Poster prepared by Getnet Taye, Enyew Adgo and Teklu Erkossa at the Nile Basin Development Challenge (NBDC) Science Workshop, Addis Ababa, Ethiopia, 9–10 July 2013
COMPUTER 10: Lesson 7 - File Storage and Online Collaboration
Characteristics and estimated onsite costs of sediment lost by runoff from Mizewa catchments, Blue Nile Basin
1. Abstract
This study was conductedin Mizewa watershedwhich is located in Blue Nile Basin (BNB) to
quantify and characterize the suspendedsediment(SSL) and to estimateonsite financial cost of
erosion in terms of yield reductiontaking maize as representativecrop. For this purpose,discharge
measurementand runoff samplingwas madeduring the rainy seasonof 2011 at the outletof three
subwatersheds(lower Mizewa (MZ 0), UpperMizewa (MZ 1) andGindenewur(GN 0)). The samples
were filtered to separate the sediment which was sub sampled for determinationof suspended
sediment concentration(SSC), total Kjeldal nitrogen (TN),organic carbon (OC),NO 3
-,NH 4
+and
available phosphorous(P) content. The onsite cost of erosion was estimatedbasedon productivity
changeapproach(PCA) focusingon available NP losses. The resultrevealedthattheSSC andits NP
contentvaried in spaceandtime, in which lower SSC occurredtowardstheendof therainy season.
The meanseasonaldischargewas found to be 2.12±0.75,1.49±0.52 and 0.57±0.20 m3/sec at MZ0,
MZ1 and GN0 stationsin thatorder while the correspondingsedimentconcentrationwas 510±370
mg/l, 230±190 mg/l and 370±220 mg/l. This lead to the total suspendedsedimentloss (SSL) of 4
ton/ha/year,2 ton/ha/yearand3 ton/ha/yearfrom therespectivesubwatersheds. The on-site financial
cost due to N and P lost associatedwith SSL was estimatedto be 200$/ha, 186$/ha and 227$/ha
from MZ0, MZ1 andGN0 watershedsrespectively. The studyrevealedthattheeconomicimpactsof
soil erosion which is variable based on the characteristics of the land resources and management
practicesdeservedueattention. The result may helpin sensitizingbothfarmers anddecisionmakers
abouttherisk of soil erosionandin targetingmanagementpracticestoovercomethechallenges.
Keywords : Blue Nile basin, Soil Erosion, Runoff, Sedimentconcentration,Nutrientloss
Introduction
Blue Nile Basin is heavily affected by land degradation problems. Overpopulation, poor
cultivation and land use practices are the major cause, which resulting in significant loss of
soil fertility, rapid degradation of the natural eco-systems, significant sediment and nutrient
depositions in lakes and reservoirs (Tamene et. al,2006).To supplement rainfed agriculture
with irrigation, a massive surface water harvesting effort has been undertaken in the dry
lands of Ethiopia in the last few years. However, most of the water harvesting schemes is
under serious siltation and dry up (Amanul, 2009) due to upstream land degradation
mainly soil erosion. Circumstances in turn lead to a reduction of productivity because of
risky land use exercise. This study was conducted in Mizewa catchment to determine loss
of SSL with associated nutrients in runoff, input to the NBDC Program on Water and Food
being implemented in the BNB. The result was helpful for estimating productivity loss and
corresponding economic cost which provide crucial evidence to inform the land users and
policy makers to take actions. Taking this into considerations, it is essential to design and
implement suitable land management practices to curtail optimum utilizations of resources.
Therefore, this study was conducted to quantify suspended sediment as well as nutrient
loss and to estimate impact of nutrient lost on crop yield along with its financial cost in
Mizewa catchments.
Results & Discussion
The averagedischargewas 2.12±0.75, 1.49±0.52 and 0.57±0.20 m3/sec with total flow volume of
18.34(106m3), 12.87(106m3), and4.92(106m3) per seasonat MZ0, MZ1 andGN0 rivers respectively.
Peak daily dischargewas observedaroundAugust17- August26 in thethreestations,may bedueto
excess in saturationand full vegetationland coverage. Mean flow rate was statistically significant
betweensites (t=2.68 andP≤0.025 betweenMZ0 andMZ0, t = 6.58 andP≤0.000 betweenMZ0 and
GN0 andt=5.54 andP≤0.000 betweenMZ1 andGN0 rivers.
