Retrospective Analysis of Land Cover and Use Dynamics in Gilgel Abbay Watershed by Using GIS and Remote Sensing Techniques, Northwestern Ethiopia

doi:10.4236/ijg.2013.47093

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International Journal of Geosciences, 2013, 4, 1003-1008

http://dx.doi.org/10.4236/ijg.2013.47093 Published Online September 2013 (http://www.scirp.org/journal/ijg)

Retrospective Analysis of Land Cover and Use Dynamics in

Gilgel Abbay Watershed by Using GIS and Remote Sensing

Techniques, Northwestern Ethiopia

Amare Sewnet Minale

Department of Geography and Environmental Studies, Bahir Dar University, Bahir Dar, Ethiopia

Email: [email protected]

Received June 1, 2013; revised July 5, 2013; accepted August 2, 2013

which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ABSTRACT

This study was aimed at examining land cover changes for the last 35 years and its causative factors in Gilgel Abbay

watershed by using GIS and remote sensing, survey and population data. The land use and cover changes study will

help to apply the appropriate land use. The land cover/use status for the years 1973, 1986, 1995 and 2008 were exam-

ined using land sat images. The changes in different land cover units such as forest, wood and bush lands, grass, wet-

lands and water bod ies, and farm and settlements were analyzed. Population change, tenure, poverty and lack of market

and credit facilities in the watershed area were analyzed as causes of land cover changes. The results of the study have

shown that during the last 35 years forest, grass lands, wetlands and lake areas were converted to farm and settlement

areas. There was rapid increase of population with growth rates of 4.9% and 3.5% (1984-1994 and 1994-2007), respec-

tively per annum which caused more land cover changes.

Keywords: Gilgel Abbay Catchment; Land Conversion; Causes of Land Cover Change; Population Increase;

Socio-Economic Factors

1. Introduction

Interpreting and conceptualizing the land cover/use

changes contribute to complex dynamics of land cover

and is important for policy and planning actions [1,2].

Land use changes are caused by both natural and socio-

economic factors [3]. Land use land/cover (LULC) is

perhaps the most prominent form of global environ-

mental change phenomenon occurring at spatial and

temporal scales. Land cover is the physical and biologi-

cal cover of the surface of land, whereas land use is the

syndromes of human activities such as agriculture,

forestry and building construction that alter land surface

processes [4]. The conversion of natural land to cropland,

pasture, urban area, reservoirs, and other anthropogenic

landscapes represents the form of human impact on the

environment [5]. Roughly 40% of earth’s land surface is

under agriculture, and 85% has some level of anthro-

pogenic influence [6]. Therefore, large-scale land cover

change is largely a rural phenomenon, but many of its

drivers can be traced to the consumption demands of the

swelling urban population [7]. Deforestation, wetland

drainage, and grassland degradation have all amounted to

a globally significant alteration of the land co ver changes.

Large scale environmental phenomena like land degra-

dation and desertificatio n, biodiversity loss, habitat des-

truction and species transfer are consequences of land

use by converting natural land covers [8].

The relationship between land cover and use change

and its causative factors is complex and dynamic. The

land cover and use change is mainly manipulated by both

natural and socio-economic factors. Some studies sug-

gested that demographic dynamics contribute more than

any other process to land cover changes [9] while others

suggested the superiority of economic factors [10]. Other

socio-economic factors of land cover change include

poverty, tenure security, and availability of market and

credit facilities. Each of th e above causes was associated

with one or more ultimate causes for land cover change,

if remedied would solve the problem. These factors have

become apparent between, as well as within, individual

regions and countries. Thus, the diversity of causative

factors of land cover change must be considered within

the regional variati o ns [11].

In Ethiopia, different micro studies from aerial photo

and satellite images have revealed that agricultural land

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has been expanded extensively at the expense of other

land uses. Population pressure with other institutional

and socio-economic factors was often cited as a primary

reason for conversion to crop lands in Ethiopia [12-14].

This in turn has an implication to dependency of the poor

people on the environmental resource in the coun try. The

land tenure system in the place so far in the country has

not been secured and the land has been privatized. In

highlands of Ethiopia, land is seriously degraded owing

to problems related to land tenure security and other fac-

tors of degradation. Gilgel Abbay watershed (GAW), sub

area of Lake Tana Basin, is one of such areas, where the

above problems could be manifested.

Accurate information on land-cover changes and the

forces and processes behind is essential for designing a

sound environmental policies and management. The

land-cover analysis provides the baseline data required

for proper understanding of how land was used in the

past and the types of changes to be expected in the future.

