Hydrological response patterns of some third order streams on the ...

Hydrological Sciences - Journal - des Sciences Hydrologiques, 28, 3, 9/1983

Hydrological response patterns of some third order streams on the Basement Complex of southwestern Nigeria J. 0. ADEJUWON, L. K. JEJE & 0. 0. 0GUNK0YA Department of Geography, Ile-Ife, Nigeria

University

of

Ife,

ABSTRACT The hydrological response patterns of some third order rivers in southwestern Nigeria are described in terms of regime, discharge variability and recession of flow from peak discharge in the rainy season to the lowest discharge in the dry season. The maximum and minimum discharges of the rivers coincide with the rainy and dry seasons respectively. Most of the rivers exhibit erratic flow with quick responses to storms and storm water abating quickly, while some have a very low groundwater contribution to streamflow, a phenomenon which accounted for their drying up for considerable periods in the year. The variability indices of discharge of the rivers range from 0.25 to 2.12 while the recession constants range from 0.64 to 0.98. The variability indices are much higher than those of basins of similar size reported from other studies. Three types of rivers are recognizable on the basis of their hydrological response patterns. It is observed that the nature of the surficial and solid geology has a greater influence on discharge characteristics than other factors such as depth of regolith and rainfall.

Types de réponse hydrologique de quelques cours d'eau de troisième ordre sur le Complexe de base du sud ouest du Nigeria RESUME On décrit le type de réponse hydrologique de quelques cours d'eau en termes de régime, de variabilité du débit et de tarissement de l'écoulement depuis le débit maximal en saison des pluies jusqu'au débit minimal en saison sèche. Les débits maximaux et minimaux de ces rivières coincident respectivement avec la saison des pluies et la saison sèche. La plupart de ces cours d'eau présentent un écoulement très irrégulier avec des réponses très rapides aux averses et le débit de ruissellement dû à l'averse décroissant rapidement alors que certains d'entre eux ne bénéficient que d'une très faible contribution des eaux souterraines a l'écoulement, phénomène qui est responsable de l'annulation totale du débit pour de longues périodes au cours de l'année. Les indices de variabilité du débit de ces rivières varient de 0.25 à 2.12 alors que les coefficients de tarissement sont compris entre 0.64 et 0.98. Les indices de variabilité sont plus élevés que ceux qui ont été signalés dans 377

378

J.O. Adejuwon et al.

d'autres études pour des bassins de superficies analogues. On peut distinguer trois types de rivères à partir de la nature de leur réponse hydrologique. On observe que la nature géologique des couches superficielles a une plus grande influence sur les caractéristiques des débits que d'autres facteurs tels que la profondeur du régolithe et la hauteur pluviométrique.

INTRODUCTION A large number of studies have produced extensive information on the hydrological response patterns of rivers and on the nature and magnitude of the influence of certain environmental factors controlling these responses (e.g. Cross, 1949; Reinhart & Eschner, 1962; Wright, 1970; Walling, 1971; White, 1977). A survey of these studies shows that except for the work of Rodier (1959) and a few others, there is little or no information on this aspect of hydrology of rivers in the humid tropics of Africa. Although some information exists on the hydrology of large rivers (e.g. NEDECO, 1959; Ledger, 1964; 1969), as these have been studied in connection with water resource development projects, this cannot give an understanding of the hydrological behaviour of the more ubiquitous small rivers. Moreover, due to their size and heterogeneity, it is very difficult to unravel the relationship between hydrological response and drainage basin environmental factors through a study of these large river basins. Studies from temperate environments have provided some understanding of the various interactions occurring in drainage basins, but their findings may not be applicable to the humid tropics especially southern Nigeria due to considerable differences in climatic, physiographic and vegetational factors. Also, the use of different indices to express drainage basin characteristics, and the use of both experimental plots and drainage basins as a research base inhibit reconciliation and comparability of results from different areas. In view of the above, useful additions to current knowledge could be achieved by examining the hydrological responses of small rivers and the relationship between these patterns and drainage basin characteristics especially on the extensive Basement Complex rocks of humid tropical Africa. The need for this becomes more pressing given the current awareness in Nigeria for water resource development to meet domestic, industrial and agricultural needs, the paucity of streamflow data, and the advantages in exploiting small streams (Bruin & Hudson, 1950; Saunders & Warford, 1976) to meet the pressing water needs of the country. Thus, apart from the documentation of the various hydrological response patterns of the basins, it would be relevant to identify the environmental factors which determine water yield patterns in these basins. This paper therefore presents the hydrological response patterns of 15 third-order streams on the Pre-Cambrian Basement Complex rocks of southwestern Nigeria in terms of (a) discharge regime patterns, (b) flow variability, and (c) recession patterns. Attempts are also made to examine the environmental factors which promote these patterns.