Mean SSC duringtherainy seasonwas 510±370 mg/l, 230±190 mg/land370±220 mg/lfrom MZ0,
MZ1 andGN0 stationsrespectivelyandsignificant variationswas observedbetweentimeandspace.
In MZ0 mean SSC varied from 67 mg/l to 900 mg/l per decade. SSC coefficient of variation
betweenperiodswas 73%, 82 % and 61% for MZ0, MZ1 and GN0 stations,respectively implying
thatSSC in MZ1 was morevariable over timethanMZ0 andGN0 rivers. Statistical testfor temporal
and spatial variability of mean SSC (mg/l) over decades has shown that there is a significant
variation betweenperiods(F=4.51 andp≤0.0032) andsites (F=5.61 andp≤0.013) betweenthethree
sites. From thegeneraltrendof hysteresisloop, it is possibleto concludefor MZ0 andMZ1 stations
for a given runoff discharge,lower SSC values occur towardstheendof therainy seasonthanat the
beginning. This may be due to an increase in vegetationcover which decreasedsedimentsources;
though,dischargehas positive correlation to SSC (Amanuel, 2009). Walling (1977) indicatedthat
scatterSSC –Q relationshipis typical of ‘supply-limited’ or sedimentsourcesconditionsin its upper
catchmentswhich can be explainedby clockwise hysteresis effects of sedimenttransportsystems.
This is mostly attributedto sedimentdepletionin upperslopesof a basin, sometimesevenbeforethe
runoff has peakedsedimentis derivedfrom thebedandbanksof thechannelor areasadjacentto the
channel(Ongley,1996). GN0 stationdisplay counterclockwisehysteresisloop early in runoff (from
D1 to D4) reversing to clockwise afterward. This may be causedby a variety of factors relatedto
sedimentsourcesanddischargeconditions; initial sedimentcontributionfrom thestreambedandits
banks, a delayedcontributionof sedimentfrom subcatchmentandoccurrenceof dry periods(Seeger
etal., 2004).
Mean clay, silt andfine sandcontentof sedimentwas 42±10, 39±5, 20±15% for MZ0; 37±13, 36±4,
27±17% for MZ1 and40±1138±3, 22±15 % for GN0 stationrespectively. In this studyconcentration
of silt and clay have a decreasing trained with time; while, the proportionof fine sand in the
sedimentincreasestowardsendof runoff sedimentmonitoringseasons(Amanul,2009). Particle size
was highly relatedto thesedimentloss, this is becausetheactive fractionof sedimentis usually cited
as thatportionwhich is smaller than63µm (silt + clay) (Lal, 998). Statistically textural contentof
suspendedsedimentwas not significant (P<=0.05) betweenthe stations. Significant correlation of
sedimenttexture was observed with all NC and SSC in all stations. This implies that fine soil
particles play greatrole in theprocessof erosionin thewatershed(Lal, 1998); reflects therate and
severity of erosion in the study watershedand it was a challenge for the livelihood of the poor
farmers. This is because nutrients are strongly adsorbed to the finer soil fractions, which are
preferentiallytransportedby thesedimentationprocessesbecauseof theirhighspecific surface areas
(Haregeweynet al.2008). Mean soil loss from MZ0 catchmentusing RUSEL modelwas estimated
to be13.21 ton/ha/year,nearly 80 % of soil loss was from 15 - 50 % slopeclass dominatedby finger
millet, nigger seed and teff cultivation, and this is above tolerable soil los limit in Ethiopia.
Cultivated land use system contributingmore than 60% of total soil los (mean=20.4 ton/ha/year)
which was about1.5 timesmeanannualsoil loss ratein Mizewa.
