Runoff and soil loss from an oxisol in Southeastern Nigeria under various management practices

Agricultural Water Management, 5 (1982) 193--203

193

Elsevier Scientific Publishing Company, Amsterdam - - Printed in The Netherlands

RUNOFF AND SOIL LOSS FROM AN OXISOL IN SOUTHEASTERN NIGERIA UNDER VARIOUS MANAGEMENT PRACTICES

M.E. OBI

Department of Soil Science, University of Nigeria, Nsuhka (Nigeria) (Accepted 22 March 1982)

ABSTRACT Obi, M.E., 1982. R u n o f f and soil loss from an oxisol in Southeastern Nigeria under various management practices. Agric. Water Manage., 5: 193--203. Water infiltration, runoff and soil loss were investigated in an oxisol in Southeastern Nigeria under six management practices. The infiltration rates ranged from 235 m m h "1 to 947 m m h "1. Annual runoff and soil loss of up to 204 m m and 55 t/ha, respectively, were recorded in the bare fallow plots, whereas in the mulched maize plots the values were as low as 12 m m and 0.9 t/ha, respectively. No runoff or soil loss was recorded under three vegetative covers tested. Surface crusting was noted to be the major cause of the runoff and soil loss.

INTRODUCTION

The rate at which water enters the soil is of considerable importance in irrigation planning and soil erosion control. In particular, it is an indication of rainfall acceptance or runoff potential. Under secondary forest conditions in Western Nigeria, a high average infiltration rate of 20--25 c m h "1 was observed by Wilkinson and Aina (1976). However, severe reduction in the infiltration rate followed land clearing and arable cropping. In an earlier investigation in Northern Nigeria, Wilkinson (1975a) also observed high rates under grass fallow rotations. He stated that the severity of erosion in this area was more a function of the high rainfall erosivity than of the inherent erodihility of the soil. Quantification of water runoff and sediment loss from areas of sloping topography continues to attract and sustain the attention of many researchers. The "epicentre" of activity in this regard has been the temperate region of the United States where the numerous findings have led to greatly improved conservation planning. In recent years, however, interest in soil erosion research has built up in the tropical regions of the world, some parts of which have degenerated into "disaster areas" following erosion. Works by Hudson (1957), Lal (1976) and Felipe-Morales et al. (1977) are but a few of the attempts to evolve meaningful conservation programmes in tropical countries.

0378-3774/82/0000--0000/$02.75 © 1982 Elsevier Scientific Publishing Company

194

Lal (1976) investigated the effects of slope, crop rotation and residue management on runoff and soil loss in an alfisol in Western Nigeria. He noted severe surface runoff and soil loss of 543 mm and 156 t/ha, respectively, from bare soil on a 5% slope. In contrast, runoff and soil loss under maize--maize mulch management were 4 mm and 0 t/ha, respectively. Aina et al. (1976) found a total annual soil loss of up to 87 t/ha under cassava monocropping at a site with a 5% slope. Soil loss quantification appeared to have been simplified with the development of the Universal Soil Loss Equation of Wischmeier and Smith (1960) and in particular with the development that enabled the prediction from some field observations and the results of some simple laboratory tests (Wischmeier and Mannering, 1969). The equation, however, is not universal and attention has been appropriately drawn to its possible misuse (Wischmeier, 1976). There is certainly a great need for systematic field plot investigation in the tropics. This is important in an area such as Southeastern Nigeria with vast areas of sandy soils which developed over false-bedded sandstones (Jungerius, 1964). In these areas soil erosion b y water has reached very severe dimensions in recent years (Obi and Asiegbu, 1980). This study was undertaken to characterize water infiltration, runoff and soil loss under various management practices in an oxisol in Southeastern Nigeria. MATERIALS AND METHODS

Experimental site The experimental plots were set up in the University of Nigeria, Nsukka, farm (06 ° 52'N, 07 ° 24'E). The area is characterized b y a humid tropical climate with wet and dry seasons. The rainfall pattern is bimodal with an annual total of a b o u t 1700 mm. The wet season with high rainfall usually begins in April and ends in October. Perennial bush burning has left fire-resistant trees and grasses as predominant vegetation. The floor canopy vegetation is dominated b y Imperata cylindrica, Andropogon spp., Eupatorium africana and Pennisetum spp.

