Evolution of the effectiveness of stone bunds and trenches ... · Rainfall, runoff and soil loss were measured using 21 large (600–1,000 m2) runoff plots at Mayleba catchment. Results

Evolution of the effectiveness of stone bunds and trenches in reducing runoff and soil loss

in the semi-arid Ethiopian highlands

Gebeyehu Taye, Jean Poesen, Matthias Vanmaercke, Bas van Wesemael,Lotte Martens, Daniel Teka, Jan Nyssen, Jozef Deckers, Veerle Vanacker,

Nigussie Haregeweyn, and Vincent Hallet

with 5 figures and 2 tables

Summary. Soil and Water Conservation (SWC) structures, in particular stone bunds andconservation trenches, have been extensively installed in Tigray since the 1980’s. As the effec-tiveness of stone bunds and trenches in reducing runoff and soil loss depends on their reten-tion capacities, it can be expected that this effectiveness declines over time due to infilling withsediment. However, little is known about the rate of this decline during subsequent years. Wetherefore assessed the effectiveness of SWC structures for two land use types, three slopeclasses and during three consecutive rainy seasons. Rainfall, runoff and soil loss were measuredusing 21 large (600–1,000 m2) runoff plots at Mayleba catchment. Results show that all stud-ied SWC structures are more effective in reducing soil loss than runoff. Conservation trenchesare generally more effective in reducing runoff and soil loss than stone bunds. However, dueto their infilling with sediment, their effectiveness quickly declines over time. By the end of thethird rainy season, their effectiveness was reduced to about one third of their initial effective-ness. The effectiveness of stone bunds remained fairly constant during three consecutive rainyseasons. These findings have important implications, as they demonstrate that many of theinstalled SWC structures (especially in rangelands) are only very effective for short periods(one to two years). Regular sediment removal from conservation trenches is therefore crucialto preserve their effectiveness over longer periods.

Key words: Tigray, soil erosion, soil conservation, storage capacity

1 Introduction

The subtropical Highlands of Ethiopia are seriously threatened by land degradation,due to a combination of biophysical (e. g. steep topography, erosive rains, poor vege-tation cover), socio-economic (e. g. population pressure, poverty) and policy (e. g.poor land use practices, overgrazing) factors (e. g. Hurni 1988, Nyssen et al. 2004,Hurni et al. 2005). High runoff production and soil loss rates are the most critical land degradation processes in the region, leading to both long-term and seasonal food,water and energy insecurity (e. g. Nyssen et al. 2004, Haregeweyn et al. 2006, Zenebeet al. 2013). The relatively short rainy season in combination with a lack of in-situ water infiltration, form important challenges for water-availability throughout theyear (e. g. Osman & Sauerborn 2001, Vancampenhout et al. 2006, McHugh et al.

Zeitschrift für Geomorphologie Vol. 59,4 (2015), 477–493 Articlepublished online February 2015; published in print December 2015

© 2015 Gebr. Borntraeger Verlagsbuchhandlung, Stuttgart, Germany www.borntraeger-cramer.deDOI: 10.1127/zfg/2015/0166 0372-8854/15/0166 $ 4.25

eschweizerbart_xxx

2007, Nyssen et al. 2010) and result in high runoff rates. These high runoff rates causesevere erosion and nutrient depletion problems on the land (Hurni 1988, Harege -weyn et al. 2008a, Girmay et al. 2009). This also leads to important off-site impactssuch as frequent flash floods, the silting up of reservoirs built for irrigation and elec-tricity supply and the deterioration of water quality (e. g. Haregeweyn et al. 2006,Vanmaercke et al. 2010, Guzman et al. 2013, Zenebe et al. 2013).

To address these problems, soil and water conservation (SWC) structures (suchas check dams, stone bunds, conservation trenches and stone bunds with trenches)have been extensively installed in the Highlands since the 1980’s, particularly in thenorthernmost Tigray region (Herweg & Ludi 1999, Nyssen et al. 2004, Desta et al.2005, Descheemaeker et al. 2006). Also micro-dams have been constructed to har-vest water and to develop irrigation (Haregeweyn et al. 2008b, Teka et al. 2013). Toreduce reservoir capacity losses due to siltation, SWC structures are often installed in the catchments of these reservoirs (Nyssen et al. 2004, 2007, Desta et al. 2005,Haregeweyn et al. 2006, Descheemaeker et al. 2006).

Several studies in Ethiopia were conducted to evaluate the effects of such SWCstructures on runoff and groundwater recharge (e. g. Hurni et al. 2005, Nyssen et al.2010, Teka et al. 2013, Taye et al. 2013) and soil loss (Herweg & Ludi 1999, Hurniet al. 2005, Desta et al. 2005, Haregeweyn et al. 2008b, Nyssen et al. 2008, 2009a,Taye et al. 2013). However, little is known about how the effectiveness of these SWCstructures changes over time for different land use types.