Hydrological response patterns of some third order streams

379

STUDY AREA For this exercise, 15 third-order basins (see Table 1) were selected from a sixth-order drainage basin covering 457 km between latitudes 7°27' to 7°43'N and longitudes 4°55' to 5°10'E in southwestern Nigeria (Fig.l). They were chosen on the basis of accessibility to their regular main channels. The basins are underlain by the Pre-Cambrian Basement Complex rocks comprising quartzites, granitic gneisses, amphibolites and some undifferentiated schists (de Swardt, 1953; Smyth & Montgomery, 1962). Soils and regolith in the basins are generally shallow and closely related to the solid geology and belong mainly to the order of Oxisols, being made up of profiles characteristic of Ferruginous Tropical Soils (Smyth & Montgomery, 1962). The soils differ mainly in their textural characteristics, with soils on the highly fractured and jointed-quartzites being coarse, sandy and shallow; while those on the granitic gneisses, amphibolites and schists are deeper, fine grained and clayey. The soils on the amphibolites and schists are the deepest and most clayey (Smyth & Montgomery, 1962). The typical mature vegetation is the tropical rain forest now found in forest reserves in the study area. The present land use consists of a kaleidoscopic pattern of farms, fallow regrowths, secondary forest, cocoa/kola/plantain plots, and patches of high forests. The climate in the study area is a drier "species" of Koppen's Af or Thornthwaite's AAr characterized by a short dry season extending from November to March. The mean total annual rainfall is about 1500 mm. Temperatures average 25°C and 27°C in the wet and dry season respectively. Table 1

Some drainage basin physiographic properties

Basins

% Area of basiri underllain by: Amph Qz Gg (Bi) (B3> (B 2 )

Okun Opapa Alura Olotun Etiokun Ohoo Erinta Mogbado Erin Arosa Anini Okorokoro Ofi Apon Orunro

96 96 78 8 36 96 58 13 33 3 29 1 1 3 1

Qz Gg

3 3 21 91 63 3 41 86 83 12 26 98 98 96 98

Quartzitic rocks. = Granitic gneisses.

1 1 1 1 1 1 1 1 4 85 45 1 1 1 1

Area (km~~2 ) (B 4 )

Dd (km - 1 ) (B s )

Rh (B 6 )

Annual rainfall (mm) |aa/vi |Biox

( t „s ) ) aBjeipsiQ

T3

r—VU~ {,_s | ) a6jGi|0Sja A|>|93M |EJOX

(aiai) nejuiBj Af>jaaiv\ |Eiox

( ,_s | | aB-ieqosirj

384

J.O. Adejuwon ef al. Table 2

Variability and recession indices of discharge

Rivers

VI

NFD

k

Qo

Okun Opapa Alura Olotun Etiokun Oh oo Erinta Mogbado Erin Arosa Anini Okorokoro Ofi Apon Orunro

0.75 0,29 0.36 1.92 0.70 0.27 0.46 0.67 0.45 0.25 0.62 2.12 1.44 0.77 0.79

0 0 0 192 106 0 0 225 0 262 0 197 147 143 101

0.94 0.98 0.95 0.81 0.90 0.98 0.96 0.87 0.96 0.88 0.93 0.64 0.82 0.92 0.92

698 113 668 520 754 405 592 261 410 14 87 281 192 201 315

Qt

1 31 19 0.02 0.6 35 15 2 9 1 0.8 0.001 0.03 1 2

t

99 68 79 61 69 99 99 36 88 21 65 28 46 73 66

VI = Variability index. NFD = Number of days during the hydrological year when there was no observable discharge. K = Recession constant. Q0 = Discharge at beginning of recession (I s"1 ). Q^ = Discharge at time t after Q 0 ( I s_1 ). t = Time of recession (days).

