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Soil water property variations in three adjacent land use types in the Rift Valley area of Ethiopia
Journal of Arid Environments 75 (2011) 1067e1071
Contents lists available at ScienceDirect
Journal of Arid Environments journal homepage: www.elsevier.com/locate/jaridenv
Soil water property variations in three adjacent land use types in the Rift Valley area of Ethiopia A. Abdelkadir, F. Yimer* Department of Natural Resources and Environmental Studies, Wondo Genet College of Forestry and Natural Resources, P.O. Box 128, Shashemene, Ethiopia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 January 2011 Received in revised form 2 June 2011 Accepted 17 June 2011 Available online 16 July 2011
This study investigated the effects of land use change on infiltration and moisture content of soils in three land use types. A total of 81 soil and core samples (3 replications 3 treatments 3 profiles 3 soil depths) were used to determine parameters that may affect the infiltration properties of soils. The infiltration rate was measured in the field using double-ring infiltrometer with three replicates in each land use type. Results showed that infiltration rates were generally slow in the open grazed and cultivated lands suggesting high potential for runoff, limited percolation, and very low amount of water available in the soil profiles. The accumulated infiltration in soils under cultivation and open-grazing was smaller than the controlled grazing by approximately 57%. Similarly, cultivation and open-grazing had reduced the soil moisture content by 29 and 33%, respectively, compared to the controlled grazing. Surface soil compaction, higher dry bulk density and lower soil organic carbon, appeared to be the principal factors for the low infiltration capacity and moisture content of the soils. Therefore, dry land management, with long term tree cover and well regulated grazing system, is very crucial for the sustainable ecosystem functioning of this environmentally fragile area. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Infiltration capacity Deforestation Land conversion Semi-arid Soil properties Abernosa
1. Introduction Physical, chemical and biological properties of soils are often affected by land cover changes (Shukla et al., 2003; Wood and Blackburn, 1981). Infiltration capacity, which is one of the important soil hydraulic properties, is influenced by particle size distribution in the soil, land use type (Fu et al., 2000), vegetation factors (including plant and litter cover and type, and organic matter content in soil), topographical and climatic influences (e.g. Gifford and Hawkins, 1978; Jiménez et al., 2006; Tromble et al., 1974; Wood and Blackburn, 1981). Continued tillage and grazing practices have a marked effect on soil bulk density, porosity, aeration, structure and aggregate stability, infiltration, water storage, water transport characteristics and runoff (Broersma et al., 1995; Cameron et al., 1981; Tollner et al., 1990). Observations indicate that most tropical soils under forest vegetation have the ability to absorb water at rapid rates (Wahl et al., 2003; Wood, 1971). However, due to land conversion practices, especially from native vegetation to cultivation and grazing systems, has meant that many tropical soils have
* Corresponding author. Tel.: þ251 461 109900; fax: þ251 461 191 499. E-mail address: [email protected] (F. Yimer). 0140-1963/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jaridenv.2011.06.012
undergone important changes in their properties, including loss of organic matter and increase in bulk density (Lepsch et al., 1994; Yimer and Abdelkadir, 2010), and as a consequence, decreases in infiltration rate (Mwendera and Mohamed Saleem, 1997). In the Rift Valley area of Ethiopia, where the present study was carried out, information about soil hydraulic properties, when subjected to different land use changes, is generally lacking. Given the fact that the Rift Valley is an environmentally fragile area, any change that may affect the infiltration capacities of the soils will have direct bearings on the hydrological properties and runoff. The recent flooding problems in different dry land areas of Ethiopia and East Africa (Achameyeleh, 2003; Alemayehu, 2007) which have resulted in the loss of life and damage to crops, may be attributable to land use changes and the resultant poor water infiltration capacity of soils. The objective of this paper is to present differences in soil hydraulic properties as influenced by different land use types that may have a profound bearing in the water budget of the drought prone Rift Valley areas of Ethiopia. The specific questions are: (i) are there differences in infiltration capacity due to differences in land uses, (ii) are there changes in factors such as soil texture, bulk density and organic carbon content that may influence infiltration capacity under different land use types?
