TROPICS Vol. 10 (4): 539-553 Issued March 10,2001

Characterization and Evaluation of Inland Valley Watersheds for Sustainable Agricultural Production: Case Study of Semi-deciduous Forest Zone in the Ashanti Region of Ghana

Kwame Osafredu kmONTENG, Daisuke KUBOTA, Keiichi HAYASHI, Tsuayuki MASIJNAGA, Toshiyula WAKATSUKI Faculty of Life and Environmental Science, Shimane University, Matse 690 8504, Japan

Ekow Issiw ANDAH Water Resources Research Institute, Accra, Ghana

ABSTRACT As a part of efforts to effectively utilize inland valleys of Ghana for sustainable agricultural production while also protecting and conserving the environment, the bio-physical nature of a benchmark site in a semi-deciduous forest zone was characterized using the integrated transect method. Different valley types, first-, second-, and third-order inland valleys, were identified within the valley system. The main soil types identified were Ferric Lixisol, Ferric Luvisol and Haplic Gleysol in the upland, fringe and valley bottoms, respectively. The soil reactions for both the upland and lowland soils were slightly acid (pH 4.8-5.9) in the topsoil to strongly acid (pH 4.0-4.3) in the subsoils. Available P (Bray 1) was lower in the upland and lowland soils. Effective cation exchange capacity (ECEC) and levels of exchangeable Ca, Mg, and K were higher for the valley bottom soils than the upland soils. The total nitrogen levels of both upland and lowland soils, however, were higher in the topsoils but lower in the subsoils (0.84- 0.02%). Generally, the soil fertility status of the lowland soils was slightly higher than the fertility status in the uplands. The upland soils were well drained, while the valley bottom soils were poorly drained. The water table exhibited a cyclical movement of surfacing in the rainy season and going into a trough in the dry season. The valley bottoms are used mainly for cultivating rice in the wet season and vegetables in the dry season, while the uplands are used mainly for cereals, root and tubers, and plantation crops. The percentage of the land overall covered by primary and secondary forest was estimated to be less than 20%. The main identified constraints on the utilization of the inland valley watersheds are soil degradation in the uplands and water control and soil fertility maintenance in the valley bottoms.

Key words: Ghana / groundwater dynamics / inland valley watershed / lowland soil / valley bottom / upland soil / West Africa

The present world population is more than 6 billion. On a global level, the increase in food production

has far outpaced the growth of population, mainly through important technological improvements.

Examples of such improvements are the use of fertilizers, the development of plant varieties that are

high-yielding and resistant to pests and diseases, and improvements in irrigation, pesticides, and

agricultural practices. In Ghana and most West African countries, despite the abundance of natural

resources to support food production, food supply continues to fall short of demand, due mainly to

unfavorable climatic conditions and natural disasters, and due partly to human activities (Okigbo,

1990). Consequently food (especially rice) requirements are met largely from increasing amounts of

540 K O. AsunonrrNc, D. Kuncnn, K Hnvesm, T.lvlnsuNRcn, T. WarcersuKr , & W. I. ANoeH

0"l"2'

10"

\-- ->!' .Kqdasi

,rrltilr. siat

Republic of phana

l.T6"

Biemso No.l

f Gold valley watershed

To Kumasi 39km

Fig.L. Research site showing the locations of transects in the watersheds.

imports (WARDA 1996).

In Ghana as in other West African countries, rice is cultivated under three main systems. namely,rain-fed upland conditions, inigated conditions, and rain-fed lowland conditions in inland valleys.Production under rain-fed upland conditions has been very risky due to unreliable rainfall as well as

shallow and erodible soil of low fertility. Although enough water is available for inigation, inigationtends to be very costly for most small-scale farmers. Fortunately, inland valleys, which have specifichydrological conditions and have been cited as having high potential for the development of rice-based, srnall-holder farming systems at the village level, occur abundantly in Ghana and in the WestAfrican sub-region in general (Moormann, 1985; Wakatsuki et al., 1989; Windmeijer & Andriese1993; Otoo & AsubontengLggq. According to Wakatsuki and Otoo (1995), the potential area forsmall- scale irrigated sawah* in intand valley watersheds is estimated at 700,000 hectares: 3Vo of thetotal land area, L-3Vo of the Guinea savanna zone, and 3-5Vo of the forest zone. If flood plains areincluded, the total potential area for inigated sawah may reach I million hectares in Ghana. Theseinland valleys can support, in addition to rice, other commonly grown cereals, vegetables, legumes,root and tuber crops, and animal production. Efficient management and sustainable utilization of theseinland valleys could therefore result in a drastic reduction in the annual importation of rice and anincrease in agricultural production in general.

Maintenance of soil fertility and control over weeds and water are major constraints on the

*The term sawah refers to bunded and leveled rice fields with both inigation and drainage gates. The term isMalayo-Indonesian in origin. Likewise, the word "paddy" originated from the Matayo-Indonesian padi, whichmeans "rice plant". In West Africa, the word "paddy" refers to unhusked rice. Most of the paddy fields in theAsian countries correspond to the definition of sawah. However, the term "paddy fields" refers to rice fieldsonly' including upland rice fields in West Africa. To avoid any confusion Uenreen rice plant paddies and theimproved man-made rice-growing environment, the author proposes to use the term sawah.