Most studies only reported the average effectiveness of these structures, whichis generally derived from plot measurements over a short (1–5 years) period (e. g.Herweg & Ludi 1999, Desta et al. 2005). Nonetheless, this effectiveness is likely todecrease over time as SWC structures are prone to damage and sediment infilling.Knowing how the effectiveness of SWC structures varies through time is crucial toassess their long-term effectiveness and to establish sustainable land use and landmanagement practices. This is not only the case for the Ethiopian Highlands. Recentcomprehensive reviews clearly highlighted the need for studies focusing on the tem-poral evolution of SWC effectiveness (Maetens et al. 2012) and on the incorporationof the effects of SWC structures in soil erosion and sediment yield models (De Venteet al. 2013). Therefore, the objective of this study is to quantify and better understandthe temporal evolution of the effectiveness of stone bunds (SB), conservation trenches(TR) and stone bunds with trenches (SBT) in reducing runoff and soil loss for crop-land and rangeland and for gentle medium and steep slope conditions.

2 Study area

The study was conducted in Mayleba catchment (fig. 1), located in Tigray (NorthernEthiopia) at ca. 40 km west of Mekelle, the regional capital. This catchment of ca.18 km2 contributes surface runoff to the Mayleba reservoir (13° 41� N, 39° 15� E)(Van De Wauw et al. 2008). The altitude of the catchment ranges between 2,290 and 2,835 m a.s. l. The climate of the catchment can be described as cool tropical andsemi-arid (Virgo & Munro 1978). Mean annual rainfall in the catchment during themeasuring period (2010–2012) ranged from 600–700 mm. Rainfall in the NorthernEthiopian Highlands is very erratic and seasonal (Conway 2000, Nyssen et al. 2005).The main rainy season (Kiremt) extends from mid June to mid September and gener-

G. Taye et al.478

eschweizerbart_xxx

ally covers more than 80% of the total annual rainfall, while October to June arecharacterized by very low rainfall and very high potential evapotranspiration rates(Nyssen et al. 2005). Most of the rainfall comes from local, but often intense and ero-sive convective thunderstorms in the afternoon and evening (Nyssen et al. 2005).Average (2010–2012) monthly air temperature measured in the catchment rangesfrom 16°C in December to 24°C in May.

Typical soils of Mayleba catchment are Calcisols, Regosols, Cambisols, and Ver-tisols which developed on limestone and basalt (Van De Wauw et al. 2008). Thesesoils contain relatively high masses of rock fragments in the soil profile (3 to 24%)and have an important rock fragment cover (6 to 38%) at the soil surface (Taye et al.2013). Dominant land use types in Mayleba catchment are cropland and rangeland.Depending on the crop type and available draft power, cropland is tilled one to fourtimes per cropping season, using a traditional ard plough (meharesha) pulled by a pairof oxen (Nyssen et al. 2000).

Evolution of the effectiveness of stone bunds and trenches 479

Fig. 1. Location of Mayleba catchment in Tigray (north Ethiopia) with indication of rainfall-runoff measuring sites: RL: rangeland sites; CL: cropland sites; G, M and S: gentle, mediumand steep slope gradients. At each rainfall-runoff measuring site a manual rain gauge is pres-ent, recording daily rainfall during the rainy season.

eschweizerbart_xxx

Mayleba catchment is intensely treated with SWC structures (most commonlySB, TR and SBT). On cropland, the majority of these structures were installed afterthe construction of the Mayleba reservoir in 1998. Installation of SWC structures onrangeland started more recently in response to increased siltation of reservoirs andwere done by community mobilization through food-for-work and free labour con-tribution programs (Taye et al. 2013). Since croplands are owned by individual farm-ers, they are generally better managed and conserved by these farmers than the com-munal rangeland.

3 Materials and methods

3.1 Studied soil and water conservation structures and installation of the runoff plots

We studied stone bunds (SB), conservation trenches (TR) and stone bunds withtrenches (SBT). Their effectiveness (and its temporal evolution) was tested by com-paring the runoff and soil loss rates from plots with these SWC structures with thecorresponding values for the control plots, i. e. plots with similar characteristics butwithout SWC structures (see fig. 2a).

Stone bunds are embankments of stone walls that are built along the contour ofthe hillslope (fig. 2b). Large rock fragments up to 0.50 m in diameter are used to con-struct the embankment while small rock fragments are used as backfill (fig. 2b). Con-servation trenches consist of ditches and earthen embankments. The embankments(ca. 0.3 m high and 0.5 m wide) are located downslope of the trenches and built fromthe soil dug up from the trenches (fig. 2c). These trenches are arranged in a staggeredmanner on the hillslope. Where stone bunds are combined with trenches, the stonebunds are built downslope of the trenches, while the excavated soil from the trenchesis used for backfilling instead of rock fragments (fig. 2d).