Experimental plots The plots were constructed on a soil exhibiting a slope of a b o u t 5%. The soil belongs to the association described b y Jungerius (1964) as an oxisol. It is a deep, porous, red to brownish-red ferrallitic soil derived from sandy deposits of false bedded sandstones and coastal plain sands. The soil is typical of the agricultural softs of the area which have suffered severe accelerated erosion b y water as a result of careless management practices. The plots were 20 m X 3 m and spaced 50 cm apart. Asbestos sheets were

195

driven into the soil to a depth of 10 cm along the t o p ends and sides of the plots. Water and sediments were collected in bins at the lower outlet of the plots and measured after each rainfall event. Soil management practices were as follows: (1) bare fallow; (2) maize on flat cultivated soil, mulched, across the slope; (3) maize on flat cultivated soil, unmulched, across the slope; (4) sweet potatoes; (5) legume (Centrosema pubescens); (6) grass (Panicum maximum). These were replicated four times except for the second management practice which was replicated twice. The experiment was laid o u t in a completely randomized design. Infiltration rates were measured with the double ring infiltrometer technique. The outer ring had an inside diameter of 40 cm and a height of 30 cm whereas the inner one measured 30 cm by 30 cm. The cylinders were carefully driven into the soil to a depth of a b o u t 15 cm. Duplicate readings were taken on each plot during the m o n t h of December. Mechanical analysis of the soft samples was carried o u t using the B o u y o u c o s (1936) h y d r o m e t e r m e t h o d with sodium hexametaphosphate as the dispersing agent. Organic matter was determined b y the Walkley--Black wet oxidation m e t h o d as described b y Allison (1965). RESULTS AND DISCUSSION Analyses of the t o p 0--15 cm of the soil under the various management practices showed that the texture was either sandy loam or loamy sand. The pH was generally low; so, t o o , were nutrient elements notably nitrogen and potassium. The changes in infiltration rate with time are presented in Figs. 1 and 2. The highest average infiltration rate was obtained under the mulched maize management followed b y continuous legume, grass, unmulched maize, and finally, the bare fallow. In 1980 the order was as follows: soft in grass > mulched maize soil > soil in legume > unmulched maize soft > bare fallow. The infiltration rate versus time for soils under mulch and, to some extent, for soil under grass and legume gave indication of textural and compaction changes in these soils. However, the occurrence of entrapped air, variations in soil--water saturation and differences in the initial wetness of the underlying strata are factors that could have contributed also to the observed shapes of the curves. In general the rates were very high. The possible reasons for this have been noted b y other workers (Wilkinson and Aina, 1976). In particular the role of the earthworms and termites in channel creation and in effecting overall soft structural changes must be noted. No consistent trend in the final infiltration rate was observed under the grass, legume or mulch treatments. This seems to suggest that there were no marked differences in the physical conditions of the soil following the three treatments. The lowest infiltration rate was obtained in the bare soil. This is consistent with the fact that infiltration is a surface phenomenon. The t o p 0--10 cm of the bare fallow plots were of weak, crumb structure. The average organic

196

1800

t600

i-

1200, z o

I000

e

e MAIZE MULCHED

< ¢z e.-J

x

800

LEGUME z -

GRASS 600

o

A

_A~,

400

A A~

MAIZE NO

o

I I

I 2

T

I

ME

o

o

'o--'-"

MULCH

BARE

FALLOW

I 3

(h)

Fig. 1. Infiltration rate as a f u n c t i o n o f t i m e under d i f f e r e n t soil m a n a g e m e n t practices in 1979.