I "55't,To Sunhani

#bbffiffi1Z-t-

4*ii\(inte Bepo 1=€ffi

Forest Reserve

,,, l,-- -Nlffi Dwinyama waterslr.d"o TD

glrY,Bi aall|$/ t

EN^.ly I (\{\iPotrikurolTlr.\.,: -

Enissiiriiir l-, vy u rJ attlol w cll\/t DI lgl"l \ \

I Rice vallev watershed \ \,

Characterization of inland valley watersheds in the Ashanti Region of Ghana 547

utilization of inland valleys for sustainable rice-based cropping (Otoo & Asubonteng, 1995). Research

is therefore needed to enhance the efficiency of fertilizer use and to promote the development ofsustainable and economically viable water control technologies. As practiced in Asian countries,

lowland sawah-based rice production is characterized by replenishing mechanisms that intrinsicallyresist erosion. Geological fertilization due to flooding compensates for the losses of nutrients(Wakatsuki, 1994; Kyuma & Wakatsuki, 1995; Greenland, 1997). Adoption of this sawah system could

help improve the fertility status of soils as well as the control of water and weeds. To be able to devise a

simple, low-cost adaptable, and environmentally friendly sawah system in the inland valleys of the sub-

region a comprehensive knowledge, based on quantified data, of the physical climate, lithology, landform,

soils hydrology, is necessary vegetation and land use (including the socioeconomic identifiers ofthe study

area). Therefore, the physical and biotic characteristics of a selected benchmark site were evaluated using

the dynamic integrated transect survey method.

MATERIALS AI\D METHODS

The detailed characterization was based on the dynamic integrated transect survey method. The main

focus of this method is the collection of information, in a given period, on the physical and biotic

characteristics of inland valley systems along transects laid across valleys. on an upland/inland

continuum at a bench-mark site.

Study AreaLocation The study area, which is part of the Mankran Valley system, is located in the Ahafo-Ano

South District of the Ashanti Region of Ghana. It is in the semi-deciduous forest zone of the country

and along the Kumasi Sunyani main road. The coordinates of the study area are 6'55'N and L" 55'W

(Fig. 1). The extent of the key area is approximately 3500 ha.

Climate The area falls within the semi-deciduous forest zone with an average temperature of 25.2" C

(23-27 "C is monthly mean). The mean annual precipitation in the three-year period January L997 to

December 1999 was 1363.1 mm. The area has a bimodal rainfall pattern. The major rainy season lasts

from mid-March to the end of July and, after a short dry spell in August, the minor rainy season goes

from September to mid-November. The dry season starts in mid-November and ends in mid-March.

Relative humidity figures range betweenST.9lVo at 900 hours and62.78Vo at 1500 hours. All these

data were obtained from the avprage of the three-year period mentioned above,1997-1999.

Data Collection and Analysis

Eight transects were laid within the study area along the different valley watersheds: the Gold Valley,

the Rice Valley, and the Dwinyama Valley (Fig. 1). Soils were identified by means of minipits (dug to

60 cm) and supplemented by augering to 100 cm (where possible). Seven soil profiles (three in the

valley bottoms, one at the fringe, and three on the upland soils) (Fig. 2) were selected for profile pits

and detailed descriptions of the main soils encountered. according to the FAO Guidelines for soil

profile (FAO, 1990). Each soil layer was sampled for laboratory analysis. Soil samples were air-dried,

ground, and passed through a 2-mm mesh sieve. Soil pH was determined using a pH meter (with a

glass electrode) with a soil-to-water ratio of 1:2.5, according to the methods described by IITA (1979)

y2 K O. Asugor.{TENG, D. KugotA, K FlRyRsHI, T.IvIASUNAGA,T. WAKAISUK , & W. I. ANDAH

Key@ Brown mottlosEl Ye[ow mottlesP.....,.. hofile numberCL..... Clay loamSCL ... Sandy clay loamL ...,.... [rOAmC .,.,.... ClaySL...... Sandy ctay

{l lX"l l8oru Pt6 n7 Plantain

Cocoa , Mixed cropping Rice . Mai?n. Rice - Cassava

L

Horizontal Dist nce (m)

Fig.2. A schematic cross-section ofthe profiles ofthe mqior soil series in the Dwinyanwatershed, in transect s, Potrikurom, Ashanti Region, Ghana.

and Mclran (L982). Total carbon content was determined by the wet combustion method (Walkley &Black 1934). and total nitrogen was deteimined by Macro-Kjeldahl method Bremner 1965). Totalcarbon and nitrogen content of some samples were determined using an NC analyzer (Sumigraph NC-90 A) as described by Geigher and Hardy (L97I). Available P was determined by the Bray No. L

method (Bray & Kurtz 1945). Exchangeable cations (Ca, Mg, K, Na) were first extracted withammonium acetate (pH 7.0, 1.0 M NH4OAc). Sodium and potassium in the extract were determinedby atomic absorption spectrophotometry as described by Thomas (1932). Exchangeable Ca and Mgwere determined by inductively coupled plasma atomic emission spectroscopy (Shimadzu ICPS2000). Exchangeable acidity was determined by first extracting with potassium chloride ( 1 m KCL)and titrating the extract with sodium hydroxide as described by Mclran (1965). Effective qation

exchange capacity (ECEC) was calculated as the sum of the exchangeable cations (K, Ca, Mg, Na)and the exchangeable acidity. Particle size analysis was conducted by the pipette method as described

by Gee and Bander (1986).

