Land Capability mapping with SPOT data and Geo-information technology South Gondar, North-

Western Highlands of Ethiopia

Getachew Tesfaye Ayehu Institute of Land Administration, Bahir Dar University Center for GIS & remote sensing, Bahir Dar University

Bahir Dar, Ethiopia e-mail: sget2006@yahoo.com

Abstract— Land degradation can be considered in terms of the loss of actual or potential productivity or utility as a result of natural or anthropogenic factors; it is the decline in land quality or reduction in its productivity. In the context of productivity, land degradation results from a mismatch between land capability and land use. Land capability mapping is the basis for sustainable development by ensuring correct land use according to its capability for sustained economic production. The purpose of this study was to assess, classify and map land according to its capability to support a range of crops on a long term sustainable basis in Deledel community watershed using on field observation and geo-information technology. The land capability for agriculture classification was developed to describe the agricultural potential of land based on the degree of limitation imposed by its biophysical properties. The classification system was based primarily on slope, land use, soil depth and the risk of soil erosion. This is not a single purpose classification made with regard to land suitability for a particular crop but a general purpose classification for possible potential crops. Then a combination of physical parameter required for the classification was defined and identified. The classification concept is based on an integrated scheme with three GIS layers of slope class, soil depth, and risk of soil erosion class. These layers were rasterized and used for layer formation and analysis. The resultant map displays the land units according to their inherent characteristics and indicates the capabilities for various types of land utilization. The analysis reveals that class I, II, III and IV (Suitable for annual crops) can be seen as the major land capability class of the community watershed. It covers an area of 267.8 ha (66.42%) with the listed major limiting factors. But class I is limited as compare to other classes of agriculturally suitable for annual crops. This class covers 4.61 ha area (1.14 %) which is dominated by gentle slope. In Deledel community watershed, this class is identified as the most suitable for annual crop production without any major limiting factors. Class VI (Suitable for perennial crops, grazing and some forestry) ranks second in the study area as it covers area about 97.5 ha (24.18%). Class VII (Suitable for forestry) has also a considerable coverage in the study area. It accounts an area of 32.19 ha (7.99%). Land not suitable for agriculture accounts an area of 5.72 ha (1.42%).

Keywords—land capability; geo-information; land use system;

I. INTRODUCTION Agriculture is an essential component of societal well-being and it occupies 40% of the land surface, consumes 70% of global water resources and manages biodiversity at genetic, species and ecosystem levels (Mulugeta & Stahr, 2010). Ever-increasing demands on the land from economic development, and growing populations are driving unprecedented land use change. In turn, land use change is driving land degradation: Deforestation, soil erosion, nutrient depletion, salinity, water scarcity, pollution, disruption of biological cycles, and loss of biodiversity. Land degradation is still an important global issue because of its adverse impact on agronomic productivity, the environment, and its effect on food security and the quality of life in general. It is increasing in severity and extent in many parts of the world, with more than 20 percent of all cultivated areas, 30 percent of forests and 10 percent of grasslands undergoing degradation. An estimated 1.5 billion people, or a quarter of the world’s population, depend directly on land that is being degraded (Food & Agricultural Organization (FAO), 2008). Yield reduction in Africa due to past soil erosion may range from 2 to 40%, with a mean loss of 8.2% for the continent (Eswaran, & Reich. 2001).

In Ethiopia 85% of the population are directly supported by the agricultural economy. However, the productivity of that economy is being seriously eroded by unsustainable land use and management practices both in areas of food crops and in grazing lands. All physical and economic evidence shows that loss of land resource productivity is an important problem in Ethiopia and that with continued population growth the problem is likely to be even more important in the future. Especially, in the highlands of Ethiopia, the area of most intense population density, the area of greatest livestock density and the area of greatest land degradation, recorded measurements of soil loss by water erosion range from 3.4 to 84.5 tons per ha per year with a mean of 32.0 tons/ha/year (Keyzer & Sonneveld 2001).

