, Life Sciences and Technologies for Developing Countries (STD 3 programme). Rennes, 22 p.

UN. 1993. Report of the United Nations Conference on Environment and Development. Rio de Janeiro, 3-14 June 1992, Vol. I, Agenda 21, Chapter 18.23. United Nations, New York.

UNEP. 1992a. Two decades of achievement and challenge. The United Nations Environment Programme 20th Anniversary. Our Planet. 4 (5). Nairobi, 23 p.

UNEP. 1992b. Proceedings of an ad-hoc Expert Group Meeting to Discuss Global Soil Database and Appraisal of GLASOD/SOTER, 24-28 February 1992, UNEP, Nairobi, 39 p.

UNSO/UNDP/UNEP. 1992. Draft report on the Expert Meeting on Desertification, Land Degradation and the Global Environment Facility, Nairobi, 28-30 Nov. 1992.

Van Engelen, V.W.P., and Wen Ting-Tiang. 1993. Global and National Soil and Terrain Digital Databases (SOTER), Procedures Manual. FAO, ISRIC, ISSS, UNEP, Wageningen, 115 p.

WRI, Cal Tech. 1992. Global Environmental Monitoring: Pathways to Responsible Planetary Management. A proposal to the World Resources Institute, Washington, D.C. and the California Institute of Technology, Pasadena. WRI, Washington DC. 17 p.

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