a geographic information system based soil loss and sediment estimation in gerdi watershed,...

8/9/2019 A Geographic Information System Based Soil Loss and Sediment Estimation in Gerdi Watershed, Highlands of Ethiopia

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Journal of Environment and Earth Science www.iiste.org

ISSN 2224-3216 !a"er# ISSN 222$-%&4' (nline#

)ol.4* No.1&* 2%14

62

A Geographic Information System Based Soil Loss and Sediment

Estimation in Gerdi Watershed, Highlands of Ethiopia

+i,achew alew

mhara /esign and Su"ervision 0ors Enter"rise /S0E#* ahir /ar* Ethio"ia

E-mailgi,achewaalew$5ahoo.com

Abstract

his stud was carried out to s"atiall "redict the soil loss rate of +erdi watershed with a +eogra"hic

Information Sstem +IS# and 7emote Sensing 7S#. 78S9E ada"ted to Ethio"ian conditions was used to

estimate "otential soil losses : utili,ing information on rainfall erosivit 7# using inter"olation of rainfall data*

soil erodi:ilit ;# using soil ma"* vegetation cover


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+igre -. Location *ap of Gerdi Watershed

2.2 Methods

he in"ut thematic data included rainfall* soil units* slo"es and land use>cover and determined as follow.

)!)! &etermination of Soil Loss factors

$ainfall Erosi/ity +actor

he monthl amounts of "reci"itation for the watershed were collected over 1$ ears : the mhara 7egional

=eteorological genc. =onthl rainfall records from these meteorological stations covering the "eriod 1&&'-

2%12 were used to calculate the rainfall erosivit Factor 7-value#. he mean annual rainfall was first

inter"olated to generate continuous rainfall data for each grid cell : H3/ nalst ools 7aster ;riging

Inter"olation in rc+IS environment. hen* the 7-value corres"onds to the mean annual rainfall of thewatershed was found using the 7-correlation esta:lished in Curni 1&'$# to Ethio"ia condition.

7 -'.12 K %.$62!LLLLLLLLLLLLLL..LLLLLLLLLLL EDuation 1#

0here 7 is the rainfall erosivit factor and ! is the mean annual rainfall mm#.

Soil Erodibility +actor

HS"atial nalst ool EAtract : =as in +IS environment was used to o:tain soil units ma" of the stud

watershed from mhara 7egional digital soil ma" at 1$%*%%% scale develo"ed : /S and S


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#able ). Soil #ypes and their Areas

Soil t"es

rea

Cectare ha# !ercent ?#

/stric Fluvisols 2&.& $&.6

/stric +lesols 6$.% $.3/stric Nitosols 14$.$ 11.&

(rthic crisols 2'$.2 23.3

otal 122$.6 1%%

Figure $ Soil =a"

Slope Length and Slope Steepness

he 3% m s"atial resolution /E= /igital Elevation =odel# was used to generate slo"e as shown figure 6 :

using HS"atial nalst ool Surface Slo"e in rc+IS 1%.1 environment. he flow accumulation and slo"e

stee"ness were com"uted from the /E= using rc+IS.

Flow accumulation and slo"e ma"s were multi"lied : using HS"atial nalst ool =a" lge:ra 7aster


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)ol.4* No.1&* 2%14

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+igre 0. Slope *ap of Gerdi Watershed

Land 'se1(o/er &ata and (rop *anagement +actor

land-use and land-cover ma" of the stud area was "re"ared from 9N/S satellite image acDuired on 2%13

and su"ervised digital image classification techniDue was em"loed using EN)I $.% software. field checingeffort also was made in order to collect ground truth information. he 9N/ S satellite image was used to

classif the current land use and land cover ma". /igital image "rocessing o"erations were carried out using

EN)I $.% software. In addition* ground truth data were used as a vital reference for su"ervised classification*

accurac assessment and validation of the result. In su"ervised image classifications techniDue* land use and land

cover t"es were classified so as to use the classified images as in"uts for generating cro" management


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a:le 3 9and cover t"es and their areas

rea

=aor land cover Slo"e ?# ha ?


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)ol.4* No.1&* 2%14

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considering local management "ractices and it was taen the weighed value for similar land use t"es. he

corres"onding ! values were assigned to each land use>land cover classes and slo"e classes and the ! factor ma"

was "roduced.

)!)!3 Soil Loss Analysis

he overall methodolog involved the use of the 78S9E in a +IS environment with factors o:tained from

meteorological stations* soil ma"* to"ogra"hic ma"* Satellite Images and /E= as shown in eDuation 4 and figure$. nnual soil loss rate was determined : a cell-:-cell analsis of the soil loss surface : su"erim"osing and

multi"ling the res"ective 78S9E factor values 7* ;* 9S* < and !# interactivel : using HS"atial nalst ool

=a" lge:ra 7aster


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"artl through the contri:ution of rain to runoff =organ* 1&&4#. he soil loss is closel related to rainfall "artl

through the detaching "ower of raindro" striing the soil surface and "artl through the contri:ution of rain to

runoff. he average annual rainfall of the watershed is a""roAimatel 1&%% mm. he result showed that rainfall

erosivit factor 7-factor# value in the watershed ranged :etween 1%$&.6' =Jmm ha1

r1

.