Three sites on rangeland and three sites on cropland were selected for installa-tion of runoff plots, each site corresponding to a gentle (5%), medium (12%) andsteep (16%) slope gradient (fig. 1). On each rangeland site, four runoff plots wereinstalled: a control plot, a plot treated with SB, a plot treated with TR and a plottreated with SBT (table 1). Three runoff plots were installed at each cropland site: acontrol plot, a plot treated with SB and a plot treated with SBT (table 1). The runoffplots had a width of 10 m and lengths varying between 60 and 100 m depending onthe available suitable land to establish them (table 1). Plots at each site were designedto test 3 to 7 SWC structures within the plot based on the recommended spacing of the SWC structures (table 1), which depends on the slope gradient of the site(BoANR 1997). Each plot was kept at least 2 m apart from adjacent plots andbounded by compacted soil bunds (0.45 m wide and 0.30 m high) covered with stoneriprap. These soil bunds were well maintained throughout the measuring campaignto prevent breaches (Taye et al. 2013). At the lower end of each plot a 17.7 m3 rectan-gular collector trench with trapezoidal cross-section was installed and lined with a0.5 mm thick geomembrane to harvest runoff and sediment. The capacity of the collector trench was calculated so that it could easily retain the largest expected daily runoff event (Taye et al. 2013). The geomembranes were regularly inspected forleakages and repaired if necessary.

G. Taye et al.480

eschweizerbart_xxx

Apart from the installation of the SWC structures and plot boundaries, each plotwas managed in the same way as the common land management practices for tillage,grazing and crop type in the region.

3.2 Data collection and analyses

Daily rainfall depths during three seasons were measured using manual rain gauges (of the double cylinder type, RG202) installed at each plot site (fig. 1). Rainfall, runoffand soil loss from the runoff plots was measured from the start till the end of eachrainy season (i. e. 15 July – 2 September 2010, 4 July – 22 September 2011 and 27 June –22 September 2012).

The water depth in the collector trench at the bottom of each plot was measureddaily (at 8:00 AM) during the measuring campaigns. These water depths were thenconverted to runoff depths by means of plot-specific depth-volume relationships andby taking into account the dimensions of the runoff plot. Daily runoff depths werealso corrected for rainfall falling directly on to the trench. Details on the runoff meas-urement procedure can be found in Taye et al. (2013). After measuring water depth


Fig. 2. Soil and water conservation (SWC) structures tested on plots in rangeland and detailsabout the installation of these structures (Mayleba, 9 September 2011). Arrows indicate runoffdirections: a) Control plot without SWC; b) Stone bund (length: 10 m, width: 0.80 m, frontheight: 0.70 m, foundation depth: 0.15 m; c) conservation trenches with soil bund downslopeof the trench (length: 3 m each, depth and width: 0.50 m, space between trenches: 0.60 m), andd) Stone bund with trenches (dimension of stone bund: same as b, and trenches: same as c).

eschweizerbart_xxx

in the collector, the runoff in the collector trench was vigorously mixed by two per-sons using floor brushes. During this mixing process, a depth-integrated runoff sam-ple of 1-liter was collected to determine the sediment concentration of the collectedrunoff (by means of filtration; see Taye et al. 2013). Next, each collector was thenemptied manually and cleaned.

Daily soil loss values were calculated by multiplying measured sediment con-centration of the runoff sample with the corresponding daily total runoff volume.Seasonal runoff depths and soil loss rates were obtained by summing up all the cor-responding daily values. Seasonal runoff coefficients (RCs, [%]), i. e. the fraction ofrainfall measured as runoff, were calculated by dividing the total seasonal runoffdepth by the total seasonal rainfall depth (including rain events that did not generaterunoff) measured at the plot site. For each plot with SWC structures, also the relativeseasonal runoff coefficient (Relative RCs) and the relative seasonal soil loss (RelativeSLs) was calculated by dividing respectively the RCs and SLs of that plot by the RCsand SLs of its corresponding control plot.

Evidently, this procedure of runoff and soil loss measurement is subject touncertainties (e. g. due to errors on the measured water volumes in the trenches, dueto uncertainties on the sediment concentrations), which affect the reliability of theresults. To account for these uncertainties, we assessed their effects on the seasonalrunoff depths, runoff coefficients and soil loss measurements by means of a MonteCarlo simulation approach. This method takes into account the integrated effect ofpotential measuring errors on the daily rainfall values, potential errors on the meas-ured water depths in the collector trench, uncertainties related to the conversion ofthese water depths to runoff values, and potential errors on the measured sedimentconcentrations (resulting from both the sampling procedure and the laboratoryanalyses). Details of this method are discussed in Taye et al. (2013).