This research was therefore, aimed at analyzing the link

between land cover changes and its causative factors in

GAW. Identifying the driving forces behind land use

changes, and developing appropriate measures to mini-

mize their ecological effec ts have great deal of importance

for land use planning. Specifically, this study determines

land cover and use status with special reference to its

causes and evaluation of their consequences through time

in GAW. Accordingly, one of the most densely popu-

lated areas of Ethiopia, the GAW, area of Lake Tana,

was proposed for the study.

2. Study Area and Methodology

2.1. Study Area

Gilgel Abbay Watershed comprises Gilgel Abbay River

and its tributaries. Gilgel Abbay Watershed is located

northwestern Ethiopia and stretches between latitudes

10˚57'N - 11˚54'N and longitudes 36˚38'E - 37˚23'E (Fig-

ure 1). The elevation ranges from 1780 m to 3400 m.

The slope is steeper at the southern part of the watershed

and declines to north wards. Gilgel Abbay River contrib-

utes more than 40% of the volume of Lake Tana’s water

and covers abou t 32% of the Lake’s total catchment with

total area of about 4865 km2. Gilgel Abbay Watershed

falls into two traditional climate zones: “Woina Dega”

(warm) and “Dega” (temperate like highland). The mean

annual rainfall and temperature for the whole watershed

was 1553 mm and 18˚C, respectively. According to [15]

there are about 1.5 million people in Watershed and

about 90% of the population live in rural areas and pri-

marily depend on agriculture.

2.2. Data Sources and Methods of Analysis

The present study used satellite images, socio-economic

Figure 1. Map of the study area.

surveys and population censuses to understand land

cover and use dynamics and factors that bring changes.

Land sat MSS (Multi-spectral Scanner) acquired on

01-02-1973 with path and row of (182p52r) with 4 bands

and, Land sat TM, 1986, 1995 and 2008 (Thematic

Mapper) acquired on 08-03-1 98 6, 1 7- 03 - 19 95 a n d 08- 0 3-

2008 with path and row of 170p52r and 169p52r,

170p52r and 170p52r respectively were used for land

cover and use analysis. These MSS and TM images wer e

geo-referenced, enhanced and transformed before actual

cover change detection. Since MSS and TM images have

different resolutions, they were re-sampled to the same

size using the nearest neighbor re-sampling technique.

Radiometric correction including correcting the images

for sensor irregularities and minimizing unwanted at-

mospheric noise and terrain effects was done by ERDAS.

The sub-setting and mos a i c k i n g to cover the study wa-

tershed was made for some images.

First unsupervised classification was conducted to get

the major land parcels in the watershed. Based on this

information, supervised classification by the help of GPS

points was used to produce thematic land cover maps.

The GPS points were collected from the study area by

frequent visit of sampled parts of the watershed. The

high resolution spot image of 2006 was also used to de-

tect the present land covers. More than 300 signatures

were used for each land unit to convert the images into

thematic land covers. However, because of low resolu-

tion of images, only major land cover types were consid-

ered. The farm and settlement areas were included in the

same land cover, because settlements in the rural areas

are near their farmlands and the swamps, ponds, riparian

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vegetation and marsh areas were also categorized under

wetlands. The major land cover classes were: forest; ag-

riculture and settlement; wood and bush lands; grass

lands; water bodies and wetlands (Table 1).

After classification, calculation of the area in hectare,

comparison of the land use land cover statistics within

and between land class units and years was made and the

trend was determined. Percentage change to determine

the trend of change then is calculated by

211

100

ttt

CA AA , where At1 is the area of one type

of land use in t1 ti me; At2 is the area of the same type in

t2 time. C is a simple and effective method of weighing

relative change of area of one type of land use.

Survey data using structured questions, interview and

focus group techniques were included to substantiate the

image analysis. Population data about districts which

covers about 75% of total watershed from three censuses

were obtained from central statistics office and were used

for change detection in catchment’s population size,

growth and spatial variation.

3. Results and Discussion

3.1. Land Cover and Use Dynamics in GAW

The forest cover includes natural densely grown trees

found in some pocket areas of the watershed. It was con-

stituted only 1.2% of the total watershed in 1973, and

had shown rapid declining during the study time. Its per-

centage share was 0.6%, 0.4% and 0.3% respectively

over the specified years (Table 2 and Figure 2). There

was 73.3% forest cover loss from 1973-2008 from wa-

tershed. The trend was mainly related to rapid population

growth and demand for farming land and construction as

well as fuel wood. Wood and bush land covers have

shown increment from 3.1% in 1973 to 5.1%, 7.6% and

7.7% respectively during the last 35 years (Table 2 and

Figure 2). This shows that, although the proportion was

small, wood and bush land covers have shown continu-

ous increment. Within the specified years grass land has

shown continuous declining change from 20.6% in 1973

to 13.9%, 11.6% and 9.3 % respectively, with very fast

and rapid conversion rate (Table 2 and Figure 2). This

was related to easy accessibility of farming activities and

redistribution of grass lands to landless farmers.