Land degradation can be considered in terms of the loss of actual or potential productivity or utility as a result of natural

or anthropogenic factors; it is the decline in land quality or reduction in its productivity. In the context of productivity, land degradation results from a mismatch between land capability and land use. Hence, development activities should be planned in a proper way so that the livelihood of the people and future generations is compromised. This can be achieved by ensuring that the sustainable development of natural resources is a major part of the future economic plans. Land capability mapping is the basis for sustainable development by ensuring correct land use according to its capability for sustained economic production. All soils are not the same and they are not of the same capability for every use. Land capability implies that the choice of land for a particular use contributes to the success or failure of that use. It further implies that the choice of land for a particular use will determine the potential impact of that use on surrounding resources such as water. To make the best use of land and to minimize the potential for negative impacts on surrounding lands, land capability analysis is needed In the past, land capability mapping for agriculture was carried out using traditional methods. There were no effective tools to help merge information from diverse sources and of different scales. Geo-information technology in combination with remotely sensed data offers the possibility of analyzing a number of different variables in order to obtain optimal land uses management. The purpose of this study was to assess, classify and map land according to its capability to support a range of crops on a long term sustainable basis using on field observation and geo-information technology. The scope of this study is delimited to Deledel community watershed in south Gondar Zone of Amhara region, Ethiopia. Our main target was to develop land capability mapping of the community watershed to ensure better land productivity; improve livelihoods, erosion control and sustainable use of agricultural lands available in the country.

II. METHODOLOGY The land capability for agriculture classification was developed to describe the agricultural potential of land based on the degree of limitation imposed by its biophysical properties. The classification system was based primarily on slope, soil depth and the risk of soil erosion. This is not a single purpose classification made with regard to land suitability for a particular crop but a general purpose classification for possible potential crops.

A. Slope Slope is a basic element for analyzing and visualizing landform characteristics. It is important in studies of watershed units, landscape units, and morphometric measures (Moore et al., 1992). When used with other variables slope can assist in runoff calculation, forest inventory estimates, soil erosion, wild life habitat suitability and site analysis (Wilson

and Gallant, 2000). Slope is a very crucial element for land capability mapping. ASTER Digital elevation model (DEM) of the study area is used to generate the slope layer using the slope functions of the Spatial Analyst toolbox of ArcGIS 9.2. The DEM has a spatial resolution of 39m. The slopes were calculated in percentage of slope. Six slope classes were designated for the studied community watershed based on FAO (1976) slope classification system as shown in TABLE I below.

B. Soil depth Soil is an important segment of our ecosystem, as it serves as an anchorage for plant and source of nutrients. Thus soil is the fundamental raw material for plant growth. The knowledge of soil resources is essential for proper watershed development and planning. Analysis of depth of soil resources is very much crucial for land capability analysis and further suggesting treatment The soil depth layer was also produced using DEM as the base map followed by sampled field data collection of soil depth of the study area. The field collected data were used as a bench mark to interpolate the whole community watershed. The soil depth was classified in to five classes based on FAO (1976).

C. Risk of soil erosion Soil erosion risk areas were delineated by identifying the compounding effect of various environmental factors which leads to erosion (TABLE III). In addition with field assessment, slope and land use/land cover types of the community watershed were priority factors that assumed to enhance or reduce erosion risk in the area. The current land use and land cover types were classified from the SPOT satellite imagery by applying supervised classification technique. Here, the advance navigation technique like GPS has been used to collect training site data and to field check classified datasets. Therefore, slope classes were assigned a number value representing its risk to erosion. Steep slopes were assigned a high value while flat slopes were given a less value for erosion. Similarly, each of the land use and land cover types was assigned values representing the likelihood of erosion. Farm land and bare soil received a high value of while forested areas had a low value for erosion.

TABLE I. SLOPE CLASSIFICATION SYSTEM

No Slope class % Code Land capability class 1 Flat or Almost flat 0-3% L1 I 2 Gently sloping 3-8% L2 IIL 3 Sloping 8-15% L3 IIILE 4 Moderately steep 15-30% L3 IVLE 5 Steep 30-50% L4 VILD 6 Very steep >50% L5 VIILD, VIILE

The Raster Calculator, available in ArcGIS 9.2, of the spatial analyst tool approximated an index of erosion vulnerability for the different overlapping areas of slope and land use and land cover type by summing the separate erosion risk values of the slope and of the land use and land cover type. In addition the value assignment was supported by field observation of land use land cover-slope –erosion potential.