3.2 Soil Erodibility Factor

he erodi:ilit of a soil is an eA"ression of its inherent resistance to "article detachment and trans"ort : rainfall.It is determined : the cohesive force :etween the soil "articles* and ma var de"ending on the "resence or

a:sence of "lant cover* the soil@s water content and the develo"ment of its structure 0ischmeier and Smith,1&'#. he soil erodi:ilit factor ;# re"resents the effect of soil "ro"erties and soil "rofile characteristics on soil

loss 7enard et al.* 1&. Erodi:ilit de"ends essentiall on the amount of organic matter in the soil* the teAture

of the soil* the structure of the surface hori,on and "ermea:ilit 7o:ert O Cil:orn* 2%%%#. he results indicated

that soil erodi:ilit value in the stud watershed ranged from %.1% =gh =J1

mm1

to %.1$ =gh =J1

mm1

ta:le 3 and figure 6#.

#able -. Soil Erodibility +actor

Soil t"e ;-value

rea

ha ?

/stric Fluvisols %.1 2&.& $&.6

/stric +lesols %.1$ 6$.% $.3

/stric Nitosols %.1$ 14$.$ 11.&

(rthic crisols %.1$ 2'$.2 23.3

otal 122$.6 1%%

Figure & Soil Erodi:ilit Factor =a"

3.3 Slope Lenth and Slope Steepness Factor

he influence of to"ogra"h on erosion is com"leA. he local slo"e gradient S su:-factor# influences flowvelocit and thus the rate of erosion. Slo"e length 9 su:-factor# descri:es the distance :etween the origin and

termination of inter-rill "rocesses. In 78S9E* the 9S factor re"resents a ratio of soil loss under given conditions

to that at a site with the TstandardT slo"e stee"ness of &? and slo"e length of 22 m "lot 7o:ert O Cil:orn* 2%%%#.

he stee"er and longer the slo"e* the higher is the erosion. Some researchers have argued that u"slo"e drainagearea is a :etter "arameter when descri:ing the influence of slo"e length on erosion* not slo"e length /esmet O


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Figure 11 /erivation of


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a:le Soil 9oss Summar of the 0atershed

Soil loss rating and class rea

ton>ha>r mm>rM class ha ?

%-$ %-%.$ Non to slight 6$.1 $.3

$-1$ %.$-1 Non to slight 41.6 3'.$

16-3% 1-2.$ =oderate 4%&. 33.4

31-$% 2.$-4 =oderate 11'.6 &.

$1-1%% 4-6.$ Cigh '4.4 6.&

1%1-2%% 6.$-16.$ Cigh 3'.$ 3.1

2%% 16.$ )er Cigh 3'.2 3.1

otal 122$.$6 1%%

Figure 13 Soil 9oss =a" of the 0atershed

3.) Sedi%ent *ieldSimilar to the soil losses* sediment ields were also ver high at the out let of the watershed. he trans"orting

a:ilit of the runoff to move all the eroded sediments was insufficient. s a result de"osition occurs inreservoirs* de"ressions* at the toe of the hills where changes slo"e. hus* the amount of erosion in the

watershed was generall more than the amount of sediment leaving the watershed at the outlet "oint. he most

common method for estimating sediment ield is sediment deliver ratio 1>%.2

#* which is develo"ed from

reservoir surve* or measurement of sus"ended and :ed loads at the gauging station and com"ared with that of

erosion in the watershed.

S &&&%.46 tons "er ear

-! (%"(L'SI%"S A"& $E(%**E"&A#I%"S

he "redicted amount of soil loss and sediment ield could facilitate com"rehensive and sustaina:le land

management through conservation "lanning for the watershed. reas characteri,ed : high to ver high soil lossshould :e given s"ecial "riorit to reduce or control the rate of soil erosion : means of conservation"lanning. he stud demonstrates that the 78S9E together with +IS and 7S "rovides great advantage to


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)ol.4* No.1&* 2%14

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estimate soil loss rate over areas though the in"ut "arameter values need to :e cali:rated to the s"ecific area.

7! $E+E$E"(ES

ililu and +raaff /e J.2%%#. /eterminants of ado"tion and continued use of stone terraces for soil and

water conservation in an Ethio"ian highland watershed. Ecological Economics. 612&4-3%2.

aramin I.* ( /engi,.* ( asan* = !arla. 2%%3#. Soil Erosion Risk Assessment with ICONA Model; CaseStudy: Beya!ari Area. ur Journal of griculture and Forestr 22%%3#.

ewet 0. and eferi E 2%%. ssessment of soil erosion ha,ard and "rioriti,ation for treatment at the

watershed level case stud in the ldr.&44#.

/esmet !.J.J. and +. +overs 1&&6#. +IS "rocedure for automaticall calculating the 8S9E 9S factor on

to"ogra"hicall com"leA landsca"e units. Journal of Soil and 0ater


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of Soil and 0ater $1*Vueen*s

"rinter for (ntario.

Sutcliff J! 1&&3#. Economic assessment of land degradation in the Ethio"ian highlands case stud. National

conservation strateg secretariat* =inistr of "lanning and economic develo"ment* ddis :a:a*

Ethio"ia.amene 9.2%%$.7eservoir siltation in the drlands of northern Ethio"ia causes* source areas and management

o"tions. !h/ hesis* Ecolog and /evelo"ment Series 3%*


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a geographic information system based soil loss and sediment estimation in gerdi watershed,...

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