3.3 Storage capacity of the SWC structures

Both SB and TR create a retention basin where runoff and sediments are trapped. Thisretention volumes of each SWC structure was topographically surveyed at the begin-ning of the first rainy season in 2010 and at the end of each rainy season, based on amethod described by Desta et al. (2005). From these measurements, the total static storage volume of each plot was calculated based on the number of SWC structures perplot and their dimensions (table 1). Static storage volume losses were calculated by sub-tracting the newly calculated from the previously measured static volume. Sediment vol-umes deposited in the retention zones upslope of the SWC structures were convertedinto mass using sediment bulk density value of 1.18 ton m–3 (i. e. the average dry finesediment bulk density deposited in nearby small reservoirs; Haregeweyn et al. 2006).

4 Results

4.1 Plot runoff

Measured seasonal runoff depths vary between 12 mm and 353 mm with an averageof 104.2 mm (table 2). Seasonal runoff coefficients range between 3.6% for croplandplots on medium slopes with SBT and 50.1% for the rangeland control plot on a gen-

G. Taye et al.482

eschweizerbart_xxx

Evolution of the effectiveness of stone bunds and trenches 483Ta

ble

1.

C

hara

cter

isti

cs o

f run

off p

lots

and

thei

r co

rres

pond

ing

test

ed s

oil a

nd w

ater

con

serv

atio

n (S

WC

) str

uctu

res.

‘Con

trol

’ ind

icat

es th

eco

ntro

l plo

t ha

ving

no

SWC

str

uctu

res.

‘Spa

cing

’ ind

icat

es t

he h

oriz

onta

l dis

tanc

e be

twee

n th

e in

stal

led

SWC

str

uctu

res.

‘NA

’ =no

t app

licab

le.

Plo

t fea

ture

sC

hara

cter

isti

cs o

f the

soi

l and

wat

er c

onse

rvat

ion

stru

ctur

es

Plo

t Cod

eL

and

use

Plo

t len

gth

Plo

t are

a Sl

ope

Trea

tmen

tSp

acin

g In

itia

l sto

rage

cap

acit

y (m

)(m

2 )(%

)(m

)(m

3 /ha)

RL

-G-C

OR

ange

land

6060

05

Con

trol

NA

NA

RL

-G-S

BR

ange

land

6060

05

Ston

e bu

nds

2010

2.8

RL

-G-T

RR

ange

land

6060

05

Con

serv

atio

n tr

ench

es20

127.

5R

L-G

-SB

TR

ange

land

6060

05

Ston

e bu

nds

wit

h tr

ench

es20

246.

4

RL

-M-C

OR

ange

land

6060

012

Con

trol

NA

NA

RL

-M-S

BR

ange

land

6060

012

Ston

e bu

nds

1213

5.0

RL

-M-T

RR

ange

land

6060

012

Con

serv

atio

n tr

ench

es12

220.

7R

L-M

-SB

TR

ange

land

6060

012

Ston

e bu

nds

wit

h tr

ench

es12

354.

7

RL

-S-C

OR

ange

land

6363

016

Con

trol

NA

NA

RL

-S-S

BR

ange

land

6363

016

Ston

e bu

nds

914

0.3

RL

-S-T

RR

ange

land

6363

016

Con

serv

atio

n tr

ench

es9

267.

4R

L-S

-SB

TR

ange

land

6363

016

Ston

e bu

nds

wit

h tr

ench

es9

446.

7

CL

-G-C

OC

ropl

and

100

1,00

05

Con

trol

NA

NA

CL

-G-S

BC

ropl

and

100

1,00

05

Ston

e bu

nds

2081

.5C

L-G

-SB

TC

ropl

and

100

1,00

05

Ston

e bu

nds

wit

h tr

ench

es20

219.

2

CL

-M-C

OC

ropl

and

9191

012

Con

trol

NA

NA

CL

-M-S

BC

ropl

and

9191

012

Ston

e bu

nds

1314

3.5

CL

-M-S

BT

Cro

plan

d91

910

12St

one

bund

s w

ith

tren

ches

1333

8.9

CL

-S-C

OC

ropl

and

7777

016

Con

trol

NA

NA

CL

-S-S

BC

ropl

and

7777

016

Ston

e bu

nds

1116

7.7

CL

-S-S

BT

Cro

plan

d77

770

16St

one

bund

s w

ith

tren

ches

1137

9.8

eschweizerbart_xxx

G. Taye et al.484

Tabl

e2.