TABLE II. SOIL DEPTH CLASSIFICATION SYSTEM

Then, a combination of physical parameter required for the land capability classification was defined and identified. The classification concept is based on an integrated scheme with three GIS layers of slope class, soil depth and risk of soil erosion class as described above. Moreover the surface and overlay analysis capabilities in GIS can effectively facilitate in handling vast amount of spatial information (Ekanayaki and Dayawansa, 2003). These layers were rasterized and used for layer formation and analysis. It should be noted that this classification is not designed for recreation or wildlife purposes and is not extended to show land capability of ornamental plants. The procedure consist of 1) organization of geographic input data 2) establishment of GIS data layers with the support of survey for ground truth 3) overlay formations and analysis 4) map output. The land capability classification (based on FAO) code, soil and water conservation classes, major limiting factors (combined slope and soil depth classes), and suitable land use are summarized in TABLE IV.

TABLE III. SOIL EROSION RISK CLASSIFICATION

TABLE IV. DEFINITIONS OF LAND CAPABILITY CLASS UNITS BASED ON THE COMBINED LAYERS OF SLOPE, SOIL DEPTH AND EROSION CLASS

III. RESULT AND DISCUSSION Slope analysis reveals that majority of gradient is moderately steep (15-30%) and steep (30-50%) and lowest gradient can be observed at the north eastern part of the basin. Effective soil depth is not a major limitation on the level and undulating slope within all study areas. In most cases, except along river banks and steep slopes on shallow and/or eroded soils the effective soil depth of a majority of the soils is at least 1meter and in many cases considerably more. The classification scheme has utilized five land use/land cover classes representing forest land, farmland, grazing land, bare land and settlements (Fig.1.).

Fig.1.The current land use/land covers of the study area

No Soil depth class cm Code Land capability class 1 Very deep >150 D1 I 2 Deep 100-150 D2 IIL 3 Moderately deep 50-100 D3 IIILE 4 Shallow 25-50 D4 IVLE 5 Very shallow <25 D5 VILD,VIILD & VIIIDE

No Erosion class

Description Code Land capability class

1 Nil No Erosion E0 I, IIL 2 Slight Some surface wash &

small rills E1 IIILE

3 Moderate Rills covers most of the surface

E2 IVLE, VILD

4 Severe Shallow gullies E3 VIILE 5 Very severe Most of the land is

dissected by gullies E4 VIIIDE

Land class unit

Soil conservatio

n class

Major limiting factor Suitable land used

I I - Suitable for annual crops

IIL II Slope 3-8%

IIILE III Slope 8-15% & slight erosion

IVLE IV Slope 15-30% & moderate erosion

VILD VI Slope 30-50% & very shallow soil depth

Suitable for perennial crops

grazing or forestry

VIILD VII Slope > 50% & very shallow soil depth

Suitable for forestry

VIILE Slope > 50% & severe erosion

VIIIDE VIII Very shallow soil depth & very severe

soil erosion

Not suitable for agriculture

The farm land covers the highest land use and land cover class. It covers an area of 107.99ha (38.57%) of the total community watershed. Farmers are growing various kinds of cereal and pulse crops including Bean, Barley, Flax, Potato, Tef, Sorghum and Finger millet. In Deledel community watershed, most of the barren land areas were cultivated and grazed in the former times now they are exhausted due to soil erosion and degradation and intensive and unmanaged grazing system. Grazing and Forestland are also covers a significant area of the community watershed. They covered an area of 82.37ha (29.42%) and 74.31ha (26.54%) respectively. Despite the farmers are now started to practice SWC structures in different land use types of the area with the support of international and local nongovernmental organizations (NGOs), most of the farms and grazing lands are still showing a degraded appearance. The dominant forest tree growing in the community watershed includes Sholla (Ficus sur/ vasta/), Bisana( Croton macrostachyus), Kulkuale (Euphorbia abyssinica), Eucalyptus, Acacia, zeqeta (Calpurina aurea), koma (Prunus africanus), etc. Bare land and settlement covers the rest of the classes. They cover an area of 8.09ha (2.89%) and 7.44ha (2.66%) respectively. “Land capability” classifications provide a ranking of the capacity of each part of a land resource to sustain broad land use classes (Rosser, et al., 1974). The community watershed has classified into eight land capability classes and summarized into four agricultural suitability classes as shown in TABLE V.