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2. Materials and methods 2.1. Study site and experimental layout The study was carried out in the Abernosa Ranch (hereafter referred to as ‘study area’), located in the central part of the Main Rift Valley area of Ethiopia, 170 km south of Addis Ababa, situated at ca. 7 510 N, 38 420 E (Fig. 1). The altitudinal range of the study site is between 1650 and 1900 m above sea level. The area is classified as semiarid eco-climatic vegetation zone (Makin et al., 1975), characterized by a distinct dry season with erratic or no rain and a mean annual precipitation ranging between 400 and 600 mm (Fig. 2). The mean minimum and maximum annual temperatures are 13.5 C and 26.5 C, respectively (National Meteorological Services Agency of Ethiopia, 2009. Geologically, the Rift Valley area of Ethiopia, generally, consists of rocks of volcanic origin such as alkaline (basalts) and acidic (rhyolites and ignimbrites, pumice, volcanic ash) and tuff (Itanna, 2005; Zewdie, 2004), riverine and lacustrine alluvium (Makin et al., 1975). Few soil studies have been conducted in the study area so far. According to Makin et al. (1975) and Zewdie (2004), Haplic Andosols (Typic Haploxerands) are identified as the major soil types found around Ziway and the study area. The existence of a pronounced dry season with high temperature and high fluctuating annual precipitation has led to a xeric soil moisture regime. The soil is characterized as loam textured (Table 1) and is generally weak in its physical structure. The natural vegetation in the study area is characterized by acacia woodland. The main acacia species include Acacia tortolis, A. seyal and
Mean monthly total precipitation (mm)
160 140 120 100 80 60 40 20 0 Jan
Feb Mar
Apr
May Jun
Jul
Aug Sep
Oct
Nov Dec
Months Fig. 2. Mean monthly total rainfall distribution between 1981 and 2006. Source: Adami Tulu meteorological station (2009).
A. senegal. Included in the tree layer is also Balanites aegyptiaca, accompanied by semi-evergreen shrubs, such as Croton dichogamus, Harrisonia, Terminalia brownie, and Sclerocarya birre (Eshete, 1999). The ground cover of this woodland site is composed of natural grasses, which are dominated by species of Hyparrhenia, especially H. anthistiriodes and H. hirta. Other common species in the grassland include Aristodia adoensis, A. adscensionis, Cenchrus ciliaris and Chloris gayana (Makin et al., 1975).
Fig. 1. Location map of the study area.
A. Abdelkadir, F. Yimer / Journal of Arid Environments 75 (2011) 1067e1071
Types of land use practice Depth (cm) 0e20
20e40
40e60
Sand Silt Clay Sand Silt Clay Sand Silt Clay Cultivated land Open-grazing Controlled grazing
39 42 40
42 37 42
19 21 18
41 43 53
37 38 36
22 19 11
44 43 43
41 41 36
15 16 21
The Abernosa ranch was established in 1954 occupying 5000 ha. Currently, however, due to high human and livestock population pressure, its size has dwindled to an area of about 2200 ha. Agricultural practices are mix of farming with rain-fed cultivation and cattle rearing for subsistence. Due to the erratic nature of rainfall and poor fertility of the soils, agricultural production is very low (Eshete, 1999). Controlled grazing has been carried out within the paddocks that are close to the Head Quarters of the ranch for controlled grazing while the open-grazing is on communal grazing fields and on cultivated lands after harvest. There is considerable over stocking in the open/communal grazing areas and hence the limited grazing fields have less capacity to maintain the everincreasing livestock population of the surrounding community. While open-grazing land commonly maintains unlimited herd of cattle and small ruminants, the stocking in the controlled grazing is usually kept at light to moderate stocking with 1250 cattle and 650 goats in the 2200 ha land and with an average stocking rate of less than one animal per hectare. 