Land use Along the eight transect lines, land use was described in terms of land cover at a width of100 m on either side of each transect line every month for a period of three years (1997-1999). Landuse maps were drawn for the various inland valleys. The valley bottom areas were also quantified byusing the Valley Bottom Ratio (VBR). The VBR is the ratio of the area occupied by the higher parts ofthe valley (crests, slopes, and fringes) (TVW) to the area of the valley bottom (VBW). Specifically,VBR = (TVW-VBW)A/BW VBR is also a measure of the potential amount of water, related to the

Charact erization of inland valley watersheds in the Ashanti Region of Ghana 543

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Depth of groundwater level (cm)

175-150-175125-150100-12575- 1 0050-752s-500-25

1

1

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1

85.24.8

192.9126.0130.8261 .0s2.263.050.7

143.351 .960.942.O65.332.7

331 .4172.4200.890.0

103.272.2

143.639.230.3s9.312.4

149.2267.3102.6224.O231 .4127.1161 .3133.569.80.0

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Fig.3. Cross-section oftopography, rainfallpattern, ground/surfacewater and land use

dynamics in first ordervalley (Transect 6),Gold Valley watershed,

Ashanti Region, Ghana.

o)o)o,

100 200 300 400 500 600 700 800 l...Begining of river flowHorizontal Distance (m) -a...End of river flow

total rainfall, that may flow as runoff or as groundwater from the higher parts of the valley into the

valley bottom.

Water table monitoring Perforated polyvinyl chloride (PVC) pipes were installed for the

monitoring of ground water and surface water dynamics along the eight transect lines for the three-

year period. The water table was measured with a wooden meter rule at two-week intervals. Changes

in groundwater depth, land use over time, and a rainfall histogram were plotted on the same graph to

show the relationship between groundwater depth, rainfall, and land use within the year. The flood

behavior in the valley bottoms and the period of stream flow were also measured.

tu+ K O.AsrrgoN'rENG, D. KuBorA, K HnvesqT. N4estnnc,noT. WAKArsutc, &W. [. ANoAn

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Depth of ground

water level (cm)

Si13:,,,| | 125-150IIT loo-125

I'ialg8I'3:i3

85.24.8

t92.9126.0

| 30.826t.0

52.263.050,7

143.351.960.942.065.332.7

33r.4t72.4200.8

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0 100 200 300 400 500 600 700 800 e00 1000Horizontal Distance (m)

The conour lines show the depth ofground or surface water level in cm.

Flg4. 6"o..-..*ton of topography, ralnfall pattern, ground/surface water and land use dynarnlcs ln streamflow Inland valley head (Transect 5), Dwlnyan watershed, Ashand Reglon, Ghana.

RESUI.,;TS AI{D DISCUSSIONValley MorphologrThe inland valleys in the study area are first-, second-, and third-order inland valley systems. We

defined, tentatively here, thal first-order systems are developed in watersheds of less than 500 ha;

second-order systems are developed in watersheds of 500-3000 ha; and third-order systems are

developed in watersheds bigger than 3000 ha. Transects l, 6, and 7 (Figs. 3 and 4) are in first-order

Characterization of inland valley watersheds in the Ashanti Region of Ghana 545

gc820(u

#10o

€ootr

Depth of ground waterlevel (cm)

Rl33:,,u| | 125-150[ , ,l 1oo-125

I'36193I ',3f3

85.24.8

192.9126.0130.8261.0

52.263.050.7

143.351.960.942.O65.332.7

331.4172.4200.8

90.0103.272.2

143.639.230.359.312.4

149.2267.3102.6224.O231.4127.1161.3133.569.80.0

0 100 200 300 400 500 600 700 800 900 1000110012001300

rhe contour rines sn"* *"13'J,?,TilBflT;:ts'", ""rface

water. -:::Ei8'3i,t 3lr@ frow

Fig.S, Cross-section oftopography, rainfall pattern, ground/surface water and land use dynamics inthird order Yalley (Transect 8), Dwinyam watershed' Ashanti rtgion, Ghana.

valley systems. They are very narrow and relatively asymmetrical. The crests are convex, with the

slopes on both sides of the valley having similar morphologies. Their valley bottom widths ranged

between L0 and 1fi) m. Transects 3 and 5 (Figs. 2 and 5) are located in second-order valley systems

and have straight slopes. The valley bottoms are almost flat (0-2Vo), have widths of between 100 and

300 m, and occur normally in the mid- sections of the valley systems. Transect 8 is in a third-order

valley system (Fig. 5). Third- order systems are also symmetrical, and they occur in the downstream

section of the valley systems. The valley fringes are much wider, though there were some

irregularities resulting from the channels created by the Dwinyama River and other intermittent

streams that flow in the valley. The width of the valley bottom is between 700 and 800 m.

Valley morphology plays a significant role in land husbandry, especially land leveling and the

establishment of measures for water control and management. Hence, valley systems with gentle

slopes, concave forms, and fairly broad valley bottoms, such as in Transects 5 and 8 described above,

No)O)

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Table L. Physical and morphological properties of the soils.