Mea

sure

d se

ason

al ra

infa

ll de

pth

(Ps)

, sea

sona

l run

off d

epth

(Rs)

, sea

sona

l run

off c

oeff

icie

nt (R

Cs)

, sea

sona

l soi

l los

s (SL

s), s

easo

nal

stat

ic s

tora

ge c

apac

ity

loss

(SC

Ls)

for

the

soil

and

wat

er c

onse

rvat

ion

stru

ctur

es a

nd e

stim

ated

rat

io (S

R) b

etw

een

the

stor

age

capa

c-it

y lo

ss a

nd th

e in

duce

d so

il lo

ss re

duct

ion

for t

he ra

iny

seas

ons o

f 201

0, 2

011

and

2012

. SR

is c

alcu

late

d by

mul

tipl

ying

the

obse

rved

stor

age

capa

city

loss

wit

h a

dry

bulk

den

sity

val

ue o

f 1.

2to

n/m

3(a

val

ue b

ased

on

Tay

eet

al.

2013

) an

d di

vidi

ng t

his

valu

e by

the

diff

eren

ce in

SL

s ob

serv

ed o

n th

e co

ntro

l plo

t an

d th

e pl

ot t

reat

ed w

ith

SWC

str

uctu

res.

‘NA

’ = n

ot a

pplic

able

. For

det

ails

on

the

plot

s, s

ee ta

ble

1.

Plo

tR

ainy

sea

son

2010

(15

July

–2

Sept

. 201

0)R

ainy

sea

son

2011

(4Ju

ly–

22Se

pt. 2

011)

Rai

ny s

easo

n 20

12 (2

7Ju

ne–

22Se

pt. 2

012)

Ps

Rs

RC

sSL

sSC

Ls

SRP

sR

sR

Cs

SLs

SCL

sSR

Ps

Rs

RC

sSL

sSC

Ls

SR(m

m)

(mm

)(%

)(t

on/

(m3 /

(mm

)(m

m)

(%)

(ton

/(m

3 /(m

m)

(mm

)(%

)(t

on/

(m3 /

ha)

ha)

ha)

ha)

ha)

ha)

RL

-G-C

O35

7.3

179.

250

.150

.0N

AN

A41

7.3

194.

246

.537

.9N

AN

A71

8.2

353.

149

.242

.5N

AN

AR

L-G

-SB

357.

312

6.4

35.4

16.0

17.5

0.6

417.

312

3.1

29.5

12.1

32.8

1.5

718.

224

8.8

34.6

16.4

18.3

0.8

RL

-G-T

R35

7.3

62.2

17.4

5.0

21.4

0.6

417.

313

0.4

31.3

9.5

40.0

1.7

718.

231

7.4

44.2

27.5

46.5

3.7

RL

-G-S

BT

357.

333

.49.

42.

034

.50.

941

7.3

53.8

12.9

2.8

53.1

1.8

718.

213

1.0

18.2

6.7

43.3

1.5

RL

-M-C

O34

8.6

147.

142

.238

.0N

AN

A41

6.6

167.

640

.238

.7N

AN

A67

4.5

285.

242

.340

.6N

AN

A.

RL

-M-S

B34

8.6

113.

532

.616

.032

.81.

841

6.6

139.

533

.512

.227

.81.

367

4.5

236.

735

.115

.922

.21.

1R

L-M

-TR

348.

649

.314

.15.

055

.12.

041

6.6

78.8

18.9

5.7

51.6

1.9

674.

521

4.5

31.8

14.8

34.6

1.6

RL

-M-S

BT

348.

621

.56.

21.

037

.71.

241

6.6

39.3

9.4

2.1

58.5

1.9

674.

510

1.2

15.0

4.0

65.1

2.1

RL

-S-C

O32

0.6

121.

637

.928

.6N

AN

A38

6.8

158.

741

.028

.6N

AN

A65

5.5

303.

046

.237

.9N

AN

AR

L-S

-SB

320.

691

.728

.610

.930

.02.

038

6.8

118.

330

.615

.423

.72.

265

5.5

218.

833

.414

.314

.40.

7R

L-S

-TR

320.

633

.610

.52.

093

.24.

238

6.8

68.6

17.7

7.4

73.5

4.2

655.

521

8.9

33.4

19.7

34.9

2.3

RL

-S-S

BT

320.

620

.06.

21.

050

.62.

238

6.8

34.4

8.9

2.1

71.8

3.3

655.

586

.013

.13.

389

.53.

1

CL

-G-C

O33

5.3

50.5

15.1

11.4

NA

NA

376.

279

.021

.015

.5N

AN

A52

1.1

140.

727

.013

.0N

AN

AC

L-G

-SB

335.

338

.811

.66.

332

.27.