2.2. Soil sampling and analysis The experimental site was divided into three area blocks, which served as replicates and each containing: controlled grazing area; open/communal grazing area and cultivated land as treatments. The controlled grazing area with its intact acacia vegetation was fenced and managed with rotational grazing for more than 50 years. Both the open-grazing and cultivated lands were under the ranch up until 1994 and later abandoned because of increased human and livestock intervention (Eshete, 1999). The locations of soil profiles in each of the treatments were randomly chosen within similar physiographic conditions such as landscape position and slope percents. A total of 81 soil samples (3 replications 3 treatments 3 profiles 3 soil depth layers e 0e0.2, 0.2e0.4 and 0.4e1.0 m) were collected during the cropping period for laboratory analyses. The soil samples were collected in January, 2007 from three excavated profiles randomly located in each of the three land use types at the above indicated varying depths for determination of dry bulk density, soil texture, organic carbon content and gravimetric soil moisture content. Gravimetric soil moisture content at sampling (in undisturbed soil cores) was determined using the standard procedures with oven drying to a constant weight at 105 C for 24 h. Dry bulk density was calculated from the oven-dry weight of identical core samples as used for soil moisture determination. Volumetric soil moisture content was determined from gravimetric moisture content and dry bulk density. Soil texture was determined using the hydrometer method (Landon, 1991).
rate was determined using double-ring infiltrometer (Bertrannd, 1965). One set of three double-rings was used for a total of three measurements at a time in each location. The obtained values were averaged to represent each location. The inner rings had diameters of 28, 30 and 32 cm and the outer rings 53, 55 and 57 cm. The rings were driven about 5 cm into the soil using a metal plate and sledgehammer. Water was filled to a level 20 cm above the soil surface. The rings were refilled to the 20 cm head level each time when the head approached 5 cm above the soil surface. Changes in water levels were recorded at the time increments 0, 1, 2, 5, 10, 15, 20, 30, 40 and 60 min for calculation of infiltration rates and cumulative infiltration values. Statistical differences were tested using two-way analysis of variance (ANOVA) following the general linear model (GLM) procedure of SPSS version 12.0.1 for Windows (Julie, 2001). Tukey’s Honest Significance Difference (HSD) test was used for mean separation when the analysis of variance showed statistically significant differences (p < 0.05). 3. Results and discussion Many of the considered soil properties showed significant differences with respect to land use type and soil depth. The soil textural analysis showed that the soil under all land use practices was loam (Table 1). This similarity in property indicates that the texture of soils was not the main reason for differences in soil water properties. Therefore, the differences in infiltration capacity and other soil water properties could be attributed singularly to the increased organic matter content and better land management under the controlled grazing. The soil organic carbon (Fig. 3) showed significantly higher in the controlled grazing than in either the cultivated or openly grazed lands (Yimer and Abdelkadir, 2010). Dry bulk density was consistently smaller in the controlled grazing than in the other land use types (Table 2). This may be related to the differences in surface soil compaction especially in moist soil conditions, discontinuity in macro pores, destruction of structure and aggregate stability that has resulted from over stocking and continued tillage in both the open-grazing and cultivated lands, respectively (Abdul-Megid et al., 1987; Broersma et al., 1995; Veldkamp, 1994). Moreover, the smaller organic carbon contents (1.24% and 1.38%), which are very low levels according to Landon (1991) may have also been responsible for the higher bulk
3.5
3.0
Soil organic carbon (%)
Table 1 Soil textural fractions (%) with respect to types of land use practice and soil depths.
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0-0.2m depth 0.2-0.4m depth 0.4-1.0m depth Overall average for each land use
c
2.5
c c
B
2.0
b b
b
A
1.5
a 1.0
b
A
a
a a
a
0.5
2.3. Infiltration measurements 0.0
In each of the land use types, three representative locations for infiltration measurements were randomly distributed (3 replicates 3 treatments 3 measurement points). The weather during all the measurements was sunny and dry. The infiltration
Cultivation
Open-grazing
Cont-grazing Overall (depth)
Land use/soil depth (m) Fig. 3. Soil organic carbon for different land use types and depths.
