hydrological processes of the Niger Delta wetland soils. I. BALOGUN ... supports a lowland tropical rain forest vegetation with mangrove swamps dominating.
Exchange Processes at the Land Surface for a Range of Space and Time Scales (Proceedings of the Yokohama Symposium, July 1993). IAHS Publ. no. 212, 1993.
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Characterization and parameterization of the physical and hydrological processes of the Niger Delta wetland soils I. BALOGUN & L. OYEBANDE Faculty of Environmental
Sciences,
University of Lagos, Lagos,
Nigeria
Abstract The physical and hydrological properties of four pedons in the Buguma-Degema wetland of the Niger Delta in the Rivers State of Nigeria were studied. The soils were grouped into organic (pedons 1 and 2), transitional (pedon 3), and mineral (pedon 4) soils. Soil texture varied from sand to clay. Bulk densities ranged from 0.24 to 0.67 g cm"3 for organic soils and 1.34 to 1.97 g cm"3 for the other two soils. Particle density was from 1.87 to 2.43 g cm"3 for organic soils and between 2.57 and 2.63 for the transitional and mineral soils. Maximum available water within 100 cm depth in the pedons ranged from 18.1-27.8 cm and 46.8-53.2 cm and values of saturated hydraulic conductivity between 752-992 cm and 1087-1532 cm day"1. The p H of the soils became strongly acidic when dry (2.9 to 4.6). INTRODUCTION Wetlands are dynamic ecosystems with a complex interrelationship of hydrology, soils and vegetation. Characteristically, wetland soils are saturated with water during all or significant portions of the year, resulting in poor oxygen penetration and chemically reducing conditions. An important reason for the urgent need to pay serious attention to the coastal zone wetlands relates to the global change, including climate change. The impact of the change is expected to alter the rate of delivery of organic matter, sediments and nutrients from land to rivers and coastal wetlands through changes in surface flows, soil erosion and other hydrogeomorphic processes. Already, floods and erosion are becoming progressively more severe in many parts of Nigeria's coastal zone, forcing settlements to shift inland at regular time intervals. Better information on the hydrophysical and chemical characteristics of these environments is needed to allow the development of coupled hydrodynamic and ecosystem models for assessing and planning mitigation of the potential impact of global change on primary and secondary production of the coastal zone wetlands. The present paper reports a study that is intended to be a preliminary contribution to these information and parameterization needs. STUDY AREA AND DESIGN OF FIELD INVESTIGATIONS Site location and characteristics The study area, Buguma-Degema swamps, is located in the Niger Delta wetlands of
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Fig. 1 Map of Rivers State and Niger Delta showing study areas (Degema and Buguma).
Rivers State in southern Nigeria (Fig. 1). The climate of this region is humid and supports a lowland tropical rain forest vegetation with mangrove swamps dominating the coast. The mean annual rainfall is 2500 to 3000 mm, and the rainy season lasts from March to November, with a short dry season from December to February. The mean annual temperature is about 26°C.
Design of field tests The study areas in Degema and Buguma are approximately 1.5 and 2.0 ha, respectively. While the Degema area is flat, the Buguma area is on a gentle slope (about 2%) from the river inland. The swamps are influenced by both groundwater and tidal water from the river. One pedon, clay loam in texture (represented by three soil profiles) was chosen in the Degema swamp; the profile pits were sunk about 40 m from the river bank and 5 m apart. Three pedons (each represented by three profile pits) were selected in the less homogeneous Buguma swamp to represent three major soil types. The first pedon (loamy sand texture) at Buguma was selected at about 30 m from the creek and the remaining two were sited up the slope at about 20 m interval. The first (pedon 3) had sandy clay loam texture while pedon 4 was sandy loam. Changes in organic matter content of the soil was a major feature used in locating the pits.
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PHYSICAL AND HYDROLOGICAL PROPERTIES AND PROCESSES For characterization of the physical and hydrological properties and processes each profile was studied at depths of 0-20, 20-40, 40-60, 60-80 and 80-100 cm. Soil properties were investigated using both disturbed and undisturbed soil samples. The soil physical properties studied were particle size distribution, particle density, bulk density, and pore-size distribution. However, the present report includes only hydrological properties. Hydrologie properties are characteristics that are used to describe the water related processes in the soil. The ones commonly used are hydraulic conductivity, soil moisture retention characteristics such as water content and water storage capacity, porosity, hydraulic diffusivity and infiltration rates or capacity and groundwater table fluctuations.