Seasonal OC concentration was 23.8±10.1 g/kg(CV=42%) in MZ0 (Fig.2.a), 19.76±9.44
g/kg(CV=48%) in MZ1 (Fig.2.b) and 10.37±6.12 g/kg (CV= 59%) in GN0 (Fig.2.c) monitoring
stations. This OC lost revealedthatarea specific organiccarbonloss of 97 kg/ha,35 kg/haand30.4
kg/ha from the respective watersheds. This may result in a serious detrimental effect land
productivityin bothshortandlong termsin which threateningthefood security of thelocal people,
this is becausein the process of erosion, loss of OC leads to depletionof soil and other nutrients
associated with the organic fraction (Lal, 1998). In addition to OC loss, TN concentrationsin
suspendedsedimentwas vary from 0.43 g/kg in MZ1 (Fig.2b) to 4.46 g/kg at MZ0 (Fig.2a) with
meanvalue of 2.05±0.87 in MZ0 (CV= 42%), 1.68±0.85 in MZ1 (CV= 50%) and0.88±0.47 in GN0
(CV= 54%) stations. Area specific TN lost throughrunoff sedimentin each monitoringstationwas
8.4 kg/ha at MZ0, 3.1 kg/ha at MZ1 and 2.5 kg/ha at GN0 only in monitoringperiod. Significant
plantavailable nutrientswere lost in associatedwith runoff and sediment. Mean area specific plant
available NP lost was (2.3 kg N/ha, 4.0 kg P2O5/ha), (1.6 kg N/ha, 4.1 kgP2O5/ha) and(2.3 kg N/ha,
4.8 kgP2O5/ha) from MZ0, MZ1 and GN0 catchmentsrespectively. Statically, there was a clear
difference in the concentrationof sedimentnitrate(F=6.23, p=0.006) and NH4
+ (F=3.85, p=0.034)
across stationsduring the study period. There was no temporalvariation in available plantnutrient
concentrationregardlessof thestationsexceptonly for soluble phosphate(F=10.47, p≤0.000). This
available nutrientspecies compositionand magnitudevaried widely within the watershedwhich
could be causedby several factors thatneedsfurther researchand detail data to come up the with
controlvariablesfor thesedifferencesamongstations.
In this study, plant available N and P lost throughrunoff suspendedsedimentwas responsiblefor
significant economic onsite costs, and this was reflected in maize grain yield reduction during
monitoringseason. Regression equationsd betweenmaize grain yield and additional N and P2O5
application based on Tilahun Tadesse et.al (2007) data source was used as a bridge to link soil
nutrientlost with grain yield loss of maize crop. R2 (in theequationsshowsa wide variationof yield
responseto thealmostequivalentamountof fertilizer level. The show thatmeangrain yield with no
P andN fertilizers were 2691kg/haand2537kg/ha,respectively. Correspondingly,thelost netmaize
grain yield due to the loss of available N and P were (134 kg/ha, 320 kg/ha, total 453 kg/ha) from
MZ0, (93 kg/ha, 328 kg/ha, total421 kg/ha) from MZ1 and(134 kg/ha,382 kg/ha, total453 kg/ha)
from GN0 watersheds. Taking 20 quintal/ha averagemaize grain yield productivityin thestudyarea
(accordingto CSA, 2011), thelost maize yield dueto available N andP2O5 account23%, 21% and
26% productivity reductionfrom MZ0, MZ1 and GN0 watershedscorrespondingly. This effect of
soil erosionon grain yield is abovetheestimatesof (Helmecke, 2009) cereal (10%), pulse(5%) and
root (12%) crops productionloss estimatedat global scale. As a result a farm enterprisehaving a
hectareof landwith maize cultivationin thestudyareahas a profit loss of about200$/hafrom MZ0,
186$/hafrom MZ1 and227$/hafrom GN0 watershedin consequenceof plantavailable N andP lost
throughrunoff soil erosionprocessonly in oneparticularrainy season.
CHARACTERISTICS AND ESTIMATED ONSITE COSTS OF SEDIMENT LOST BY RUNOFF
FROM MIZEWA CATCHMENTS, BLUE NILE BASIN
CHARACTERISTICS AND ESTIMATED ONSITE COSTS OF SEDIMENT LOST BY RUNOFF
FROM MIZEWA CATCHMENTS, BLUE NILE BASIN
Getnet Taye 1, Enyew Adgo 1, Teklu Erkossa 2
1 Bahir Dar University, collage of Agriculture and Environmental science, P.O.Box 78,
2 International Water Management Institute, Adiss Ababa, Ethiopia, P.O.Box 5689
Getnet Taye 1, Enyew Adgo 1, Teklu Erkossa 2
1 Bahir Dar University, collage of Agriculture and Environmental science, P.O.Box 78,
2 International Water Management Institute, Adiss Ababa, Ethiopia, P.O.Box 5689
Methods
The study was conductedat Mizewa catchment; Fogera, Northwest of Ethiopia, drained by
Mizewa River a tributaryof the Rib River thatfeeds to Lake Tana; covering a total area of 2664
ha, located between latitude 11.88°–11.94°N and longitude 37.78°-37.86°E. Chromic-
Luvisols,Chromic Vertisols and Leptosols are the most common soil types with basaltic rock
formations(Birhanu et.al.,2012). Mixed crop-livestock farming system with maize, rice, finger
millet, teff, groundnutand barley are the principal crops grown in the area (Birhanu et.al.,2012).