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Table 2 Soil bulk density (B.d, g cm3) and gravimetric soil moisture (GSM) as influenced by land use and soil depths (mean SE). Overall means followed by the same letter(s) are not significantly different (p ¼ 0.05) as influenced by land use and soil depths. Variables
Depth (m)
Land use types Cultivation
Open-grazing
Cont. grazing
Overall
Bd
0.0e0.2 0.2e0.4 0.4e1.0 Overall 0.0e0.2 0.2e0.4 0.4e1.0 Overall
0.93 (0.03) 0.89 (0.01) 0.91 (0.02) 0.91 (0.01) b 7.87 (0.74) 4.29(0.72) 4.88 (0.74) 5.68 (0.47) a
0.96 (0.02) 0.89 (0.01) 0.90 (0.02) 0.91 (0.01) b 4.54 (0.71) 4.98 (0.74) 6.49 (0.70) 5.34 (0.35) a
0.84 (0.01) 0.82 (0.03) 0.87 (0.03) 0.85 (0.01) a 8.14 (0.70) 6.70 (0.69) 7.16 (0.72) 7.33 (0.55) b
0.91 (0. 02) b 0.87 (0. 01) a 0.90 (0.01) ab
Table 3 Summary of two-way ANOVA results for water infiltration rates (cm min-1) and cumulative infiltration (cm) under different land use types. Source of variations
Land use (LU) Time (T) LU T Error
Infiltration rates
6.86 (0.53) b 5.32 (0.45) a 6.17 (0.45) ab
compaction in the soil surface, especially grazing on moist soils, as is the case in this study(Taboada and Lavado, 1988) influencing the size, amount and distribution of pores, and thereby the rate of water infiltration and percolation (Abdul-Megid et al., 1987). The bulk density is a commonly used parameter to relate to soil porosity, giving an effective indication of the compaction level and infiltration capacity of soils (Taboada and Lavado, 1988; Tollner et al., 1990). In our study, the continued cultivation and unmanaged communal grazing also seem to have altered the organic carbon content. The smaller organic carbon content, possibly resulting in poorer soil structure, could partly be responsible for the higher dry bulk density and poor infiltration observed in cultivation and open-grazing. We physically observed, right after the infiltration measurements, that the maximum depth the water moved down the soil profiles under cultivation and grazing on average ranged from 0.10 to 0.15 m, whereas it penetrated more than 0.40 m under the controlled grazing. The extremely low amount and slow movement of water in the studied soils are good indicators of the effect of land changes from native woodlands coupled with poor land management practices on water movement in the soil system. The fact that the soil moisture content in the soil profiles was higher in the controlled grazing than in the other land use types could be attributed to the lower dry bulk density and higher soil organic carbon contents resulting in higher infiltration and water holding capacities, and thereby lower surface runoff of water. It appears that open-grazing and cultivation had negative effects on the soil structural stability and macropore continuity and thus on the rate of water entry and percolation in the soil system.