Methods of determination Moisture retention characteristics The soil moisture retention curve was determined in the laboratory by a method based on the method of Richards (1941). An experimental variation to the method of Richards introduced here was the use of both suction plates for suctions ip = 0 to 330 cm water column (pF 0-2.52) and pressureplate extractors for suctions of \f/ = 1000, 3000 and 15 000 cm water (pF 1, 3 and 4.2). The water content was determined at each stage (saturation, draining and weighting) and converted into volumetric water content using the average bulk density for the depth. Disturbed samples (two replicates per depth of each profile) were used for the low suction range (0 to 330 cm) and to determine water retention at 1000, 3000 and 15 000 cm water suction in a Richards pressure-plate apparatus. The gravimetric content was converted also into volume water content. Paired values of volumetric water content and suction were obtained from this determination to yield the soil moisture retention characteristics.
Water storage capacity Maximum water storage capacity was estimated from moisture characteristics data (as the difference between moisture retained between p F 2.0 and 4.2). Diffusivity Diffusivity is the rate at which water is transmitted within the soil. This soil characteristic is moisture-content dependent. The evaporation method by Arya et al. (1975) was used. This method has been described by Opara-Nadi (1979). Eight replicate samples of the 10 cm long cores taken from each of the 5 depths in each pedon were analysed. The water content distribution and bulk density were determined. Since the water content gradient was greatest towards the end of the soil core, thinner segments were cut from this end. The initial
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water content was calculated from the difference between the initial weight of the sample and weight of the dry sample. The moisture content distribution of each sample was plotted. The diffusivity was calculated with the equation of diffusivity (Bruce & Klute (1956) cited by Opara-Nadi, 1979). The equation is:
D(«) = i. 2t
dx d6
"'xdfl
CD
From the plot of moisture content distribution, the derivative and integral expressions of the equation of diffusivity were graphically evaluated, and the diffusivity, D as a function of moisture content, O was calculated for different levels of water content.
Hydraulic conductivity Hydraulic conductivity, K is a function of the pore system of the soil and, therefore, it is both structure and texture dependent. But basically K is controlled by the water content of the soil and is determined either as a function of water content through the soil moisture retention curve or a function of matric suction. Saturated conductivity was determined in the laboratory from the core samples using the modified falling-head method suggested by Klute (1965). Unsaturated hydraulic conductivity was determined to enable one explain the flow of water under unsaturated condition at certain depths of the soil. This obtains especially at certain periods of the day when tidal water had receded. In order to derive K(0) functions, from the diffusivity values as a function of moisture content calculated above, use was made of the equation
K(0) = D(8)f-
(2)
a\j/
DISCUSSION OF RESULTS Moisture retention characteristics Figure 2(a)-(d) show the soil-moisture retention curves of 0-20, 20-40, 40-60, 60-80 and 80-100 cm depths of the four pedons. For the acid sulphate, clay loam textured, hemists of pedon 1 (Fig. 2(a)), the curves show a gentle slope from p F 2.0 to 4.2 and very steep slope from p F range 0.0 to 2.0, a small increase in suction corresponds to a very large change in water content. On the other hand, a large change in suction corresponds to a small change in water content for the p F range 0 to 2.0. Figure 2(b) also shows that the curves for the similar soils found in pedon 2 that are also acid sulphate in nature but with a loamy sand texture have a gentle slope over the entire suction range of p F 0.7 to 4.2. For the sandy clay loam soils of pedon 3, that are transitional between the organic pedons 1 and 2, and the purely mineral soils of pedon 4, the curves were very steep over the p F range of 0 to 2.4, flat over p F range
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LEGEND »—. = 0 - 20cm
•—.
= 20-40
a—a à—ù
= 40 - 6 0
"
= 60-80
•'
o—0
= 80 - 1 0 0 »
••
(b)
4-4 4-0
\ ^ \
3-6
^^5îÇ
3-2 2'8 2-4 2-0
_ ^
1 -6 1 -2 0-8 0-4
o-n
0-1
0-2
^
.
^™
. . 1
0-4
0-6
VOLUMETRIC
0-9
MOISTURE
CONTENT
-9 (crri
cm"J;
Fig. 2 Moisture characteristics of mangrove soils of Niger Delta, Nigeria.
of 2.4 to 3.0 and steep over p F 3.0 to 4.2. The moisture retention curves of pedon 4 soils with sandy loam texture did not show any definite shape. However, the curves were more or less fairly steep over the entire suction range. Water storage capacity The data for the maximum available water storage capacity show that the values for pedon 1 ranged from 7.0 to 11.6 cm; for pedon 2, 10.2 to 11.2 cm, for pedon 3, 3.8 to 7.8 cm and for pedon 4, 2.2 to 6.8 cm. On the basis of profile total, pedons 1, 2, 3 and 4 had 48.6, 53.2, 27.8 and 18.1 cm, respectively. This shows that the organic soils had higher available water storage capacities than the transitional and mineral soils within the 100 cm depth. The higher storage capacities for the organic pedons could be attributed to the higher porosities of the organic soils. The available water storage capacity plays a significant role in the supply of water to the plants especially during periods of dry spell and low tides.