Profile No.& Soil Vpe

Depth Horizon Boun- Sand Silt Claydary ,ft,

Textural MatrixClass colours

Mottles Structure

PtlAkumadan

seriesFerricLixisol

Pt2Bekwai

seriesFerricLixisol

Pt3Nzima

seriesFerricLixisol

Pt4Kokofu

seriesFerricLuvisol

Pt5Oda seriesHaplicGleysol

Pt6Oda seriesHplicGleysol

Pt7Oda seriesHplicGleysol

0-7 Ap as

7-29 B2Ltc ca29 - 54 B22tc cs54 - 70 F.23 as

70 -1,00 B.24 as100 -160 825

0-8 Ap as8-96 BA as

35 - 56 Btcsl as

56 - 74 Btcs2 as74 -1.02 Btcs3 as

102 -153 Bcl as0-9 Ap cs9-22 Bts cs

22 - 39 Btsl gir39 - 61, Bts2 cs61- 84 Btsgl cs

84 -120 Btsg20-9 Ap cs9-24 AB cs

24 - 48 Btsgl cs48 - 89 Btsg2 as89 -103 Btsg3 gir

103 -134 Cgl0-8 Apg cs

8-19 Bacg cs19 - 46 BCgl cs46 - 60 BCg2 cs60 - 87 Cgl cs87 -137 Cg2 c

0-5 Apg cs5-18 Bacg cs

18-37 Bcgl cs37 - 62 Bcg? cs62 - 89 Cgl cs

89 -I20 CgZ cs0-6 Apg cs6-17 Bacg cw

17 - 34 B.g cs34 - 75 Bcg2 cs75 - 89 Cgl cs89 -1,40 Cg2 cs

34.5 37.030.2 30.224.3 27.230.2 35.030.2 38.340.8 33.2

43.3 28.9

39.6 L7.7

27.5 1,4.7

24.3 19.5

16.5 33.012.2 40.049.2 37.835.8 31.525.L 27923.5 29.5

26.0 25.428.1, 25.9

37.0 42.044.5 41,.5

49.5 4r.041.0 34.536.5 39.044.5 32.0

60.0 29.0

62.5 29.0

54.5 38.053.5 25.0

64.5 21.5

56.0 14.0

57.0 31.558.5 29.0

67.4 21.6

53.4 25.1,

s5.0 15.0

54.0 16.0

62.0 26.5

58.1 30.065.3 23.7

55.4 24.1.

56.0 1,4.0

53.0 16.0

28.0 cl39.6 cl48.5 cl33.8 cl31.5 cl26.0 cl27.8 scl32.7 cl57.8 c

56.2 c

50.5 c

47.8 scl13.0 1,

32.7 Ic47.0 c

47.0 c46.6 c

46.6 c

21.0 1

14.0 1

10.5 1

24.5 1

24.5 1

23.5 1

11.0 sl10.5 sl7.5 sl

21.5 scl1,4.0 scl30.0 scl13.5 sl12.0 cl11.0 sl21.5 scl30.0 scl30.0 scl11.5 sl11.1 sl11.0 sl19.5 scl30.0 scl31.0 scl

s YR4/47.s YR 6l 8s YRs/82.s YRs/ 82.s YR s l82.s YR 5l8s YR 5147.5 YR6l 6

sYR 618sYR 618sYR 618sYR 61810 YR 51310YR 614t0YR7l610YR7l47.s YR 7 l67.s YR 61810YR sl310 YR 6122.sY 7162.sY 7 l82.sY7lA10YR7l810 YR 41210 YR 31210 YR 61210 YR 6l 1,

10YR 61110 YR 7 11,

10 YR 3l 1.

10 YR 31210YR6/210 YR 6l 1.

10 YR 61210 YR 7 11,

10 YR 41210 YR 31210 YR 61210 YR6/ 1

10 YR 61210YR7lt

m2grmlsbkm2sbkm2 sbkflsbkflsbkmlgrm2grm2sbkm2sbkm2sbkm2sbkmlgrm2gr

Rusty m2sbkflr,m2sbk

m2sbk

Rusty

RustyRustyYellowYellowYellow rustym2grRustyYellowYellowRustyRusty

RustyYellowYellowRustyRusty

mlgrflgtm2sbkm2sbkm2sbkm2sbkm2sbkm2grm2crm2crflgt

flsbkm2grm2crflgtm2crflgtm2sbkm2grm2crflgtm2crfl9.

Structure: as. abru_pt smooth; cw, dear wavy; cs dear smooth; gs, gradual smooth; gir, gradual irregular; sl,sandy loam: Is. Ioamy sand: s, sand; q clay; sd, sandy clay loam; f, fine; m, mediuin; rl weak 2" m-oderate,sblg subangular blocky; cr, crumbly; gr granular.

offer great potential for sawah development.

Soils

Figure 2 shows a schematic description

toposequence of Transect 5. Tables 1 and

of the profile of the major soil series appearing on the

2 show some of the physico-chemical parameters analy zed

Characterization of inland valley watersheds in the Ashanti Region of Ghana 547

Table 2. Chemical characteristics of the soils (nutrient levels).