1.0
Cultivated land Open grazing Controlled grazing
0.8 -1
density observed in the open-grazing and cultivation since the correlation between the soil organic carbon and bulk density was highly significant in both land use types (0.88, p < 0.001) (Fig. 3). The bulk density was significantly higher in the top surface soil than in the sub surfaces (p < 0.05, Table 2) indicating the impact of surface soil compaction by animal trampling. The gravimetrical moisture content showed a significant variation with land use type (p ¼ 0.003); high in the controlled grazing than in the other land use types (Table 2). There was also a significant difference in soil moisture across all depths (p ¼ 0.046); higher in the top surface soil than in the layers below. This may have been due to the increased water holding capacity of soil that has resulted from increased soil organic matter in the upper soil layers. The infiltration rates, through out the measurement period, were significantly higher (p < 0.001,) for controlled grazing (0.45 cm h1 0.04) than for cultivation (0.20 cm h1 0.02) and open-grazing (0.19 cm h1 0.03) (Table 3 & Fig. 4). The cumulative infiltration also paralleled the infiltration rate (Fig. 5). Taking the hydraulic gradient into account, saturated hydraulic conductivity can roughly be estimated at <1 cm h1 across all land use types. According to rating classification of Landon (1991), soils under the control grazing and lash grass vegetation are high in their infiltration rate (15.6e52.2 cm h1). While soils under cultivation and open-grazing are in many cases slow in their infiltration rate. Slow and/or low infiltration rate may mean high surface water losses, limited deep percolation, and depressed yield (Landon, 1991). These observations would corroborate the view that the reduction in infiltration, rate caused by a change in land use type from woodland to cultivation or grazing, may be attributed to degradation of the surface horizon structure and compaction (Jiménez et al., 2006). Cultivation, for example, may shear and pulverize the soil, reduce the macropore space and produce a discontinuity in pore space between the cultivated surface and the subsurface soil (Abdul-Megid et al., 1987; Broersma et al., 1995), thereby reducing the infiltration rates and water percolation in the soil profile and, consequently, increasing potential for surface water runoff (Broersma et al., 1995). Trampling by cattle is reported to cause increases in bulk density by
Infiltration rate (cm min )
GSM
0.6
0.4
0.2
Cumulative infiltration
d.f.
MS
p
MS
p
2 8 16 54
0.585 0.171 0.009 0.003
<0.001 <0.001 0.003
20.774 3.671 0.377 0.026
<0.001 <0.001 <0.001
0.0 0
10
20
30
40
Time (min) Fig. 4. Infiltration rate curves for land use types.
50
60
A. Abdelkadir, F. Yimer / Journal of Arid Environments 75 (2011) 1067e1071
References
25 Cultivated land Open grazing Controlled grazing
Cumulative infiltration (cm)
20
15
10
5
0 0
10
20
1071
30
40
50
60
Time (min) Fig. 5. Cumulative infiltration curves for land use types.
4. Conclusion Soils under the three land use types in the study area showed differences in their organic carbon contents, soil texture, bulk densities, moisture contents and infiltration capacities. Assuming that the controlled grazing locations are appropriate ecological references, uncontrolled and open access grazing and cultivation practices had to a large extent decreased the infiltration rate (approximately by 56 and 58%, respectively). Surface soil compaction (shown in 13% increase in bulk densities under cultivation and open-grazing), due to tillage and animal trampling coupled with lower contents of soil organic carbon in cultivation, and opengrazing appeared to be the principal factors for the decline of infiltration and available soil moisture causing higher surface water runoff. Under such circumstances, occasional heavy rain storm events may easily give rise to destructive surface runoff forces resulting in loss of humans, animals and crops, as has been the case, in many instances, in the Rift Valley areas. Therefore, dry land management, aimed at rehabilitating the land through long term tree cover and well regulated grazing system to enhance the recovery of the physical and chemical soil qualities, is very crucial for the sustainable ecosystem functioning of this environmentally fragile area. Acknowledgments This study was financed by Wondo Genet College of Forestry and Natural Resources, Ethiopia. We would like to thank Dr. Enrique Vivoni, Associate Editor of the Journal of Arid Environments and the two anonymous reviewers for their valuable comments. We are grateful to the late Sileshi Kifle and Estifanos Mathewos for their assistance in the field, Tigneh Eshete for producing the location map of the study area; and to the staff members of the Abernosa Ranch Head Quarters and the surrounding communities for allowing the excavation of soil pits in their fields.