Saturated hydraulic conductivity Generally, saturated hydraulic conductivities were high in all the pedons studied. Pedons 1 and 2 had conductivities ranging from 99.7 to 2014 cm day"1 and 341.7 to
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278 voo
(c)
0-3
0-56 1-00|-
0-6
0-5 VOLUMETRIC
0-4
0-3 MOISTURE
_!_
0-50 0-44 0-38 0-32 0-26 0-20 0-14
0O8
td)
0-38 0-32 0-26 0-14
0-08
CONTENT fl- ( cm" 3 cm" 3 ) •
Fig. 3 Diffusivity-moisture content relationship of the soils of the Degema (pedon 1) and Buguma (pedons 2-4) mangroves.
2575 cm day"1, respectively. Conductivities were between 254.1 and 2168.2 cm day"1 for pedon 3 and 302.9 to 2032.7 cm day"1 for pedon 4. The results show that a large variability exists both between and within the pedons. The wide variability could be partly attributed to inherent variability associated with saturated hydraulic conductivity determination.
Diffusivity Figure 3 shows the plot of diffusivity D(0) as a function of volumetric moisture content, 8. Generally, the range of transmission was greater in the organic soils. Diffusivity was measurable over a moisture range of 0.30 to 0.80 cm3 cm"3 in pedons 1 and 2, while it was measurable from 0.18 to 0.46 cm3 cm"3 and 0.10 to 0.30 cm3 cm"3 in pedons 1 and 2 (organic soils), 0.34 to 0.46 cm3 cm"3 in pedon 3 (transitional soil) and 0.22 to 0.30 cm3 cm"3 in pedon 4 (mineral soil). In terms of magnitude of diffusivity, the order was: pedon 4 > pedon 3 > pedon 2 > pedon 1.
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LEGEND
1 SUCTION
•—•
=
»—«
=
0 -
20cm
o—a
= 4 0 - 6 0 •>
t—t.
= 60-80
a—o
= 8 0 - 100 •'
20-40
•• "
2 (cm)
Fig. 4 Unsaturated hydraulic conductivity-soil suction relationship of the soils of the Degema (pedon 1) and Buguma (pedons 2-4) mangroves in Niger Delta, Nigeria.
Unsaturated hydraulic conductivity The unsaturated hydraulic conductivity functions of the four pedons are shown in Fig. 4. The results show conclusively that the form and slope of the moisture characteristic curve of soils determine, to a large extent, the form and steepness of the conductivity function. Something to note is that K(ip) values are large (10-1000 cm day"1) indicating high rate of water conduction of these soils even under unsaturated condition. One may conclude from the management point of view, that the soils can be easily drained to lower contents without any appreciable reduction in water conductivity of the soils. In general, the figures illustrate some of the problems associated with derivation of hydraulic conductivity from diffusivity values for some soils including organic and very sandy soils. CONCLUSIONS It has become necessary to undertake systematic collection and analysis of physiochemical and hydrologie data on the soils of the Niger Delta and of the rest of the West African coastal wetlands in order to provide adequate data base for comprehensive parameterization and modelling. Such data base will facilitate effective management of the soils and associated resources of the important hydro-geomorphic
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environment. The need is even more urgent in view of the threat of climate change which may cause sea-level rise of 20 to 50 cm by 2030 and 65-100 cm by 2100. REFERENCES Arya, L.A., Farrell, D.A. & Blake, G.R. (1975) A field study of soil water depletion patterns in the presence of growing soyabean roots: I. Determination of hydraulic properties of the soil. Soil Sci. Soc. Am. Proc. 39, 424-430. Balogun, I.I. (1967) Some physical and hydrological properties of some wetland soils of Rivers State. M. Phil. Thesis, Rivers State University of Science and Technology, Port Harcourt, Nigeria, 100 pp. Klute, A. (1965) Laboratory measurement of hydraulic conductivity of saturated soils. In: Method of Soil Analysis (ed. by C.A. Black), Am. Soc. Agron. Inc., N . Y . , 210-233. Opara-Nadi, O. A. (1979) A comparison of some methods for determining the hydraulic conductivity of unsaturated soils in the low suction range. Ph.D. Thesis, Inst. Soil Sci. Forest Nutrition, GeorgAugust Univ., Gôttingen, Germany. Richards, L.A. (1941) A pressure membrane extraction apparatus for soil solution. Soil Sci. 57, 377.