537

6.2

62.9

16.7

8.4

28.0

4.7

521.

199

.719

.15.

911

.31.

9C

L-G

-SB

T33

5.3

21.6

6.4

1.9

76.0

9.5

376.

234

.39.

12.

046

.04.

152

1.1

60.4

11.6

2.9

10.0

1.2

CL

-M-C

O33

1.9

44.2

13.3

4.6

NA

NA

380.

073

.819

.49.

8N

AN

A45

4.4

104.

222

.99.

3N

AN

AC

L-M

-SB

331.

923

.77.

13.

454

.253

.338

0.0

55.0

14.5

4.3

26.8

5.8

454.

481

.417

.96.

216

.76.

5C

L-M

-SB

T33

1.9

12.0

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188

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Fig. 3. Relative seasonal runoff coefficient (RCs) and Relative seasonal soil loss (SLs) for plotswith soil and water conservation structures (SWC) tested during the rainy seasons of 2010,2011 and 2012. Relative values were calculated as the ratio between RCs or SLs for the plotwith SWC structures and the RCs or SLs for the corresponding control plot. Error bars indi-cating the calculated 95% confidence interval, based on Monte Carlo simulations (section 3.2).RL-SB: Rangeland plot with Stone bunds; CL-SB: Cropland plot with Stone bunds; RL-SBT:Rangeland plot with Stone bunds and trenches; CL-SBT: Cropland plot with Stone bunds andtrenches; RL-TR: Rangeland plot with conservation trenches.

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tle slope in 2010 (table 2). The average RCs for all runoff plots over the three seasonsis 21.4%. Plots with SB overall show the smallest RCs reductions compared to thecontrol plots, while plots with SBT show the largest reductions (fig. 3). In terms oftemporal evolution, the effectiveness of TR and SBT in reducing RCs clearly declinesover the three consecutive years for all rangeland plots (fig. 3). Other plots show noconsistent decline in the reduction of RCs (fig. 3).

Very similar patterns in runoff reduction can be noted at the event scale (fig. 4).Daily runoff values on plots with SB amount to about 73% of the daily runoff on thecorresponding control plots and this ratio changes very little over the consecutiverainy seasons (fig. 4). For plots with both SBT this ratio is initially very low (17%),but increases through the three consecutive rainy seasons to about 34% (fig. 4). Plotswith only TR in rangeland show an event stronger decline in their runoff reductioneffectiveness (fig. 4).

4.2 Plot soil loss

Seasonal soil loss (SLs) values range between 0.61 ton/season and 50 ton/season, withan average of 11.5 ton/season (table 2). Overall, the highest SLs values are observedon the control plots, with plots in rangeland showing clearly higher soil losses thanplots in cropland (table 2). Overall, SLs tend to be smallest for plots on steeper slopes(table 2).

Similar to runoff reduction, SB tend to be the least effective in reducing SLs(fig. 3). This effectiveness shows no consistent evolution over the consecutive rainyseasons (2010 to 2012), but is slightly higher for plots in rangeland compared to plotsin cropland (fig. 3). The combination of both stone bunds and trenches results inmuch stronger reductions of SLs (fig. 3). Here also, the reduction tends to be largestfor rangeland plots. In cropland this reduction ratio (fig. 3) remains more or less con-stant, while for rangeland it declines over the three rainy seasons (fig. 3). Conserva-tion trenches installed in rangeland plots prove to be initially very effective in reduc-ing SLs. However, this effectiveness strongly decreases during the consecutive years(fig. 3). At the event (i. e. daily) scale, very similar patterns of runoff and soil lossreduction can be noted (fig. 4).

4.3 Storage capacity of the SWC structures

The initial storage capacity of SWC structures was the highest for plots with SBT,while plots with only SB had in general the lowest initial storage capacity (table 1).Initial storage capacities are larger for plots on steeper slopes because of the largernumber of SWC structures per unit area (table 1). For each of the three tested SWC


Fig. 4. Daily runoff (left) and soil loss (right) for plots with soil and water conservation struc-tures versus daily runoff and soil loss from the corresponding control plot for the same event.These graphs include all runoff events registered on the control plots. Data for both land usetypes (cropland and rangeland) and all slope classes are pooled per year. ‘n’ indicates the num-ber of observations on which each regression is based.

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structures, the static storage capacity clearly declines over the three consecutive rainyseasons (table 2). This decline is most pronounced for TR in rangeland. Three yearsafter installation, TR retain on average only 25% of their initial storage capacity,while this is 35% for SB and 54% for SBT (table 2). Regardless of the land use orSWC measure, plots on gentle slopes show the relatively largest storage capacitylosses (table 2).