Abdul-Megid, A.H., Schuman, G.E., Hart, R.H., 1987. Soil bulk density and water infiltration as affected by grazing systems. Journal of Range Management 40, 307e309. Achameyeleh, K. 2003. Integrated Flood Management: Case Study in Ethiopia. WMO and Global Water Partnership, Technical Report. p. 14. Alemayehu, D. 2007. Assessment of Flood Risk in Dire Dawa Town, Eastern Ethiopia, Using GIS. MSc Thesis, Addis Ababa University. p. 86. Bertrannd, A.R., 1965. Rate of water in take in the field. In: Black, C.A. (Ed.), Methods of Soil Analysis, Part I. American Society of Agronomy, Madison, WI, pp. 197e209. Broersma, K., Robertson, J.A., Chanasyk, D.S., 1995. Effects of different cropping systems on soil water properties of a Boralf soil. Communnications in Soil Science and Plant Analysis 26 (11 & 12), 1795e1811. Cameron, D.R., Shaykewich, C., deJong, E., Chanasyk, D., Green, M., Read, D.W.L., 1981. Physical aspects of soil degradation. Edmonton, Canada. In: Agricultural Land e OurDisappearing Heritage-A Symposium. Proceedings of the 18th Annual Alberta Soil Science Workshop, pp. 186e255. Eshete, G. 1999. The impact of different land use type on structure, regeneration and soil properties of Abernosa Acacia woodland. MSc thesis. Skinnskatteberge, Swedish University of Agricultural Sciences, Sweden. Fu, B.J., Chen, L.D., Ma, K.M., Zhou, H.F., Wang, J., 2000. The relationships between land use and soil conditions in the hilly area of the Loess Plateau in northern Shaanxi, China. Catena 36, 69e78. Gifford, G.F., Hawkins, R.H., 1978. Hydrological impact of grazing on infiltration. Water Resources Research 14, 305e313. Itanna, F., 2005. Sulfur distribution in five Ethiopian Rift Valley soils under humid and semi-arid climate. Journal of Arid Environments 62 (4), 597e612. Jiménez, C.C., Tejedor, M., Morillas, G., Neris, J., 2006. Infiltration rates in andisols: effects of changes in vegetation cover (Tenerife, Spain). Journal of Soil and Water Conservation 61, 153e158. Julie, P., 2001. A Step by Step Guide to Data Analysis Using SPSS for Windows. Open University Press. Landon, J.R., 1991. A Handbook for Soil Survey and Agricultural Evaluation in the Tropics and Subtropics. Longman, Harlow. Lepsch, I.F., Menk, J.R.F., Oliveria, J.B., 1994. Carbon storage and other properties of soils under agriculture and native vegetation in São Paulo State, Brazil. Soil Use and Management 10, 34e42. Makin, M.J., Kingham, J.J., Waddams, A.E., Birchall, C.J., Tamene, T., 1975. Development Prospects in the Southern Rift Valley of Ethiopia. Land Resource Study 21. Tolworth, Surrey, England.. Ministry of Overseas Development, Land Resources Division. Mwendera, E.J., Mohamed Saleem, M.A., 1997. Infiltration rates, surface runoff and soil loss as influenced by grazing pressure in the Ethiopian highlands. Soil Use and Management 13, 29e35. National Meteorological Services Agency of Ethiopia, 2009. Addis Ababa, Ethiopia. Shukla, M.K., Lal, R., Owens, L.B., Unkefer, P., 2003. Land use and management impacts on structure and infiltration characteristics of soils in the North Appalachian region of Ohio. Soil Science 168 (3), 167e177. Taboada, M.A., Lavado, R.S., 1988. Grazing effects of the bulk density in a Natraquoll of the flooding Pampa of Argentina. Journal of Range Management 41 (6), 500e503. Tollner, E.W., Calvert, G.V., langdale, G., 1990. Animal trampling effects on soil physical properties of two Southeastern U.S. Ultisols. Agriculture, Ecosystem and Environment 33, 75e87. Tromble, J.J., Renard, K.G., Thatcher, A.P., 1974. Infiltration for three rangeland soilvegetation complexes. Journal of Range Management 27 (4), 318e321. Veldkamp, E., 1994. Organic carbon turnover in three tropical soils under pasture after deforestation. Soil Science Society of America Journal 58, 175e180. Wahl, N.A., Bens, O., Schäfer, B., Hüttl, R.F., 2003. Impact of changes in land-use management on soil hydraulic properties: hydraulic conductivity, water repellency and water retention. Physics and Chemistry of the Earth 28, 1377e1387. Wood, H.B., 1971. Land