Discharge recession The discharge regimes depict the recession pattern in terms of the slope of fall of discharge from peak to minimum discharge and the duration of recession from the peak discharge to the first day of observation of the lowest discharges. Table 2 shows the values of the recession constants and the data used in their derivation. The recession constants (K) range from 0.64 to 0.98 with high values depicting basins with large groundwater storage and a slow decrease in this storage during the recession period, while low values show sharp decline from peak to baseflows, thus implying not only a low groundwater storage in such basins, but also a low groundwater contribution to streamflow.

Differences

in the aggregate

hydrological

response

patterns

Figure 3 shows the clustering patterns of the 15 rivers on the basis of their values of variability index (VI), recession constant (K) and number of days during the hydrological year in which there was no observable discharge (NFD). On this basis and on that of their discharge regimes, the rivers can be classified into three groups: (a) Rivers characterized by perennial flow, low variability of discharge with VI ranging from 0.29 to 0.75 and gentle recession of flow from peak discharge in the rainy season to minimum discharge in the dry season with K values ranging from 0.93 to 0.98. These rivers could further be classified into two groups - those with high perennial discharges (Okun, Opapa, Alura, Ohoo, Erinta, Erin; e.g. Fig.2(a) and those with low discharges even during the rainy season (Anini, Fig.2(b)).

Hy'drologieal response patterns of some t h i r d order streams

385

I! •o.e °

variability index recession constant zoo

220

2-*0

260

Number of days without flow (I\IFD) Fig. 3

Groupings of the Rivers based on V I , K and N F D .

(b) Rivers with high discharges during the rainy season but which dry up completely for between 100 and 150 days in the dry season. These rivers have variability indices ranging from 0.70 to 1.44, and recession constants between 0.82 and 0.92 (Etiokun, Ofi, Apon, Orunro; e.g. Fig.2(c)). (c) Seasonal rivers which dry up for between 190 and 270 days in the year. There are however two groups of rivers in this class. Two of the rivers, Olotun and Okorokoro, have high variability indices (1.92 and 2.12), low recession constants (0.81 and 0.64), high discharges during the rainy season and dry up for about 195 days in the hydrological year. The remaining two rivers, Mogbado and Arosa, although dry for 225 and 262 days respectively, have low discharges even during the rainy season (Fig.2(d)) which promote low variability indices (0.67 and 0.25) and high recession constants (0.87 and 0.88). These recession constants show the problem in the use of K value derived using equation (1). As the constant is based on Q 0 and Q^, the magnitude of these discharges strongly affects the constant.

Relationships physiographic

between hydrological properties

response

and the drainage

basin

The stepwise regression analysis yielded equations (3) and (4) on the relationships between variability index (VI) and recession constant (K) respectively on the one hand and the drainage basin physiographic properties on the other. VI = -15.3 - 0.3 log Bx - 0.2 log B3 - 0.6 log Bg + 2.8 log B 8 - 2.1 log B 9

(3)

(R2 = 0.84) K = 2.2. + 0.1 log B1 + 0.1 log B 6 - 0.4 log B 8 + 0.2 log B c

(4)

(R^ = 0.97) where B^ is percentage of basin area underlain by quartzitic rocks; B2 is percentage of basin area underlain by granitic gneisses; B3 is

386 J.O. Adejuwon ef al. percentage of basin area underlain by amphibolites and schists; B4 is basin area; B 5 is drainage density; Bg is relief ratio; B7 is total annual rainfall; Bg is total dry season rainfall, and Bg is maximum weekly rainfall. Table 3 shows the R2 (coefficient of determination) values of the influence of each basin parameter on the variability index and recession constant. (Only variables that accounted for more than 1% of the variance in the dependent variable were included in the equations.) Table 3

VI K

R2 values due to each basin parameter in the regression equations

Bi

B2

B3

B4

B5

B6

B7

B8

B9

Total R2

0.15 0.48

0.01 0.00

0.02 0.00

0.01 0.00

0.00 0.00

0.38 0.06

0.01 0.00

0.25 0.38

0.04 0.05

0.87 0.97

Equations (3) and (4) and Table 3 show that factors of geology and soil, basin relief and rainfall significantly influence discharge variability and recession. Among others, equation (3) shows that the higher the percentage area of basin underlain by amphibolites and basic schists (B3), the less variable the discharge of the river, and that the higher the dry season rainfall (Bg) the more variable the discharge. Equation (3) shows that the higher the dry season rainfall in a basin, the sharper the recession. These are unusual relationships peculiar to the study area.

