REPUBLIC OF RWANDA

RWANDA WATER AND FORESTRY AUTHORITY P.O.BOX: 7443 KIGALI Department of Water Resources Management

Mapping prone areas to Soil

Erosion in 20 districts of

Northern, Southern and

Western Provinces

August 2019

1. Background

Erosion risk map of Rwanda has been

developed in June 2018 using the

Catchment Restoration Opportunity Mapping

(CROM) Spatial Decision Support tool.

CROM model was developed based the

Universal Soil Loss Equation (USLE model)

originally introduced by Wischimeier and

Smith in 1978. The USLE model counts five

input parameters derivable from Rainfall

(R), Soils (K), Topography/ Relief (LS),

Land cover and crop management (C), and

conservation practices (P), each having a

multiplier effect as follow:

A=R×K×LS×C×P

Where A is the average annual loss (T/Ha);

R is the rainfall-runoff erosivity factor; K is the

soil erodibility factor; LS is the slop length (L)

and steepness (S) factor; C is the cover and

management factor; P is the land

management and conservation practices

factor. Combining these factors in the GIS

model builder, the CROM helped to identify

six erosion risk classes: (1) No erosion risk,

(2) Low erosion risk, (3) Moderate erosion

risk, (4) high erosion risk, (5) very high

erosion risk and (6) the extremely high

erosion risk. CROM report is available at

RWFA/IWRM department and ESRI

Rwanda office and priority restoration

measures are describe in the report.

However this risk map did not show the

areas already covered by erosion control

measures in place and the areas not

protected yet. The map does not neither

indicate the erosion features as proof of

risk, nor recommended practices to

mitigate those risks. This makes it hard to

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know the progress made to fight against

erosion and their results. Moreover, the

plan for the future interventions becomes

difficult because the erosion risk map

shows the potential while districts need to

know where exactly the problem still and

what is appropriate measures taking into

account current land use. Therefore, the

team which was recruited to do the forest

mapping was also tasked to use the same

World View images and map erosion the

control practices currently in place using

visual image interpretation techniques. The

team was also tasked to recommend

appropriate control measures where are

required. This study covers the 20 districts

in Rwanda from West, North and South

Province.

2. Brief on methodology used to map

erosion control and recommended

practices

Defining the areas with no risk to high risk soil

erosion, observation on land use/cover type

is very important. For example, areas

covered by forests, pasture, perennial crops

(tea, coffee and banana), and agricultural

land with erosion control measures such as

terraces, agroforestry in alley cropping and

hedgerows are mapped as areas with

minimum or no risk though CROM model has

previously identified such areas as high risk

areas. High risk zones occupied by

settlements (built-up areas) are considered in

the interpretation and delineated to ensure

storm water management interventions are

planned in the future.

In the erosion control mapping, three high

erosion risk categories identified by CROM

model were prioritized: (1) high risk, (2)

very high risk and (3) extremely high risk.

Using World View images areas of four key

erosion factors has been mapped: 1) land

cover/use, 2) occurrence of erosion

feature, 3) erosion control measures in

place, and 4) Recommended erosion

measures.

The first step was to clean-up the CROM

raster data using Geo-processing tools of

ArcGIS/ArcMap. This step consists of

reducing individual pixels of different erosion

risk categories and remaining with large

areas with the similar risks. This

generalization process ensures that original

risk categories modelled by CROM are kept

as much as possible. It is just to allow the

merge of the neighboring cells (at least three

neighbor pixels) in order to produce a map

easy to be interpreted. The processed raster

data was then converted to vector data, a

format which is easy to use in spatial multi-

criteria analysis and produce statistics. The

geodatabase was finally created which

contains (1) three risk categories identified by

CROM-DSS model, (2) Possible erosions

features observable on the WV images, (3)

existing land use/land cover classes, (4)

observed erosion control practices, and (5)

recommended erosion control practices for

risk categories which are currently without

erosion control measures or where the

existing measures are judged inadequate

and require the revision.

3. Summary findings

The following tables show erosion risk

categories, areas, erosion features

currently in place, land uses factors and

erosion control measures that are

observable; and recommended

interventions ensure better land protection

against erosion in the 20 districts of

Northern, Southern and Western

Provinces.