Urea and DAP are the commonly used chemical fertilizers. Mizewa lie between1852m and
2360m, dominatedby hill to rolling undulatingplain land forms and characterizedby unimodal
rain fall (mean,1204mm) pattern,peaksaroundAugust 20th.Mean annualtemperaturerangesfrom
16.730C to19.320C.
Flow height(h), surface flow velocity (Vs) measurementand suspendedsedimentsamplingwere
conductedat thethreemonitoringstationsthreetimes a day from July8, 2011 throughOctober16,
2011 to calculate discharge (Q), suspendedsedimentconcentration(SSC), suspendedsediment
load (SSL), nutrientconcentration(NC) and nutrientloss (NL) . Fertilizer maize grain yield (GY)
response data were obtained from research results under similar agro-ecological conditions.
Surface flow velocity was measuredusing a plastic bottleand convertedto the averagevelocity
(V) using Graff methoda (1996). Record of h was convertedinto flow cross-sectional area (A)
usingan empirical relationshipb betweenh andA. The volumeof water(Q) passinga cross section
per unit of time was calculated using the area-velocity methodc (Hudson, 1993). Sample runoff
sedimentwas bulkedin to oneas decade(D) accordingto thedateof samplingstartingfrom July8,
2011 throughOctober 16, 2011 in order to have enoughsedimentfor laboratoryanalysis and to
reducecost of laboratory. A compositesub-sampleof onelitter was takenfrom bulkedsamplesfor
analysis. In the laboratory, the decade runoff sediment sample was filtered using Whatman
(0.42mm) filter paper to have SSC . The filtered water was analyzed for dissolved nitrate and
dissolved phosphate. The sedimentleft on the filter paperwas air dried and weightedto analyze
texture,OC, TN, NO-
3, NH+
4, andavailable P concentration. In laboratory,OC was assessedusing
Walkley and Black (1934), texture of sediment was determined using hydrometer method
following Sahlemedhin and Taye (2000), Jackson (1958) method was used for TN, while
Gregorich and Ellert(1993) methodwas applied for NO-
3 and NH+
4 analysis and Olsen et.al.
(1954) procedurewas applied for available P. Dissolved nitrate and dissolved Phosphatewere
determinedusingspectrophotometer(Bache andWilliams, 1971). Load of sediment(SSL), load of
dissolved NO3
-and PO4
3- load was productof Q(m3/D) and their concentrationin mg/l. Sediment
boundedloads of OC, TN, NH4
+, NO3
- andavailable P was theproductof mg/kgof each species
with SSL massin Kg/D. Seasonalloadwas thesumof 10 decadesof eachspecies.
RUSLE modelestablishedby Wischmeier andSmith (1978) was appliedto identify hotspotareas
for sedimentsources. Hysteresis loop was developingto assess the temporaleffect on sediment
and dischargeinteractionover time. Productivity changeapproach(PCA) techniqueBojo (1995)
was used to estimate on-site cost of soil erosion. Available NP lost with runoff suspended
sediment,NP fertilizers responseof maize GY andmarketprice of GY werethebasic datasources
for evaluatingthecost of nutrientloss throughrunoff soil erosion. Effect of soil loss on crop yield
andits onsitefinancial cost estimationwas calculatedtakingtheloss of available NP nutrientsand
puta value onit usingtheequivalentestimatednetmaize grain yield loss. Effect on cropyield was
simply calculatedas thenetgrain yield lost betweenpotentialgrain yield dueto lost available NP
onfittingcurve andmeangrainyield with noNP fertilizers.
Statistical comparisons were performed using SPSS 16. Analyses were performed to make
comparison with in groups of runoff sediment and nutrient loss between sites and decades.
Significance differences in sedimentload, rate of discharge, and nutrientloss betweensites was
determinedby t-test at 95% confidence limit. Pearson correlation analysis was done for effect
analysis amongsedimentparameters.