A. Land Suitable for Annual Crops The analysis reveals that class I, II, III and IV (Suitable for annual crops) can be seen as the major land capability class of the community watershed. It covers an area of 267.8 ha (66.42%) with the listed major limiting factors. But class I is limited as compare to other classes of agriculturally suitable for annual crops. This class covers 4.61 ha area (1.14 %) which is dominated by gentle slope.

Fig.2. Land capability based on land class unit

TABLE V. LAND CAPABILITY CLASS OF DELEDEL COMMUNITY WATERSHED

Land Capability

class

Description Deledele

Area (Ha) %

I Suitable for annual crops

4.61 1.14 IIL 34.37 8.52

IIILE 74.28 18.42

IVLE 154.54 38.33

VILD Suitable for perennial

crops grazing or forestry

97.5 24.18

VIILD Suitable for forestry

30.96 7.68

VIILE 1.23 0.31

VIIIDE Non-suitable for agriculture

5.72 1.42

TOTAL 403.21 100

In Deledel community watershed, this class is identified as very high suitable for annual crop production without any major limiting factors. Class II covers 34.37 ha area (8.55%) in the community watershed. This class is highly suitable for annual crop production. Soils in this class require careful soil management, including conservation practices, to prevent deterioration or to improve air and water relations when the soils are cultivated. The soils in this class provide the farm operator less latitude in the choice of either crops or management practices than soils in class I. The limitations are few and the practices are easy to apply for this class. Class III is extended between class II and IV. It’s spatial and areal extent is much better in the community watershed as compare to Class I and II. The total area covered by this class is 74.28 ha (18.47%). This class is identified as moderately suitable for annual crops. Soils in class III have more restrictions than those in class II and when used for cultivated crops, the conservation practices are usually more difficult to apply and to maintain. Limitations of soils in class III restrict the amount of clean cultivation; timing of planting, tillage, choice of crops and harvesting. Class IV, this is a dominant class with respect to areal extent in the study area, which accounts for 154.07 ha (38.32%). The restrictions in use for soils in class IV are greater than those in class III and the choice of plants is more limited. When these soils are cultivated, more careful management is required and conservation practices are more difficult to apply and maintain. Soils in class IV may be used for annual crops, pasture, range or wildlife food and cover. Soils in class IV are well suited to only two or three of the common crops or the harvest produced may be low in relation

to inputs over a long period of time. Many sloping soils in class IV are suited to occasional but not for regular cultivation.

Fig.3. Land capability based on its suitability for agriculture

B. Land Suitable for Perennial crops, Grazing and Forestry The result has shows that class VI (Suitable for perennial crops, grazing and some forestry) ranks second in the study area as it covers area about 97.5 ha (24.18%). Physical conditions of soils placed in class VI are such that it is practical to apply perennial crops, range or pasture improvements and some forestry, if needed, such as seeding, liming, fertilizing, and water control with contour furrows, drainage ditches, diversions or water spreaders. The major limiting factors are steep slopes and very shallow soil depth.

C. Land Suitable for Forestry Class VII (Suitable for forestry) has also a considerable coverage in the study area. It accounts an area of 32.19 ha (7.99%). soils have very severe limitations that make them unsuited to cultivation and that restrict their use mainly to grazing, forestland, or wildlife.

D. Land Not-suitable for Agriculture Land not suitable for agriculture accounts an area of 5.72 ha (1.42%). Soils and miscellaneous areas have limitations that preclude their use for agriculture.

IV. CONCLUSION AND RECOMMENDATION The methodology premised here to indicate land capability classes for decision-making intervention. The analysis reveals that Class I, II, III, IV, VI, VII and VIII are present in the study area. Out of that Class I which is much suitable for agriculture accounts an area of 4.61 ha area (1.14 %). Class IV is a dominating class as far as the areal extent is concerned with 38.32 percent. The Class VI is most susceptible to land degradation which accounts for 24.18 percent. Integrated watershed development should be planned by considering land capability classification of Deledel community watershed. In addition future study needs to include additional biophysical and socioeconomic factors that determine the land capability of this specific community watershed or other basins in the region.

ACKNOWLEDGMENT Special thanks, for funding the field data collection, goes to

Tana-beles integrated watershed development at Bahir Dar, Ethiopia.

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