Table 1 shows the top three districts most

susceptible to erosion with over 50% of

their land currently at high risk of water

erosion; the first being Muhanga with 63%

of its land prone to water erosion,

Ngororero district with 61% and Rutsiro

with 53% of its total land identified as high

erosion risk zones. Other districts such as

Gakenke, Burera, Karongi, Rulindo and

Nyamagabe districts need also

considerable attention as the risk accounts

more than 40% of the district land to high

erosion risk zones. An in-depth analysis

and characterisation of each high risk

mapping unit, with the related erosion

control recommendations will be

demonstrated in the full report.

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Table 1: District level erosion risk and proportion of land affected

District Name District High risk Very Extremely Total % Land

land (Ha) high high risk with

risk Erosion

risk

Muhanga 64,772 17,805 15,532 7,264 40,601 63

Ngororero 67,899 17,302 15,918 8,269 41,490 61

Rutsiro 65,995 11,175 15,933 8,075 35,183 53

Gakenke 70,325 16,055 13,116 5,615 34,787 49

Karongi 79,298 17,929 11,179 5,462 34,571 44

Rulindo 56,699 12,141 8,542 2,298 22,981 41

Nyamagabe 109,036 19,487 17,926 6,219 43,632 40

Burera 58,856 11,465 8,417 3,216 23,098 39

Nyaruguru 101,027 16,695 14,201 6,989 37,885 37

Nyabihu 52,958 9,295 6,766 2,323 18,384 35

Kamonyi 65,553 9,980 7,014 2,204 19,198 29

Huye 58,153 9,036 4,823 2,166 16,025 28

Ruhango 62,678 9,835 3,806 1,230 14,870 24

Nyanza 67,215 9,889 4,365 901 15,155 23

Gisagara 67,920 11,106 3,139 354 14,599 21

Rubavu 34,090 2,489 2,420 1,694 6,603 19

Nyamasheke 94,802 8,512 5,830 3,648 17,990 19

Musanze 50,717 4,493 3,431 1,164 9,087 18

Gicumbi 82,721 10,029 3,722 601 14,352 17

Rusizi 91,731 4,167 1,717 200 6,084 7

Grand Total 1,402,446 228,885 167,798 69,893 466,576 33

Of One million four hundred thousand

hectares (1,402, 446 ha) of country land

located in the Northern, Western and

Southern Provinces, about 466,576

hectares (33%) have shown high to

extremely high erosion risk of which 49%

are at high risk (228,885 hectares), 36% at

very high risk (167,798 hectares) and 15%

at extremely high risk (69,893 hectares).

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Table 2: District areas affected by erosion with features confirming the current risk

District Total Gullies Severe Landsli- Rill Not Grand Percenta- Name Land (ha) gullies des (ha) erosion Visible Total ge (%)

(ha) (ha) (ha) on WV

(ha)

Muhanga 64,772 72 121 34 7,377 32,998 40,601 63%

Ngororero 67,899 377 597 31 30 40,456 41,490 61%

Rutsiro 65,995 10,913 1,818 105 13,497 8,851 35,183 53%

Gakenke 70,325 8,296 1,957 238 138 24,158 34,787 49%

Karongi 79,298 6,516 518 5 14 27,518 34,571 44%

Rulindo 56,699 1,581 561 5 0 20,833 22,981 41%

Nyamagabe 109,036 2,604 951 710 16,064 23,304 43,632 40%

Burera 58,856 965 2,416 48 128 19,542 23,098 39%

Nyaruguru 101,027 116 46 27 670 37,026 37,885 37%

Nyabihu 52,958 306 283 14 1,081 16,699 18,384 35%

Kamonyi 65,553 3,721 829 115 501 14,030 19,198 29%

Huye 58,153 4,063 413 41 0 11,508 16,025 28%

Ruhango 62,678 849 129 3 0 13,888 14,870 24%

Nyanza 67,215 25 0 0 4,934 10,196 15,155 23%

Gisagara 67,920 1,109 7 19 10,066 3,398 14,599 21%

Rubavu 34,090 1 0 0 0 6,602 6,603 19%

Nyamasheke 94,802 16 17 0 1,089 16,868 17,990 19%

Musanze 50,717 0 0 0 0 9,087 9,087 18%

Gicumbi 82,721 2,827 21 0 22 11,482 14,352 17%

Rusizi 91,731 814 69 109 61 5,030 6,084 7%

Grand Total 1,402,446 45,170 10,752 1,506 55,673 353,475 466,576 33%

The table 2 shows that of the high risk land

(466,576 ha), already 12% contain gullies

(55,922 hectares) and some of them are

severe gullies (10,752 hectares) in Burera,

Rutsiro and Gakenke Districts and 12% have

shown rill erosion (55,673 hectares).