DISCUSSION The seasonal variations in the discharge of a drainage basin depend on the relationship between climate, vegetation, soils and rock structure and basin morphology. The erratic nature of flow of these rivers is a characteristic to be expected of the small rivers studied as their basins have little or no channel and flood plain storage capacity which can significantly regulate flow and maintain high discharges for some period after storm events. Variations in the length of period of high discharges within the rainy season however appear to depend on the size of the basin and most especially on the nature of the underlying geology (cf. Table 1 and Figs 2-5). The seasonal variation in streamflow response to rainfall reflects the influence of soil moisture depletion. The regular rainfall and the high humidity during the rainy season generally maintain soil moisture status at capacity levels. Thus, there is always a quick streamflow response to rainfall events. The low humidity and the high evapotranspirative losses during the dry season cause such severe depletion of soil moisture storage that most rainfall during this season and even at the beginning of the rainy season is used to replenish this storage. The extent of the depletion is exemplified by the River Mogbado (Fig.2(d)). A cumulative rainfall of 350 mm spread over 13 weeks beginning from 1 April produced little or no observable discharge in the river.

Hydrological response patterns of some third order streams

387

In addition, it took a storm of 110 nun in the lourteenth week to generate the first observable discharge. This low influence of dry season rainfall on hydrological response of these streams is shown by equations (3) and (4), The equations show that high dry season rainfall is associated with greater flow variability and sharper recession. This is contrary to expectation, as a large amount of rainfall during this season should have produced discharges similar to those of the rainy season and thus promote low variability and gentle recession. It should be noted, however, that the dry season rainfall occurred in the last two months of the season when most streams had already dried up or reached their lowest discharges. The variations among the basins, in the quantity of annual rainfall, also have little influence on discharge regimes. For instance, given their low rainfall values, Ohoo and Erin would be expected to have similar traits to Arosa (Table 1 ) , but the former maintain high discharges throughout the year while the latter is seasonal and with low discharges. Also, Mogbado (Fig.2(d)) whose basin received a much higher amount of rainfall dried up for 225 days. For a stream to have perennial discharge, low discharge variability and gentle recession, enough water must be stored during the rainy season to provide discharge during the dry season. The 15 rivers drain areas underlain by the Pre-Cambrian Basement Complex with associated saprolite which have varying structural and textural characteristics. The quartzitic rocks which are highly fissured and jointed, are overlain by shallow, coarse, highly permeable and porous saprolite, which characteristically promotes high rates of infiltration and storage in rock crevices out of the reach of agents of évapotranspiration. As these rocks promote delayed flow and good aquifer characteristics, rivers draining them are characterized by high discharges throughout the year, low discharge variability and gentle recession (e.g. Okun, Opapa, Ohoo, Alura). On the other hand, rocks such as granitic gneisses, amphibolites and schists underlying the other basins are poorly jointed, also, the saprolite derived from them, though relatively deep, is too clayey especially on the amphibolites and schists to serve as an efficient aquifer. Rather, they tend to favour quickflow, or where the relief is low, retention of water on or near the surface and its subsequent loss through évapotranspiration. Thus rivers draining these rocks generally have high discharges during these rains but dry up during the dry season, high discharge variability and sharp recession. These influences of geology are reflected in equations (3) and (4) especially, as recession constants or slope of recession depend on groundwater storage capacity and upon the transmissivity of the rocks and saprolite which the rivers drain {cf. Cross, 1949; Todd, 1953). However, equation (3) shows that increasing percentage of basin area underlain by amphibolites and basic schists (B3) is associated with low discharge variability. This is because Arosa, which is associated with the geological type has a low VI due to its low discharges during the period it had discharge. Low gradients are expected to favour even discharges throughout the year while high relief favours more variable flow (Maxey, 1964; Carlston, 1968). In the rivers studied, low gradients are associated with high discharges during the rainy season and little

388 J.O. Adejuwonef al. or no discharges during the dry season, high discharge variability and sharp recession (e.g. Mogbado, Okorokoro, Olotun). This however reflects the geology as high relief occurs on the quartzitic rocks (correlation coefficient, r, between quartzitic rocks, B-^, and relief ratio, Bg, is 0.70) while relief is much lower on the other rocks (r between granitic gneisses, B2, and relief ratio, Bg, is 0.63). The higher variability index values obtained for the 15 rivers as compared with those obtained by Walling (1971) in Devon, southwest England, may be due to the considerable differences in the climate of the two different study areas. Unlike the generally cool and moist climate of temperate regions, the high input of solar radiation coupled with about five months of low humidity cause high évapotranspiration and rapid desiccation in the basins studied.