5 Discussions

5.1 Representativeness of the seasonal measurements

Runoff and soil loss measurements were only conducted during the main rainy sea-son and therefore represent only about two thirds of the annual rainfall. Nonethe-less, the measured runoff and soil loss values probably represent a larger fraction oftheir corresponding annual values. As other studies indicate (e. g. Descheemakeret al. 2006, Girmay et al. 2009, Nyssen et al. 2009b, 2010), runoff will only occur during rainfall events that exceed the plot’s hydrological deficit. For this study, theminimum daily rainfall that generated runoff on any of the plots over the three meas-uring seasons was 3 mm, while several larger rainfall events (� 5 mm/day) did notgenerate runoff due to low soil moisture contents. Rainfall events that occur outsidethe main rainy season are generally characterized by smaller depths and intensitiesthan those occurring during the rainy season (Nyssen et al. 2005). Furthermore, soilsare generally dry and have well-developed shrinkage cracks outside the main rainyseason, leading to lower runoff responses (Descheemaeker et al. 2006, Nyssen et al.2009b, Vanmaercke et al. 2010, Zenebe et al. 2013, Guzman et al. 2013). Hence it isreasonable to assume that the conducted runoff and soil loss measurements compriseat least 80% of the annual runoff and soil loss.

5.2 Spatial differences in runoff and soil loss

Large differences in terms of runoff coefficients and measured soil loss can be notedbetween the different plot treatments (table 2). Taye et al. (2013) provided a detailedanalyses and discussion of these differences for the rainy season of 2010. Overall, theresults of the rainy seasons in 2011 and 2012 confirm these findings. The higher RCsand SLs on the rangeland plots compared to the cropland plots are mainly explainedby the fact that cropland is commonly tilled just before or at the start of the rainy sea-son. Tillage creates furrows and ridges that strongly enhance depression storage andthe infiltration capacity of the soils (Bewket & Sterk 2003, Guzha 2004), while the soil structure of rangeland in this area is generally less favourable for infiltration,due to continuous trampling by large numbers of livestock. Furthermore, the vege-tation cover in cropland strongly increases from 0% at the start to over 84% towardsthe end of the rainy season, while grazing prevents a substantial vegetation cover todevelop on rangeland (Taye et al. 2013). The observation that RCs and SLs values aregenerally smaller for plots on steeper slopes (table 2) may seem counter-intuitive.However, this is explained by the fact that the plots on steeper slopes generally havea much higher rock-fragment cover at the soil surface (Taye et al. 2013). Higher rock-fragment covers generally increase infiltration rates and reduce runoff and soil

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detachment (e. g. Poesen et al. 1994, Nyssen et al. 2001). These effects most likelyoutweigh the commonly expected positive influence of slope on runoff and erosionrates.

Regarding the effectiveness of the studied SWC structures, it is noteworthy thatthese structures are generally much more effective in reducing SLs than in reducingRCs (fig. 3, 4). This is in line with other studies (e. g. Hessel & Tenge 2008, Maetenset al. 2012). Overall, TR are more effective in reducing runoff and soil loss than SB(fig. 3, 4). This is most likely explained by the fact that TR trap sediment-loadedrunoff more effectively forcing the runoff to either infiltrate or evaporate. Stonebunds are less effective in trapping runoff, but mainly reduce the runoff velocity(Desta et al. 2005, Vancampenhout et al. 2006, Nyssen et al. 2007).

5.3 Effectiveness and storage capacity of the SWC structures

As indicated in the introduction, the effectiveness of SWC measures can be expectedto be proportional to its storage capacity. A comparison of the relative RCs and SLsand their average storage capacity confirms this (fig. 5). Nonetheless, differencesbetween the studied SWC structures can be noted (fig. 5): while TR show a clear cor-relation between their static storage capacity and their effectiveness in reducingrunoff or soil loss, such trend is not apparent for SB. As discussed in section 5.2, thisis most likely because SB are partially permeable for (sediment-loaded) runoff, whileTR and SBT trap the runoff more effectively. SBT were found to be most effective in


Fig. 5. Relative reduction in runoff (left) and soil loss (right) for stone bunds (SB), stone bundswith trenches (SBT) and conservation trenches (TR) on both cropland and rangeland versustheir mean static storage capacity for the rainy seasons of 2010, 2011 and 2012. Relative reduc-tions were calculated as the difference in seasonal runoff depth or area-specific soil loss for thecontrol plot and the corresponding value for the plot with SWC, divided by the seasonal runoffdepth or area-specific soil loss for the control plot. Mean static storage capacities were calculat-ed as the average static storage capacity before and after the indicated rainy season.

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reducing runoff and soil loss (fig. 3, 4), but also have the highest static storage capac-ity (table 1). It is noteworthy that for static storage capacities above 200 m3/ha, theeffectiveness of the SWC structures in reducing soil loss or runoff is no longer cor-related with storage capacity (fig. 5).