However about 76% of this land did not show,

on the image the erosion features. This does

not mean that there is no erosion but rather it

is because images used were mostly taken

during the summer time (May-August) the

winter images were clouded.

From table 3, it is shown that land in the

high risk areas is mostly used for

agriculture with seasonal crops (61.3%).

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This exposes land to splash erosion and

further detachment as land is not

permanently covered. In fact, the crop

management and cover factor (C) is very

high for seasonal crops with conventional

(regular) tillage. Forests with good canopy

density occupy only 25%. This means that

land will continue to be eroded if no serious

measures are taken in agricultural lands.

Mining areas in high risk zones accounts for

0.2% of the total high risk land mapped. Built

up area occupy 4.3% of the risk area. Built up

area accelerate water velocity, the runoff and

the flow accumulation which creates severe

gullies downstream.

In such areas water harvesting infrastructure

should be established to collect storm

water from houses in agglomerated zones

which are also mapped.

Table 3: Different land use/cover types observed in high erosion risk zones

Land use / cover types Total (Ha) Percentage

Seasonal crops 285,841 61.3

Dense forest (good canopy density) 117,554 25.2

Build-up area 19,976 4.3

Degraded forest 15,424 3.3

Banana 9,242 2.0

Tea 5,384 1.2

None 4,949 1.1

Water body 4,714 1.0

Coffee 2,603 0.6

Mining concession 856 0.2

Pasture or prairie grass 33 0.01

Total 466,576 100.00

The table 4 summarizes the erosion control

techniques and proportion of land at high risk

which are today protected against erosion for

each district. This table shows that about

311,613 hectare (about 66.8 % of total land

identified as vulnerable to

erosion) are to date not protected yet. Only

25.5% are protected by forests (119,119 ha),

3.8% are protected by bench terraces

(17,812 ha) and 3.8% only are also protected

with contour banks terraces i.e. progressive

terraces (17,292 ha).

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Table 4: High risk areas already protected against erosion and type of protection

District District Bam Bench Conto Forest Gras Hedgero Unprote Total

Name land (ha) boo terrac ur (ha) sed ws trees cted land % of

plant es bank water or land under unprot

ation (ha) terrac ways shrubs (ha) risk ected

(ha) es (ha) (ha) (ha) land

(ha)

Burera 58,856 523 689 4,506 12 17,368 23,098 75%

Gakenke 70,325 12 642 1,864 8,786 16 23,467 34,787 67%

Gicumbi 82,721 1,653 497 4,390 7,812 14,352 54%

Gisagara 67,920 18 926 408 3,059 10,187 14,599 70%

Huye 58,153 1 330 1,044 6,312 469 7,869 16,025 49%

Kamonyi 65,553 16 3 158 3,694 57 1 15,269 19,198 80%

Karongi 79,298 3 445 427 10,062 23,634 34,571 68%

Muhanga 64,772 366 28 9,287 30,920 40,601 76%

Musanze 50,717 11 346 275 1,866 5 6,585 9,087 72%

Ngororero 67,899 2,769 54 7,142 31,525 41,490 76%

Nyabihu 52,958 2,958 244 2,505 2 12,676 18,384 69%

Nyamagabe 109,036 2,254 3,999 15,725 8 37 21,609 43,632 50%

Nyamashek 94,802 303 6 1,139 16,542 17,990 92%

e

Nyanza 67,215 945 3 3,436 10,771 15,155 71%

Nyaruguru 101,027 427 121 16,873

20,464

37,885

54%

Rubavu 34,090 45 895 5,664 6,603 86%

Ruhango 62,678 29 3,134 0 11,707 14,870 79%

Rulindo 56,699 3 1,853 3,363 7,176 11 10,576 22,981 46%

Rusizi 91,731 1 64 1,658 4,361 6,084 72%

Rutsiro 65,995 1,024 4,021 7,474 58 22,608 35,183 64%

Grand Total 1,402,446 65 17,812 17,292 119,119 67 608 311,613 466,576 67%

The table 5 (a, b) show the recommended

erosion control practices depending on the

type of risks found the area and the level of

protection presented in table 4. The contour

bank terraces are recommended in

high risk agricultural lands and in the

forested area without ditches. The total

area that need contour banks or ditches

construction is about 199,124 hectares

(43%) of the total high risk areas.

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Table 5a: Recommended erosion control practices where are not currently existing.