carbon content was low (0.55%). The soil is vulnerable to surface sealing during the intense storms in the rainy season. Lawes (1961) pointed out that many surface soils with such characteristics were subject to crusting from raindrop impact. Visual observation showed crusts of up to 3 mm thickness 3 days after heavy rains. The condition in the unmulched maize soil closely resembled that in the bare soil. Noteworthy is that the peak rainfall intensity in the area is about 200 mm h -1 and occurs at short time interval of about 6 min. The observed infiltration rates all exceeded this value. Although the contribution of lateral f l o w in the infiltration process was not evaluated, it appears that runoff or soil loss would not be expected to occur in this soil except as a result of soil crusting. The rainfall histograms are shown in Fig. 3. Surface water runoff and soil

197

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400

; ; MAIZE, NO MULCH

ZOO

o

~- J~--..l:L_~

J

i

~ T i ME

s

~

,,

BARE FALLOW

~,

(h~

Fig. 2. Infiltration rate as a function o f time under different soil management practices in 1980.

loss under the various management practices are given in Figs. 4 and 5. Table I shows the r u n o f f as percentage of rainfall. The total rainfall at Nsukka in 1979 was 1946 mm. Of this quantity, an average of 177 mm or 9% appeared as surface r u n o f f under the bare fallow management. The average soil loss under the same management was 5.5 kg m -2 with the highest m o n t h l y values recorded in July and August, the months of peak rainfall. In contrast, no r u n o f f or soil loss was observed under legume cover, grass and sweet p o t a t o treatments. These management practices apparently provided sufficient c a n o p y vegetation to dissipate the energy of the rainstorms. Under the unmulched maize system r u n o f f and soil loss were 93 mm and 1.6 kg m -2, respectively, or 53% and 29%, respectively, of those recorded under the bare fallow system during the same period. Finally, under the mulched

198

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ANO

MONTHS

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M 1980

1979

Y E ARS

Fig. 3. Monthly rainfall in 1979 and 1980.

maize management runoff and soil loss occurred only during the months of July, August and September which constituted the period of heaviest rainfall. It should be pointed out, however, that termite activity was not checked in these plots. Observation revealed that up to 10% of the originally applied mulch was destroyed by July. The average total runoff recorded under the management system was 12 m m whereas soil loss amounted to about 0.2 kg m "2. These values represented 7 and 3% of the average runoff and soft loss, respectively, recorded under the bare fallow treatments. In 1980, the total rainfall was 1 8 3 0 mm. Of this quantity an average of 204 m m or 11% was lost as surface runoff from the bare fallow plots. The average total soil loss was 3.5 kg m "2, the highest monthly loss being recorded in September, the month of heaviest rainfall. Under the unmulched maize management runoff and soil loss were 76 m m and 1.2 kg m -2, respectively, or 37 and 35%, respectively, of the losses under the bare fallow system. Under the mulched maize treatment surface runoff

199

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FALLOW

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MONTHS Fig.

4. Cumu]ative

l~inoff

in

1979

and

1980.

and soil loss were 17 mm and 0.1 kg m "2, respectively. These losses took place only during the period of the heaviest rainfall and represented 8 and 2%, respectively, of the losses from the bare fallow plots. As in the first year, no losses were recorded in the legume, grass and sweet potato plots during the second year. The peculiarities of the rainstorms in the area under study have been discussed by a number of researchers including Wilkinson (1975b), Kowal (1976) and Lal (1976). As pointed out by these workers, many of the rainstorms tend to be erosive. Furthermore, they are often accompanied by high wind speeds which further aggravate their erosive effect. Table II shows the number o f storm events at Nsukka in 1979 and 1980 and

200 BARE FALLOW

c~

~

- -

1979

----

t980

lIAR[

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S

i

/

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-J

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/

I

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/

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J'~

=c:

•

MA I Z E , MULCHED

MONTHS

Fig. 5. Cumulative soil loss in 1979 and 1980.

the number that resulted in soil loss under the selected management practices. In 1979, about 4 1 % of the rainstorms caused soil loss in the bare fallow plots. O n the average there were about nine erosive events in these plots each month between M a y and October. In the unmulched m ~ e plots, 3 6 % of the ~ a l l events turned out to be erosive. During each of the months of May to October there were about eight erosive events. Finally, in the mulched maize plots only 10% of the total rainfall events gave rise to soil loss primarily during the intense storms of July to September. The results obtained in 1980 showed a similar trend. There were 134 rainfall events. The percentages of the total rainstorms that caused soil loss were 47, 41 and 18 for the bare fallow, unmulched maize and mulched maize soils, respectively.