A static storage capacity of 200 m3/ha therefore seems to represents an optimumfor TR and SB in the northern Ethiopian highlands, while SWC structures with largerstatic storage capacities may be seen as a waste of effort but their effectiveness is sus-tained over several seasons. This seems especially the case when comparing the reduc-tions in SLs that the studied SWC structures induce with their mean static storagecapacity losses (SCLs; table 2). SCLs values of these SWC structures are commonlytwice as high as their SLs reduction for SWC structures in rangeland and even eightto ten times higher for SWC structures in cropland. These high values correspondwell with findings of Desta et al. (2005) who, for the same region, reported an aver-age sediment accumulation rate behind SWC structures of 58 ton ha–1y–1.

Although the differences between SLs reduction and SCLs may be partlyexplained by the fact that the SLs values underestimate the total annual soil loss, thisalone cannot explain the observed differences as SLs represents 80% of the annualsoil loss (see section 5.1). A more relevant explanation is probably that a large frac-tion of the eroded sediments is deposited again before reaching the collector trenchat the bottom of the runoff plot. For plots with SWC structures, transported sedi-ments will be mainly deposited in TR or SB. However, also on the control plots, alarge fraction of the sediments will be deposited within the plot and will therefore notcontribute to the measured SLs. As earlier studies indicate, tillage may further helpexplaining why static storage capacity losses on cropland plots are much higher com-pared to the static storage capacity losses on untilled rangeland (Nyssen et al. 2000,2007, Desta et al. 2005). Also trampling by cattle likely contributes to the storagecapacity losses in rangelands.

6 Conclusions and implications

Our results show that each of the studied SWC structures i. e. stone bunds (SB), conservation trenches (TR) and stone bunds with trenches (SBT) reduced runoff andsoil loss significantly in both cropland and rangeland compared to untreated land.Nonetheless, this reduction was generally much smaller for runoff than for soil loss(fig. 3, 4). This has important implications for watershed management using SWCstructures. Since most of the sediment export in the northern Ethiopian Highlandsoccurs during a few short but intense flash floods, effective catchment managementrequires measures that reduce runoff production and hence the magnitude of thesefloods (e. g. Vanmaercke et al. 2010, Zenebe et al. 2013). Stone bunds, for example,were found to reduce runoff by only 20 to 30% compared to the control plots (fig. 3,4). Conservation trenches or SBT seem initially more effective in reducing runoff and soil loss in rangeland sites. However, their effectiveness in reducing both runoffand soil loss, generally declines over time (fig. 3, 4). The reduced effectiveness of theSWC structures is attributed to the storage capacity loss due to sediment deposition(fig. 5). To remain effective, the SWC structures need to have a sufficiently large stor-age capacity. Our data indicate that a storage capacity of ca. 200 m3/ha is an optimumvalue in this regard (fig. 5).

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Acknowledgements

This study is part of the project “Improving Water Resource Planning at the Scale of Micro-dam Catchments in Tigray, Northern Ethiopia: Learning from Success and Failure (WAREP)”.This project is financed by the Conseil Interuniversitaire de la Communaute Française de Belgique Commission Universitaire pour le Développement CIUF-CUD. The financial sup-port is gratefully acknowledged. We thank Mekelle University, authorities of Dogu’a TenbienDistrict and local farmers in the study area for their cooperation and for facilitating fieldwork.M. Vanmaercke acknowledges his research grant from the Research Foundation Flanders(FWO).

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Addresses of the authors: Gebeyehu Taye (corresponding author), Daniel Teka, Departmentof Land Resources Management and Environmental Protection, Mekelle University PO Box231, Mekelle, Ethiopia. – Jean Poesen, Matthias Vanmaercke, Lotte Martens, Jozef Deckers,Department of Earth and Environmental Sciences, KU Leuven, Celestijnenlaan, 200E, B-3001Heverlee, Belgium; Research Foundation Flanders (FWO), Brussels, Belgium. – Bas van Wese-mael, Veerle Vanacker, Georges Lemaitre Center for Earth and Climate Research, Earth andLife Institute, Universitè Catholique de Louvain, Belgium. – Jen Nyssen, Department ofGeography, Ghent University, Belgium. – Nigussie Haregeweyn, Arid Land Research Center,Tottori University, Japan. – Vincent Hallet, Department of Geology, University of Namur,Belgium. Corresponding author’s: E-Mail: [email protected] or [email protected]


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Evolution of the effectiveness of stone bunds and trenches ... · Rainfall, runoff and soil loss were measured using 21 large (600–1,000 m2) runoff plots at Mayleba catchment. Results

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