Profile No . Horizon pH& Soil Type HzO

Org.C T-N O.M. C/N Exchangeable cationsCaMgKNa

TEB Ex.Acid. ECEC Base Avail. ECEC/clayAl+H Sat. Pz0s cmol( )

cmol(+) kg ,/r, mg/kg kg-t.luPtl 0-7 5.7

7-29 4.9Ferric 29-54 4.8Lixisol 54-70 4.7

70-1,00 4.6100-160 4.3

3.5 0.31 6.01.3 0.13 2.20.7 0.09 1.2

1.0 0.11 1.8

1.3 0.13 2.20.5 0.07 0.8

3.2 0.31 0.1 1,3.6

7.4 0.09 0.1 5.61.0 0.06 0.1 3.8

1,.9 0.35 0.1 5.12.4 0.46 0.1 6.5

1,.4 0.19 0.1 3.4

0.5 1,4.2 95.81.1 6.7 83.51.5 5.3 71.40.5 5.6 91,.1

0.8 7.3 89.1

I.2 5.2 69.6

11.2 10.09.7 4.07.4 2.69.4 2.8

9.8 3.66.6 1.8

5.327.083.52 10.8

1,.44 1,6.7

1.00 23.2

1.00 20.1,

Ptz 0-8 5.28-36 4.3

Ferric 36-56 5.5Lixisol 55-74 5.7

74-102 5.8102-153 4.7

3.4 0.47 5.9 7.30.8 0.09 1,.4 9.70.5 0.05 0.8 9.40.5 0.08 0.6 6.40.5 0.05 0.8 9.20.4 0.06 0.7 7.2

5.4 2.5 0.54 0.8 10.33.0 1,.4 0.09 a.2 4.61.6 1.1 0.04 0.0 2.84.7 1,.7 0.20 0.3 6.91,.9 2.1 0.04 0.0 4.12.5 2.1 0.07 0.1 4.8

0.8 LL.L 92.8 4.800.5 5.1" 90.1 8.000.5 3.3 84.6 1.85 5.60.6 7.5 92.0 0.67 1,3.4

1.8 5.9 59.3 0.50 1,r.61.9 6.7 7L.6 0.00 14.0

Pt3 0-9 5.1

9-22 4.9Ferric 22-39 4.8Lixisol 39-61. 4.2

61,-84 4.284-120 5.1

2.9 0.33 4.9 9.6 1,4.9

1,.2 0.2 2.1, 6.2 10.10.4 0.03 0.6 12.0 3.10.3 0.05 0.6 6.8 4.30.4 0.04 0.6 9.3 3.90.4 0.04 0.6 8.8 4.6

3.7 0.21, 0.2 1g.g

1.1 0.16 0.2 11.50.3 0.02 0.1 3.61.0 0.06 0.1 5.41.5 0.03 0.1 5.52.7 0.50 0.1 6.9

0.6 19.5 96.9

0.5 L2.0 95.80.4 4.0 89.9

0.4 5.8 93.r0.5 6.0 91,.7

0.6 7.5 92.0

5.007.122.10 8.41.00 1,2.4

0.96 12.9

0.21 1,6.1,

Pt4 0-9 5.99-24 5.9

Ferric 24-48 5.1Luvisol 48-89 4.3

89-103 4.3103-134 4.7

2.4 0.3L 4.1 7.6 14.81.2 0.15 2.0 7.4 5.91.0 0.84 1,.7 1,.2 2.70.3 0.05 0.5 5.8 3.80.2 0.04 0.4 5.0 5.20.2 0.02 0.3 8.0 2.3