This land cover has shown increasing change during

the specified years. The percentage change was 26.1% in

1973 and 34 .9%, 36.3% and 41.2% r espectively on tho se

years (Table 2 and Figure 2). This was the larger expan-

sion and share than other land use types. This implies

that agriculture and settlement areas have been exten-

Table 1. Land classification and their descriptions.

No. Land class Description

1 Forest Tree-covered land where the trees cover density is greater than 10%.

2 Wood and bush lands Areas with sparse trees mixed with short bushes, grasses and open areas; less dense than the forest with

little useful wood, mixed with some grasses

3 Grass lands Land predominately covered with gra sses, forbs, grassy areas used for communal grazing.

4 Agriculture and

settlement Areas used for crop cultivation, both annuals and perennials, and the scattered rural settlements that are

closely associated with the cultiv a ted fields.

5 Water b o dies Areas covered by La k e , Rivers and stream s in the catchment permanently

6 Wet lands Wetlands include areas that waterlogged and swampy in the wet season, and dry in the dry season, perennial

marshy areas and riparian vegetations.

Source: Amare and Kameswara, 2011.

Table 2. Land cover and use changes in GAW from 1973 to 2008.

Land cover classes Year 1973 Year 1986 Year 1995 Year 2008 1973-2008

Area in ha*. % Area in ha. % Area in

ha. % Area in

ha. % Total loss or

gain

Forest 9328 1.2 4527 0.6 3298 0.4 2581 0.3 −6747

Wood and bush lands 24,645 3.1 39,980 5.1 60,148 7.6 60,863 7.7 36,218

Grass land 162,481 20.6 109,550 13.9 91,748 11.6 73,026 9.3 −89,455

Farm and settlement

land 205,993 26.1 274,947 34.9 286,261 36.3 324,536 41.2 118543

Lake Tana 301,899 38.3 302,946 38.4 301,082 38.1 282,990 35.9 −18,909

Wet lands 84,069 10.7 56,465 7.2 45,878 5.8 44,419 5.6 −39,650

Total 788,415 100 788,415 100 788,415 100 788,415 100

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Figure 2. Land cover and use maps of GAW in 1973, 1986, 1995 and 2008. Source: Amare and Kameswara, 2011.

sively expanded from the conversion of grass, forest and

wetlands. Because, each member of the family added has

to get land for farming and settlement from other land

cover units. The share of Lake Tana in 1973 was about

38.3% and on next period it showed slight increase to

38.4%. But after 1986, the Lake Tana area had shown

declining trend from 38.1% in 1995 to 35.9% in 2008

(Table 2 and Figure 2). There was 6.26% loss of Lake

Tana area for the last 35 years to other land covers. The

rate of conversion of Lake Tana to other land cover is

recent event. From the watershed wetlands have declined

from 10.7% in 1973 to 7.2%, 5.8% and 5.6% over the

specified years respectively (Table 2 and Figure 2).

From 1973-2008 about 47.16% of wetlands were con-

verted to other land units. This implies that wetlands

degradation and their ecological and economical impor-

tance was lost.

The land cover changes that have been recognized in

GAW had shown continuous expansion of arable land in

order to meet the increasing food demands of the grow-

ing population. Arable land expands at the expense of

forest, grass and wetlands. In the watershed, there was

declinging of grass, wetlands and forest land covers from

1973-2008. But the Lake Tana area change was increas-

ing on the first period but decreased for the last two pe-

riods. The land covers of wood and bush lands and farm

and settlement areas have been increasing for the whole

periods.

3.2. Causes of Land Cover and Land Use Change

in GAW

The most cited causes of land cover change in many lit-

eratures are population increase. However, the relation-

ship between population and land cover change is debat-

able issue. Some such as [16] argues that increase in

population has positive effect on resource available.

However, [17] on the other hand finds in Ethiopian high-

lands that population pressure lead to land degradation.

Similarly, studies in different parts of Ethiopia have

shown that population pressure has been found to have

negative effect on scrublands, riparian vegetation and

forests [12,18]. In Gilgel Abbay watershed districts

population for the last 25 years has been growing very

rapidly. It has doubled itself in less than 20 years. Age

proportion of young population under 15 was 48.3%.