CONCLUSION The discharge characteristics of some third-order rivers in southwestern Nigeria are described in terms of regime, discharge variability and recession patterns. Most of the rivers exhibit poor discharge regulation, thus some dry up for more than 200 days though the rainy season covers seven months. An important observation is the major influence of the nature of the solid geology on discharge characteristics. Among the rivers studied, those which drain areas underlain by the highly fractured quartzitic rocks and their associated shallow regolith have markedly different characteristics from those which drain other rocks like granitic gneisses, amphibolites and schists. The former are characterized by perennial flow whereas rivers draining the other rocks have seasonal flow, higher discharge variability and sharper recessions.

REFERENCES Bruin, J. & Hudson, H.E., Jr (1950) Discussion on "Streamflow variability" by E.W.Lane & K.Lei. Trans. Am. Soc. Civ. Engrs 115, 1084-1134. Carlston, C.W. (1968) Slope discharge relation for eight rivers in the US. USGS Prof. Pap. 600D, D45-D47. Cross, W.P. (1949) The relation of geology to dry weather streamflow in Ohio. Trans. Am. Geophys. Un. 30 (4), 563-566. de Swardt, A.M.J. (1953) The geology of the country around Ilesha. Geol. Survey of Nigeria Bull. no. 23. Lane, E.W. & Lei, K. (1950) Streamflow variability. Trans. Am. Soc. Civ. Engrs 115, 1084-1134. Ledger, D.C. (1964) Some hydrological characteristics of West African rivers. Trans. Inst. Brit. Geogr. 33, 73-90. Ledger, D.C. (1969) The dry season flow characteristics of West African rivers. In: Environment and Landuse in Africa (ed. by M.F.Thomas & G.W.Whittington), 83-102. Methuen, London. Lull, H.W. (1950) Discussion on "Streamflow variability" by E.W.Lane & K.Lei. Trans. Am. Soc. Civ. Engrs 115, 1084-1134.

Hydrological response patterns of some third order streams

389

Maxey, G.B. (1964) Hydrogeology. In: Handbook of Applied Hydrology (ed. by V.T.Chow), section 4-1, 1-33. McGraw-Hill. Morgan, R.P.C. (1971) A morphometic study of some valley systems of the English Chalkland. Trans. Inst. Brit. Geogr. 54, 33-43. Morisawa, M.E. (1957) Accuracy of determination of stream lengths from topographic maps. Trans. Am. Geophys. Un. 38, 86-88. NEDCO (1959) Rivers Studies and Recommendations on Improvement of Niger and Benue. North Holland Publishing Co., Amsterdam. Reinhart, K.G. & Eschner, A.R. (1962) Effects of streamflow of four different forest practices in the Allegheny Mountains. J. Geophys. Res. 67 (6), 2433-2445. Rodier, J. (1959) Results obtained from experimental basins in French overseas territories. La Houille Blanche no. B, 952-963. Saunders, R.J. & Warford, J.J. (1976) Village Water Supply. John Hopkins Univ. Press, Baltimore. Schumm, S.A. (1956) The evolution of drainage system and slopes in badlands at Perth Amboy, New Jersey. Geol. Soc. Am. Bull. 67, 597-646. Smyth, A.J. & Montgomery, R.F. (1962) Soils and landuse of Central Western Nigeria. Government Printer, Ibadan. Todd, D.K. (1953) Streamflow frequency distribution in California. Trans. Am. Geophys. Un. 34 (6), 897-905. Walling, D.E. (1971) Streamflow from instrumented catchments in South East Devon. In: Exeter Essays in Geography (ed. by K.J.Gregory & W.L.D.Ravenhill), 55-81. Exeter. White, E.L. (1977) Sustained flow in small Appalachian watersheds underlain by carbonate rocks. J. Hydrol. 32, 71-81. Wright, C.E. (1970) Catchment characteristics influencing low flows. Wat. Wat. Engng 74, 468-471. Received

25 February

1982;

accepted

24 January

1983.