4.5 0.55 0.2 20.02.1. 0.26 0.L 8.21.3 0.02 0.2 4.21,.2 0.03 0.2 5.91.9 0.07 0.4 7.6

1.8 0.02 0.6 4.7

0.5 20.5 97.6 6.681.0 9.2 89.2 5.021.0 5.2 8A.7 4.04 49.3

L.4 7.3 81.5 3.08 29.7

1.9 9.4 80.6 2.11, 38.42.0 6.7 70.3 2.00 28.6

Pts 0-8 5.78-L9 5.2

Haplic 1,9-46 4.7Gleysol 46-60 4.6

60-87 4.687-1.37 4.5

2.5 0.38 4.31.2 0.13 2.0

0.4 0.07 0.60.0 0.07 1.80.1 0.04 0.20.1 0.1,4 0.1

6.6 27.2

8.8 12.8

5.1 12.0

0.4 12.02.5 13.6

0.4 1,4.9

0.1 24.9 99.6

0.1 L7.4 99.4

0.2 1,4.3 98.6

0.2 16.3 98.20.6 L9.6 96.9

0.2 20.3 99.0

8.01

9.01

4.04 190.5

3.08 75.62.11, 139.82.00 67.8

2.8 0.61, 0.2 24.8

4.0 0.22 0.3 17.31.6 0.1,6 0.3 1,4.1

3.2 0.22 0.5 16.04.4 0.25 A.7 19.04.0 0.35 1.0 20.1

Pt6 0-5 5.45-18 5.4

Haplic 1,8-37 4.6Gleysol 37-62 5.7

62-89 5.289-120 5.2

1.5 0.13 2.6 11.6 1,1,.9

0.2 0.03 0.4 7.7 6.9

0.2 0.05 0.2 3.6 5.5

0.5 0.02 0.8 23.0 4.5

0.1 0.02 0.2 7.0 3.5

0.1 0.01 0.2 10.0 2.5

1.5 0.13 0.1 1,3.5

1,.9 0.03 0.1 g.g

1,.4 0.06 0.1 7.0

2.2 0.06 0.1 6.9

3.4 0.06 0.1 7.0

3.0 0.05 0.1 5.6

0.1 13.6 99.610.1L0.5 9.4 94.7 9.02 77.90.5 7.5 93.3 5.04 67.90.5 7.4 93.2 5.01 34.4

1,.2 8.2 85.3 3.02 27.21,.2 6.8 82.3 4.01, 22.7

Pt7 0-6 5.36-17 4.7

Haplic 17-34 4.6Gleysol 34-75 4.6

75-89 4.589-140 4.2

2.9 0.21 5.0 13.8 12.60.5 0.15 1.0 3.4 7.40.3 0.08 0.6 4.0 4.70.2 0.07 0.4 2.9 7.70.1 0.07 0.2 1.6 5.60.1 0.06 0.2 1,.7 5.4

3.5 0.29 0.5 16.81.5 0.20 0.4 9.41.2 0.1,4 0.4 6.5I.7 0.29 0.2 9.9

0.9 0.55 0.1 7.0

0.8 0.s5 0.1 6.8

0.9 17.6 95.2 9.700.9 10.3 91.6 8.401,.2 7.7 84.3 6.04 69.5

2.7 1,2.6 78.5 4.01. 61,.6

3.6 10.6 56.5 3.20 35.33.0 9.8 69.5 4.00 3'1..7

and used in the classification of the soils encountered. Soils of the summit, with a relatively gentle

slope, belonged to the Akumadan series, Ptl, which had a strong red colour. 2.5 YR, of the B horizons

(Table 1). The Bekwai soil series, PtZ, and the Nzima series, Pt3. appeared in the next-lowertopographic position. The yellow colour of their B horizons was increased, i.e., 5 YR and 7.5-10 YRfor the Bekwai and Nzima series, respectively. These upland soils (Ptl to Pt3) were classified as Ferric

K O. AsunovmNc, D. KunorR, K llRynsm, T. IVIASLJNAGA, T. WAKATSUK , & W. I. ANDAH

Lixisol, showing low-activity clay with discrete iron nodules (ISSS 1994). They are well drained on

the summits and upper slopes. and become moderately well drained on the middle slopes. From the

particle-size analysis, the clay contents of the soils of Ptl and Pt2 under citrus or cocoa plantation

increased down the profile to values ranging between 28 and 587o.The effective cation exchange

capacity (ECEC) of clay of the subsoil layers ranged from 6 to 23 cmol (+) kg-l. Textures varied from

loam (L) at the top through clay loam (CL) to clay (C) within the subsoil. The topsoil of ft3, of the

Nzima series, however, showed only L3Vo of clay under maize, cassava, cocoyam, and plantain mixed

cropping. This may mean that once forest cover is removed, upland topsoils are susceptible to erosion.

Table 2 shows that the upland soils had reactions varying from slightly acidic (pH 5.4-5.9) in the

topsoil to strongly acidic (pH 4.0-4.3) in the subsoils. ECEC values were between 6 and 2O cmol (+)

kgJ in the top 30 cm. However, ECEC decreases within the profile. showing the importance of organic

matter. The organic matter content was high only on the topsoil (2-6Vo) but decreased sharply in the

subsoil, up to 0.057o. The CA.{ ratio was below 10 and the nitrogen content was higher in the topsoil but

decreased in the subsoils (O.84-0.O2Vo) in all the horizons. The topsoils showed low to moderate values

of phosphorus. The topsoil values were much higher than the subsoil values. This is due to the effect of

mineral cycling in citrus and cacao plantations. The fringe soil, Pit No. 4, shows higher clay activity, in

the range of 29-49 cmol (+) kg-' of clay. The soil was classified as Luvisol (ISSS 1994)'

The colours of the B horizons were dominated by yellow (Table 1). Although some morphologies,

such as mottle characteristics, were similar to those of the upland soils, soil texture and general

fertility characteristics, including available phosphorus, were more similar to the valley bottom soils

(Table 2). The soils of the valley bottoms P6-Pt7 were slightly acidic in the topsoil (pH 5.2-5.9) but

more acidic in the subsoils @.2-a.\. The valley bottom soils are better supplied than the upland with

exchangeable cations, especially tlre more mobile cations Ca and Mg. The soils have light to medium

textur€s. The higber level of exchangeable cations may be attributed to the frequent supply of basic

cations by runoff and floodwater, and through ground water. The moderate to high levels of organic

matter associated with the soils' hydromorphism may contribute partly. Activitie$ of clay, which in

Table 2 are calculated as ECEC divided by clay content, were much higher in the lowland soils than in

the upland soils. This also explains the richness in basic cations of the lowland soils in spite of their

coarser texture than the upland soils. This suggests a difference in mineralogical characteristics

between lowland and upland.