use effects on the hydrologic characteristics of some Hawaii soils. Journal of Soil & Water Conservation 26, 158e160. Wood, M.K., Blackburn, W., 1981. Grazing systems: their influence on infiltration rates in the Rolling Plains of Texas. Journal of Range Management 34 (4), 331e335. Yimer, F., Abdelkadir, A., 2010. Soil property changes following conversion of acacia woodland into grazing and farmlands in the Rift Valley area of Ethiopia. Land Degradation and Development. doi:10.1002/ldr.1022. Zewdie, E., 2004. Characterization and genesis of some soils of the Ethiopian Rift Valley showing andic properties. Ethiopian Journal of Natural Resources 6 (2), 215e235.
Contents lists available at ScienceDirect
Journal of Arid Environments journal homepage: www.elsevier.com/locate/jaridenv
Soil water property variations in three adjacent land use types in the Rift Valley area of Ethiopia A. Abdelkadir, F. Yimer* Department of Natural Resources and Environmental Studies, Wondo Genet College of Forestry and Natural Resources, P.O. Box 128, Shashemene, Ethiopia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 January 2011 Received in revised form 2 June 2011 Accepted 17 June 2011 Available online 16 July 2011
This study investigated the effects of land use change on infiltration and moisture content of soils in three land use types. A total of 81 soil and core samples (3 replications 3 treatments 3 profiles 3 soil depths) were used to determine parameters that may affect the infiltration properties of soils. The infiltration rate was measured in the field using double-ring infiltrometer with three replicates in each land use type. Results showed that infiltration rates were generally slow in the open grazed and cultivated lands suggesting high potential for runoff, limited percolation, and very low amount of water available in the soil profiles. The accumulated infiltration in soils under cultivation and open-grazing was smaller than the controlled grazing by approximately 57%. Similarly, cultivation and open-grazing had reduced the soil moisture content by 29 and 33%, respectively, compared to the controlled grazing. Surface soil compaction, higher dry bulk density and lower soil organic carbon, appeared to be the principal factors for the low infiltration capacity and moisture content of the soils. Therefore, dry land management, with long term tree cover and well regulated grazing system, is very crucial for the sustainable ecosystem functioning of this environmentally fragile area. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Infiltration capacity Deforestation Land conversion Semi-arid Soil properties Abernosa
1. Introduction Physical, chemical and biological properties of soils are often affected by land cover changes (Shukla et al., 2003; Wood and Blackburn, 1981). Infiltration capacity, which is one of the important soil hydraulic properties, is influenced by particle size distribution in the soil, land use type (Fu et al., 2000), vegetation factors (including plant and litter cover and type, and organic matter content in soil), topographical and climatic influences (e.g. Gifford and Hawkins, 1978; Jiménez et al., 2006; Tromble et al., 1974; Wood and Blackburn, 1981). Continued tillage and grazing practices have a marked effect on soil bulk density, porosity, aeration, structure and aggregate stability, infiltration, water storage, water transport characteristics and runoff (Broersma et al., 1995; Cameron et al., 1981; Tollner et al., 1990). Observations indicate that most tropical soils under forest vegetation have the ability to absorb water at rapid rates (Wahl et al., 2003; Wood, 1971). However, due to land conversion practices, especially from native vegetation to cultivation and grazing systems, has meant that many tropical soils have
* Corresponding author. Tel.: þ251 461 109900; fax: þ251 461 191 499. E-mail address: [email protected] (F. Yimer). 0140-1963/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jaridenv.2011.06.012