IN Erosion control practices and place where are Total Percen

recommended (Ha) tage

1 Afforestation 6,948 1%

2 Agroforestry /Alley cropping systems 30,061 6%

3 Bamboo to close gullies (where gullies, landslides are found) 4,824 1%

4 Bench terraces 18,295 4%

5 Contour bank terraces (ditches) 199,124 43%

6 Contour banks/ditches (for existing forests without ditches) 4,351 1%

7 Grassed waterways (for bench terraces already in place) 3,174 1%

8 Hedgerows/ shrubs 29,286 6%

9 No till agriculture (recommended for existing perennial crops) 15,518 3%

10 No recommendation (i.e. forests, bench terraces, contour bank

terraces are already in place see table 4) 116,961 25%

11 Perennial crops 64 0.01%

12 Reforestation (recommended for very degraded forests) 15,028 3%

13 River side buffers (with bamboo or any other appropriate shrubs) 4,924 1%

14 Storm water management facilities (in Built up area) 17,811 4%

15 Waterways infrastructure (for bench terraces already in place) 207 0.04%

16 Total area with erosion control recommendations 466,576 100%

Maps

Soil erosion measures – Southern province.

Soil erosion measures – Western province.

Soil erosion measures – Northern Province.

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Table 5b: Recommended erosion control practices and Districts where are to be implemented.

IN 1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

BURERA 204

1,659

2,544

170

10,572

5

385

825

338

4,242

56

86

2,012

0

23,098

GAKENKE 89

205

161

60

20,838

1,523

870

1,378

496

7,996

16

592

270

293

34,787

GICUMBI 107

180

0

970

5,134

7

2,118

39

4,388

853

27

529

14,352

GISAGARA 32

0

13

28

9,491

1,281

6

3,055

8

4

681

14,599

HUYE 94

32

10

140

6,271

98

59

1,320

662

6,263

7

651

43

374

16,025

KAMONYI 267

990

272

32

8,989

35

7

268

2,680

3,665

1,409

98

477

9

19,198

KARONGI 210

1,563

336

318

18,273

693

89

647

980

9,944

620

439

452

6

34,571

MUHANGA 423

2,972

119

183

18,754

227

380

3,116

9,294

2,272

837

1,997

28

40,601

MUSANZE 36

93

11

157

240

3

4,742

8

2,054

14

19

1,710

0

9,087

NGORORER 734

1,294

832

9,681

12,610

71

2,775

1,943

7,104

1,183

1,821

1,406

35

41,490

O

NYABIHU 1,317

2,198

419

76

5,989

446

81

2,976

765

2,439

474

249

928

28

18,384

NYAMAGAB 868

574

2

4,032

15,931

398

579

1,991

1,022

15,582

29

1,849

86

681

10

43,632

E

NYAMASHE 79

2,558

31

184

7,445

9

302

1,024

1,152

3,607

364

1,223

11

17,990

KE

NYANZA 876

4

0

9,144

8

1,075

11

0

3,270

2

597

54

114

15,155

NYARUGUR 80

1,801

64

288

16,500

478

0

378

1,477

16,270

6

375

70

98

37,885

U

RUBAVU 0

1,445

4,168

0

45

21

923

0

0

1

0

6,603

RUHANGO 19

1

0

11,171

93

29

46

3,073

0

309

104

23

1

14,870

RULINDO 316

655

5

796

7,792

5

5,172

456

7,260

50

136

339

22,981

RUSIZI 94

76

2,970

6

36

115

1,683

72

6

1,026

6,084

12

IN 1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

RUTSIRO 1,978

10,890

2

1,181

6,841

244

30

2,613

322

7,303

4

37

210

3,448

80

35,183

Grand Total 6,948

30,061

4,824

18,295

199,124

4,351

3,174

29,286

15,518

116,961

64

15,028

4,924

17,811

207

466,576

Highlights

Of 1,402, 445 ha located in 20 districts of Northern, Western and Southern Provinces, about 466,576 hectares (33%) have shown high to extremely high erosion risk;

High risk districts are Muhanga (63%), Ngororero (61%), Rutsiro (53%), Gakenke (49.5%), Karongi

(44%), Rulindo (40.5%) and Nyamagabe (40%);

Landslides and severe gullies have been found in Gakenke, Rutsiro, Burera and Karongi and have damaged about 29,500 hectares.

About 67% of identified high erosion risk land is not to date protected and 61% of it is mainly located in Agricultural land.

From area identified as high erosion risk, already 25% are protected by forests (plantations and natural forest).

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