The nitrogen levels in the lowland soils, however, showed a trend similar to that of the upland

soils. Although the topsoils were relatively rich in nitrogen, the subsoils were extremely poor in both

total N and C. The soils had low to moderate values of phosphorus, ranging from 2-10 mg kgJ, with

decreasing levels from the topsoil to the subsoils.

However, the available phosphorus in subsoils of the valley bottoms was much higher than that in

subsoils of the uplands, showing the effect of alluvial and colluvial deposition. By virtue of their

geomorphic position, the soils are poorly drained, as reflected in their matrix colors (Iable 1). The

poor drainage of the valley bottom soils indicates that these soils can hold water. These soils would

therefore be very useful for rice cultivation in the wet season and vegetable cultivation in the dry

season. The most deficient nutrient was phosphorus followed by nitrogen. Some lowland soils, such

as in Pt6 soil, however showed very low available potassium.

Characterization of inland valley watersheds in the Ashanti Region of Ghana 549

- - - Rrinfill +- Potrikrom(T5) -# R.Vrllcy(T7)

+ G. Vrllcy (T6) * Dwlnyam (Tt)

t\EEv

.It

,Etr

aIl

6l&

3s0

300

250

200

150

100

50

0

350

300

250

200

rs0

100

50

0

3s0

300

250

200

150

100

50

0

0

-20

-40

-60

-80

-t00

- 120

- l.{0

-160

0

-24

-40

-60

-80

-r00

-t20

- 140

-t60

0

-20

-40

-60

-80

-100

- 120

- 140

-t60

AEC'

\J

-a€)

t)slr(|)t6l

'Etr=oLa0

lloSI-IELq)

o

Fig.6. Graphs ofmean values ofground water movement al valley bottom in dilferent valleyorder systems within the study area.

Water lhble Dynamics

Figure 6 shows the mean depth of the ground water level at first-, second-, and third- order of the

valley bottom in relation to monthly rainfall patterns. In1997, the driest of the three years monitored,

topsoil saturated (i.e., ground water level) was nearly equal to 0 cm and the saturation started only in

June to July, except for Gold Valley. However, in the other two years topsoil started to saturate in

May, an approximately one-month delay from the first peak of the rainfall. The months of lowest

ground water levels were February and March. The water table at the study area was effectively

monitored for 3 years from January t997 to December 1999. Three years of results are shown in the

following: Fig. 3 of Transect 6 for Gold Valley, about 100 ha of catchment area, the first-order valley;

t/l/ \ l, \ /,/\-

550 K O. AsunovrnnG, D. KusorA, K FInyRSHr, T. IvlnsuNnce, T. WerRtsuKr , & W. I. ANpau

Fig. 4 of Transect 5 for the Potrikrom site, about 2500 ha of catchment area, the second-order valley;

and Fig. 6 of Transect 8 for the Dwinyama site, about 3500 ha of catchment area, the third-order

valley. The results of T1-T4 and T7 are similar to T5-T6. Monitoring points along each transect line

are shown on the lines of the topography survey in Figs. 3-5. The depth of ground water dynamics

was also shown with the monthly distribution of rainfall.

At the time when the measurements started in January, the water table was still falling due to a

lack of rain, increased solar radiation, and a rise in temperatures. Therefore, the evapo- transpiration

effect was enhanced as a characteristic of this season (the dry season). The water table started to rise

steadily at the beginning of the rains, from toward the end of March to May, despite the fact that

rainfall did not stabilize until June, when the first peak was attained in all the valleys (Figs. 3-5).

The movement of the water table conesponds approximately to the flow of water in the seasonal

streams; the streams dry up in the dry season, until the ground water table rises to the surface in the

riverbed in late May or early June in this area, depending on the intensity of the rainfall that year. As

the water table graph shows, water starts flowing in the riverbeds when the rains stabilize in June, but

water flows in May if, in any year, the rains are heavy and stabilize early. The flow of water stops as

soon as the rain stops in Rice and Gold Valleys, because their catchment areas are more exposed. The

water flow in the Dwinyama watershed remains up to the end of December, depending on the time the

rain stops. The stream in the first-order valley flowed for about four months during June to October

(Fig. 3) whereas the second-order valley stream flowed about 6 to 7 months during May to November

(Fig. a). The third-order valley stream flowed about 7 to 8 months during May to December (Fig. 5).

The pattern of movement of the water table appears to be cyclical, surfacing in the rainy season and

going into a trough in the dry season (Fig. 6). The water table rises to the surface, and sometimes

above it, from the end of May to November, depending on the rainfall pattern, the relative size of the

catchment area, and how exposed the watershed area is. The valleys, in terms of how long and how

high the water stays on the surface ofthe lowland, are the second- and third-order valleys in Transects

5 (Potrikrom) and 8 (Dwinyama) (Figs. 4, 5 and 6). These are followed by the first-order inland

valleys in Transects 6 (Gold Valley) and 7 (Rice Valley) (Figs. 3 and 6). The driest valley is Gold

Valley. Although Rice Valley and Gold Valley are first-order valleys, Rice Valley has a smaller VBR,which means it has a bigger valley bottom and higher amounts of rainfall flowing as runoff.or as

groundwater from the higher parts of the valley.