The total population of water shed districts in 1984 was

580,258 and in 1994 popu lation of the same districts was

863,432 and in 2007 population of districts became

1,162,956 [15,19,20]. The growth of population per year

from 1984 to 1994 was 4.9% and from 1994-2007 it was

3.5 % (Table 3). Although its growth is declining, it is

still increasing rapidly and brought the scarcity of land,

deforestation and soil erosion in the watershed. This im-

plies that the land cover and use change that is obtained

from image analysis was because of population increase

in the Gilgel Abbay watershed. Furthermore, shortage of

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Table 3. Population of districts in GAW from 1984-2007.

Census years Growth rate

Districts 1984 1994 2007 1984-1994 1994-2007

Sekela 65,699 85,950 138,652 3.1 6.1

Achefer 160,274 238,255 329,074 4.9 3.8

Mecha 156,904 244,801 292,210 5.6 2.0

Bahir

Dar

Zuria 197,581 294,424 403,020 4.9 3.7

Total 580,258 863,432 1,162,956 4.9 3.5

Source: (CSA, 1984; 1994; 2008).

land has forced farmers to cultivate steep slopes and

shallow soils that are vuln erable to degrad ation and leads

to cover change to any one form.

Intensive interview and focus group discussion was

held with community in the stu dy watershed to find other

causes of land cover change other than population in-

crease. It was believed that land is not individually

owned and has affected investment and managed on this

resource. Prior to 1975 in Ethiopia land was in hands of

lords but after 1976 the land became the prime property

of the government [21]. As the result of this, there was

mismanagement of land, including overgrazing and

clearing of forests for different purposes. Poverty is also

linked to land cov er changes in that the poor over use the

natural resources to escape from poverty. [22] has shown

that in Ethiopian highlands, rural poor households have

caused land degradation.

The empirical evidence is obtained by comparing the

food produced and required especially, at the upper part

of the catchment because of degraded land the gap is

very wide. In most cases actual production of variety of

crops per household per year was less than the amount of

food crops needed to feed their family per year. In most

cases, the actual production was lower than the required

food crops by 16.6%, 4.1%, 4.2% and 5.0% per year,

respectively (Table 4). By monetizing the agriculture

output, the income generated from the land by individual

household had a range from US $97 to US $1090/annum,

with a mean income of US $630. As the result of this,

many people in the watershed have to get income for

living from other sources such as selling fire wood, cow

dung and others that are obtained from exploitation of

environmental resources. From the interview held be-

cause of uneven distribution of income and lack of good

management practices over the natural resources, poverty

was prevalent, especially at the upper part of the catch-

ment. Most respondents expressed insecurity of the ten-

ure and rights over the land and better productivity can

be achieved thr ough secured ownership rights to the land

holders.

Table 4. Average required and actually produced crops by

sample households.

Sample kebeleRequired average crops per quintals

1 - 5 6 - 10 11 - 15 16 - 20 21 - 25Total

Abbay sengab 12 9 13 4 38

Lijome 11 20 11 42

Ambo mesk 6 17 14 3 40

Total 6 40 43 27 4 120

Sample kebeleActual average crops

produced per ye ar per quintals

1 - 5 6 - 10 11 - 15 16 - 20 21 - 25

Abbay sengab13 14 8 1 2 38

Ambo mesk2 8 6 19 7 42

Lijome 5 13 9 7 6 40

Total 20 35 23 27 15 120

Because of small land holding size and shortage of

land in highlands of Ethiopia, plowing steep slopes is

contributing to land degradation [17]. In Gilgel Abbay

watershed the average land size of farmers is 0.89 ha.

Farmers didn’t have enough land to plough and tried to

expand their plot by clearing forests and communal

grazing near their plot of farm lands. Farmers are poor to

buy agricultural input and there are no roads which pene-

trate to rural villages and farmers’ knowledge about the

market was very limited. Thus, land tenure insecurity,

poverty, lack of land and inaccessibility to market and

road facilities were the causes of land cover changes in

the watershed.

4. Conclusion

There was land cover change in the watershed because of

demand for agriculture and settlement land for increasing

population, problems related to land policy and lack of

infrastructures. The conversion was very rapid on forest;

wetlands and grass because of lack of strategies to con-

trol and manage these resources. Hence, there should be

strategies of managing open access resources through

participation of local people in the management. There

should be also land use planning by identifying the

proper land for specific purpose so that the marginal

lands will not be put into use. Population increase has

played a major role on land cover changes and there

should be strategies that are proposed to strengthen fam-

ily planning programs. Catchment management should

involve the well-judged use of natural resource with ac-

tive participation of institutions, individuals, organiza-

tions, in harmony with the ecosystem components.

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