Conclusions & Recommendations
During
water
which
understand
during
study
downstream
The
OC/ha,
3
2
GN
transport,
measuring
sediment
The
characterized
great
and
the
indicated
the
though
natural
This
and
MZ
to
nutrient
considered
The
an
same
study
benefit
letting
measures,
Therefore
national
to
impact
productivity
expedite
prevent
Acknowledgements
This
Challenge
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
Conclusions & Recommendations
During the monitoringperiod 18.34x106m3, 12.87x106m3 and 4.92x106m3 of
water was lost from MZ0, MZ1 and GN0 watershedsin the form of runoff
which has a potentialto irrigate a significant hectareof land, so thatonecould
understandthe valuable benefits gained by farmers if this water was used
duringdry seasonthroughwaterharvestingtechnologiesthoughit needsdetail
study for recommendations,as the off-site costs from sedimentationand other
downstreamimpactswerenotinvestigatedin this paper.
The meanSSL of 4 ton/ha,2 ton/haand 3 ton/hain associationwith ((97 kg
OC/ha, 8.4 kg TN/ha, 2.3 kg available N/ha, and4 kg P2O5/ha), (35 kg OC/ha,
3.1 kg TN/ha, 1.6 kg available N/ha, and4.1 kg P2O5/ha) and(30.4 kg OC/ha,
2.5 kg TN/ha, 2.3 kg available N/ha, and4.8 kg P2O5/ha)) from MZ0, MZ1 and
GN0 watershedsrespectively. The sediment lost did not consider bed-load
transport, which might be important in the Mizewa catchments. Hence,
measuringbedload in futureis importantin orderto obtainmorerealistic total
sedimentandnutrientloadcalculation.
The relationshipbetweendischarge and suspendedsedimentconcentrationis
characterizedby clockwise hysteresis for MZ0 and MZ1 stations, despitethe
great differences betweenthe decades,i.e. differences in terms of pre-decade
anddecade. Estimatedsedimentloss (RUSEL, 13.2 ton/ha/year)dataregarding
the loss of SSL and associated plant nutrientsduring the monitoringperiod
indicatedthattheridges of MZ1 andGN0 rivers alongwith themiddlepartof
thewatershedandlower partof MZ0 werethemostcritical sourceof sediment;
thoughit needsfurther investigationas of the complex interactionof multiple
naturalandanthropogenicfactors.
This study concludethat, a reductionin maize gain productivityof 23%, 21%
and 26% and equivalentfinancial cost of 200$/ha, 186$/ha and 227$/ha from
MZ0, MZ1 andGN0 catchmentsrespectivelywas estimated. However, in order
to obtain a better picture of erosion impacts in the area, studies on other
nutrient losses like calcium and magnesium, off-site effects need to be
considered.
The study also recommenda detail study as of runoff water harvestingalso is
an opportunityfor enhancingrural livelihoods and food security and at the
same time minimizes the risk of erosion in the Mizewa watershedsand as the
study translatesthe onsiteeffect of soil erosion into economicterms; this will
benefitthe understandingof the problemby land users and/orpolicy makers,
letting them see the need to promote and/or implement soil conservation
measures,as thatis thelanguagethattheyusually understandbest.
Therefore, the researcher recommends considering cost of soil erosion in
nationaleconomyaccountingis importantto show significance of soil erosion
to policy makers andto land users; evenif, moreeconomicandenvironmental
impact analyses at the country level are neededto help set priorities for land
productivityissues, to assess the costs and benefitsof policy decisions, and to
expedite identification of the type of investments that will be required to
preventlandresourcesdegradationandincreaseproduction.
Acknowledgements
This paper presents findings from IWMI Nile4 project of the CGIAR
ChallengeProgramonWater andFood ; Blue Nile Basin, East Africa, Ethiopia.
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Nile
a V=0.6*Vs, b A=5.5h2+3.6h for MZ0 , A=2.9h2+0.95h+0.05 for MZ1 andA=9.95h2+7.44h for GN0, c Q=V*A
d GY =-0.29N2+58.6N+2537.3(R2=0.75) and GY= -0.55(P
2
O
5
)2+82.25P
2
O
5
+2690.7 (R2=0.88) regression equations between GY of maize to N and P
2
O
5
.
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