201 TABLE I R u n o f f as percentage of rainfall under different m a n a g e m e n t practices in 1 9 7 9 and 1 9 8 0

Month

Runoff (% rainfall) Bare

Maize

fallow

no m u l c h

1979

mulched

1980 1979 .

.

1979

1980

7 2

---

--

January

.

.

February

.

.

March April May

-1 12

4 6 10

-0.4 6.0

June July

14

4

6.0

4 4

---

11 August 9 S e p t e m b e r 10 October 9 November 5 December .

18 12 12 Ii 8

4.0 7.0 7.0 3.0 2.0 .

6 3 5 4 1 .

2 1 0.4 ---

.

.

.

1980

.

.

.

.

.

--

3 1 1 --

.

Early in the year (February to April) sporadic high intensity rainfall could easily be absorbed by freshly tilled dry soil. Thus r u n o f f and soil loss are generally negligible during this period. Later with successive rainstorms, there is appreciable soil water recharge. Compaction and crusting eventually develop with consequent r u n o f f and soil loss. Detailed examination of daily records showed t h a t rainfall of up to 11 m m failed to produce r u n o f f in the freshly tilled bare soil in February. Furthermore, following a cessation of rain for any 24-h period during the rainy period, the threshold rainfall a m o u n t required to initiate r u n o f f in the bare soil was about 5 mm. It should be noted that in the bare fallow plots appreciable soil losses of 55 and 35 t / h a occurred in the first and second years, respectively. Corresponding losses from the unmulched maize plots were 16 and 12 t/ha, respectively. Much smaller losses of 1.6 and 0.9 t / h a occurred in the mulched maize plots in 1979 and 1980, respectively. Clearly illustrated in Figs. 4 and 5 is t h a t higher r u n o f f did n o t necessarily mean higher soil loss. The complex interaction of the soil surface condition, antecedent moisture and the rainfall pattern cannot, therefore, be overemphasized. The soil under investigation has a porous topsoil. The depth to the parent material exceeds 200 cm. Preliminary maize production records (unpublished) suggest t h a t a loss o f up to 10 t / h a of this soil can be tolerated w i t h o u t much appreciable loss in production capacity. The major management problem from the erosion viewpoint appears to be t h a t of attempting to offset rainfall

202 TABLE II Monthly erosive rainfall events in 1979 and 1980 under different management practices Month 1979

Rainfall Erosive rainfall events events Bare Maize fallow no-mulch

mulch

1979 January February March April May June July August September October November December

0 6 2 11 18 20 20 27 23 19 8 0

0 0 0 1 10 10 10 9 8 8 3 0

0 0 0 1 9 8 8 8 8 8 3 0

1980 January February March April May June July August September October November December

0 1 5 7 11 18 24 25 19 19 5 0

0 0 3 2 4 7 9 12 14 9 3 0

0 0 2 1 4 6 8 11 11 9 3 0

Legume

Grass

Sweet potatoes

0 0 0 0 0 0 5 6 4 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 1 1 0 7 8 6 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0

i m p a c t b y e n s u r i n g a d e q u a t e v e g e t a t i v e c o v e r . T h i s is p a r t i c u l a r l y c r i t i c a l a t t h e e a r l y s t a g e s o f g r o w t h o f a c r o p l i k e m a i z e w h e n c a n o p y c o v e r is u s u a l l y very sparse. Increasing the plant density would, no doubt, increase rainfall interception but beyond an optimum density, the benefit may be offset by decreased yield. Planting across the contour will reduce particle transportation and, therefore, soil loss to a considerable extent. However, the most obvious management practices are mulching and timely introduction of crops t h a t p r o v i d e a d e q u a t e v e g e t a t i v e c o v e r . I n t h i s c o n n e c t i o n i t is p e r t i n e n t t o note that a mulch rate of 3 t/ha reduced the runoff and soil loss to tolerable levels w h e r e a s l e g u m e , g r a s s o r s w e e t p o t a t o e s r e d u c e d t h e m t o z e r o .