The toposequence of the inland valley watershed was classified as an upland slope with a pluvial

water regime, fringed with a phreatic water regime, and a valley bottom with a fluvial water regime

(Moorman, 1985). The upland slope has no ground water in the soil profile. The valley bottom has

saturation or flooding of the whole soil profile during some of the rainy season. The fringe has ground

water in the soil profile during some of the rainy season in any given year. The water table dynamics

and the flooding regimes uue very good indicators for planning appropriate water management and

developing sustainable cropping systems in the inland valleys.

Interaction of water table and land use

From the schematic cross-sections of the different valley orders, a close relationship between water

table movement and land use is apparent, especially in the valley bottoms. The largest valley bottom

was found on Transect 8 (Fig. 5), a third-order valley. It has the best moisture regime during both the

Characterization of inland valley watersheds in the Ashanti Region of Ghana 551

wet and dry seasons (Fig. 6). The valley bottom is used and suitable for the cultivation of rice fromApril to December. It is also used and suitable for the cultivation of vegetables during the dry season

because of the residual moisture available during that season (Fig. 5). The valley fringes are used

mainly for cereal, vegetables, root and tuber crops, and legumes. In the second valley order found on

Transect 5 (Fig. 4), the land uso pattern follows that of Transect 8. Rice is cultivated in the valleybottoms and the fringes during the wet season, but the valley bottoms are left to fallow during the dry

season. This is because the water level drops more sharply after the rainy season (Fig. 6). The first-order valleys on Transects 6 and 7 are the driest and, as such, the valley bottoms are used mainly inthe wet season for the cultivation of upland rice, maize, and sometimes oil palm (Figs. 3 and 6). The

uplands of all of the valley systems are used to cultivate cereals like maize, root and tuber crops

(cassava and cocoyam), and perennial crops (cocoa, citrus, oil palm, plantain). The lower slopes and

fringes are used mainly for upland rice, vegetables, and sometimes cocoa cultivation (Figs. 3-5).

CONCLUSION

From the above results and observations it can be concluded that, given their assured water supply and

relatively fertile soils, inland valleys can contribute to greater stabilization of food production (especially

rice). It must be noted, however, that even though the valley bottom soils are slightly richer than those ofthe uplands and fringes of the area, their nutrient reserves are still low for intensive crop production.

Observed nutrient levels reflect the need for improved nutrient management. Other observed constraints

were water control and weed infestation. These valley may have a high potential for increased and

sustainable food production (especially rice), ifcunent production systems and practices could be further

improved through the adoption of systems that promote nutrient accumulation and retention and improved

water management systems (sawah). Nutrient retention, weed control, and water management could be

improved under submerged conditions.

The sawah system, as implemented in some parts of Asia, is known to be sustainable and is

characterized by nutrient-replenishing mechanisms through geological fertilization (Hirose &Wakatsuki 1,997, Greenland 1997) with intrinsic resistance to erosion (better water control, nutrient,

and weed management). To improve rice production, the implementation of African adaptive sawah,

based on rice farming systems (Wakatsuki, L994; Hirose & Wakatsuki, 1997; Wakatsuki et al., 1998),

could be important for future farming systems in Ghana and the West African subregion.

ACKNOWLEDGEMENTS We thank the Japan International Co-operation Agency (JICA), for

providing the necessary support in carrying out this study. Part of this study was supported by Grant-

in-Aid for International Scientific Research Program (08041150) from the Ministry of Education,

Science, Sports and Culture, Japan.

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Received Aug. 3,2000 Accepted Nov. 25,2000

Characterization of inland valley watersheds in the Ashanti Region of Ghana 553

K ・ O ・ ASUBONTENG ，久保田大輔， 林慶 ―， 増 永二 之 ， 若 月利 之 ， E ・ I ・ ANDAH 持続的農業 坐 産のための内陸 谷地 集水域の特徴付けと評価―ガーナ ア シヤンテ 地域の 半 落葉樹林におけるケーススタディ

持続的農業 生 産のためのガーナ内陸各地の効果 的な利用と環境保全を目指す 努力の― 部として， トランセ クト法を用いて半乾燥樹林帯のべンチマークサイトの 坐 物物理的特徴付けを行った。 第 1 ， 第 2 ， 第 3 番 とした異なった各地形が認められた。高地及び低地土壌とも，表土は弱酸性 (pH4.8-5.9) で下層 土 は強酸 性 (pH4.0-4.3) あった。 可給態 リン 酸 (B 『 myl) は高地及び低地土壌とも低かった。有効 陽 イオン交換容 量 ( 胱 ) と交換 性 MLK のレベルは谷地底地で高地よりも高かった。しかし高地及び低地の土壌 と も全窒素レベルは，表土で高く下層 十 で低かった (0 ・ 02-0.84%) 。全般 的に低地土壌の肥沃 度 状態は高地 土壌よりも高かった。高地土壌は排水がよく，― 方 低地土壌は排水が悪かった。地下水面 は雨期に浅く， 乾期に深い循環型の動態を示した。各地低地は主に雨期の稲作と乾期の野菜作に利用されており，―市高 地 ほ 主に短い休閑の後に混作と カ カオ畑に利用されている。― 次 及び二次林に被われている土地 ほ 全体の 川 お以下と推定された。内陸各地利用の主な制約 ほ ，高地の土壌， " と谷地低地の 水 制御と土壌肥沃 度維 持である。