203 ACKNOWLEDGEMENTS T h e a u t h o r is i n d e b t e d t o t h e Soil Science D e p a r t m e n t f o r t h e installation o f t h e r u n o f f facilities a n d t o t h e a g r o m e t e o r o l o g y staff o f t h e C r o p Science D e p a r t m e n t f o r t h e provision o f t h e rainfall data. Warm t h a n k s are d u e also t o Mr. Felix O g b u f o r assistance in m e a s u r i n g t h e r u n o f f a f t e r each rain. REFERENCES Aina, P.O., Lal, R. and Taylor, G.S., 1976. Soil and crop management in relation to soil erosion in the rain forest region of Western Nigeria. Symposium Proceedings of the National Conference 25--26 May, Lafayette, IN. Allison, L.E., 1965. Organic carbon. In: C.A. Black, D.D. Evans, J.L. White, L.E. Ensminger and F.E. Clark (Editors). Methods of Soil Analysis, Part 2. Am. Soc. Agron., Madison, WI, pp. 1367--1378. Bouyoucos, G., 1936. Directions for making mechanical analysis of soils by the hydrometer method. Soil Sci., 42: 225--229. Felipe-Morales, C., Meyer, R., Alegro, C. and Vitorelli, C., 1977. Losses of water and soil under different cultivation systems in two Peruvian locations. In: Proc. Int. Conf. Role of Soil Physical Properties in Maintaining the Productivity of Tropical Soils, 6--10 Dec., 1977, IITA, Ibadan, Nigeria. J. Wiley, New York, NY, pp. 489--499. Hudson, N.W., 1957. Erosion control research. Rhod. Agric. J., 54: 297--323. Jungerius, P.D., 1964. The soils of Eastern Nigeria. Publ. Serv. Geol. Luxemb., 14: 185--198. Kowal, J.M. and Kassam, A.H., 1976. Energy load and instantaneous intensity of rainstorms at Sarnam, Northern Nigeria. Trop. Agric., 53(3): 185--198. Lal, R., 1976. Soil erosion problems on an alfisol in Western Nigeria. In: HTA Monograph 1, Ibadan, Nigeria, pp. 25--76. Lal, R., 1977. Analysis of factors affecting rainfall erosivity and soil erodibility. In: D.J. Greenland and R. Lal (Editors), Soil Management in the Humid Tropics. J. Wiley, New. York, NY. pp. 49--56. Lawes, D.A., 1961. Rainfall conservation and the yield of cotton in Northern Nigeria. Emp. J. Exp. Agric., 29: 307--318. Obi, M.E. and Asiegbu, B.O., 1980. Physical properties of some eroded soils of Southeastern Nigeria. Soil Sci., 130: 39--48. Wilkinson, G.E., 1975a. Effect of grass fallow rotations on the infiltration of water into a Savanna zone soil of Northern Nigeria. Trop. Agric. (Trinidad), 52: 97--108. Wilkinson, G.E., 1975b. Rainfall characteristics and soil erosion in the rain forest area of Western Nigeria. Exp. Agric., 11: 247--255. Wilkinson, G.E. and Aina, P.O., 1976. Infiltration of water into two Nigerian soils under secondary forest and subsequent arable cropping. Geoderma, 15(1 ): 51--59. Wischmeier, W.H., 1976. Use and misuse of the universal soil loss equation. J. Soil Water Conserv., 31(1): 5--9. Wischmeier, W.H. and Smith, D.D., 1960. A universal soil loss equation to guide conservation farm planning. Trans. 7th Int. Congr. Soil Sci., Madison, WI, Vol. 1, pp. 418--425. Wischmeier, W.H. and Mannering, J.V., 1969. Relation of soil properties to its erodibility. Soil Sci. Soc. Am. Proc., 33: 131--137.