undergone important changes in their properties, including loss of organic matter and increase in bulk density (Lepsch et al., 1994; Yimer and Abdelkadir, 2010), and as a consequence, decreases in infiltration rate (Mwendera and Mohamed Saleem, 1997). In the Rift Valley area of Ethiopia, where the present study was carried out, information about soil hydraulic properties, when subjected to different land use changes, is generally lacking. Given the fact that the Rift Valley is an environmentally fragile area, any change that may affect the infiltration capacities of the soils will have direct bearings on the hydrological properties and runoff. The recent flooding problems in different dry land areas of Ethiopia and East Africa (Achameyeleh, 2003; Alemayehu, 2007) which have resulted in the loss of life and damage to crops, may be attributable to land use changes and the resultant poor water infiltration capacity of soils. The objective of this paper is to present differences in soil hydraulic properties as influenced by different land use types that may have a profound bearing in the water budget of the drought prone Rift Valley areas of Ethiopia. The specific questions are: (i) are there differences in infiltration capacity due to differences in land uses, (ii) are there changes in factors such as soil texture, bulk density and organic carbon content that may influence infiltration capacity under different land use types?
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A. Abdelkadir, F. Yimer / Journal of Arid Environments 75 (2011) 1067e1071
2. Materials and methods 2.1. Study site and experimental layout The study was carried out in the Abernosa Ranch (hereafter referred to as ‘study area’), located in the central part of the Main Rift Valley area of Ethiopia, 170 km south of Addis Ababa, situated at ca. 7 510 N, 38 420 E (Fig. 1). The altitudinal range of the study site is between 1650 and 1900 m above sea level. The area is classified as semiarid eco-climatic vegetation zone (Makin et al., 1975), characterized by a distinct dry season with erratic or no rain and a mean annual precipitation ranging between 400 and 600 mm (Fig. 2). The mean minimum and maximum annual temperatures are 13.5 C and 26.5 C, respectively (National Meteorological Services Agency of Ethiopia, 2009. Geologically, the Rift Valley area of Ethiopia, generally, consists of rocks of volcanic origin such as alkaline (basalts) and acidic (rhyolites and ignimbrites, pumice, volcanic ash) and tuff (Itanna, 2005; Zewdie, 2004), riverine and lacustrine alluvium (Makin et al., 1975). Few soil studies have been conducted in the study area so far. According to Makin et al. (1975) and Zewdie (2004), Haplic Andosols (Typic Haploxerands) are identified as the major soil types found around Ziway and the study area. The existence of a pronounced dry season with high temperature and high fluctuating annual precipitation has led to a xeric soil moisture regime. The soil is characterized as loam textured (Table 1) and is generally weak in its physical structure. The natural vegetation in the study area is characterized by acacia woodland. The main acacia species include Acacia tortolis, A. seyal and
Mean monthly total precipitation (mm)
160 140 120 100 80 60 40 20 0 Jan
Feb Mar
Apr
May Jun
Jul
Aug Sep
Oct
Nov Dec
Months Fig. 2. Mean monthly total rainfall distribution between 1981 and 2006. Source: Adami Tulu meteorological station (2009).
A. senegal. Included in the tree layer is also Balanites aegyptiaca, accompanied by semi-evergreen shrubs, such as Croton dichogamus, Harrisonia, Terminalia brownie, and Sclerocarya birre (Eshete, 1999). The ground cover of this woodland site is composed of natural grasses, which are dominated by species of Hyparrhenia, especially H. anthistiriodes and H. hirta. Other common species in the grassland include Aristodia adoensis, A. adscensionis, Cenchrus ciliaris and Chloris gayana (Makin et al., 1975).
Fig. 1. Location map of the study area.
A. Abdelkadir, F. Yimer / Journal of Arid Environments 75 (2011) 1067e1071
Types of land use practice Depth (cm) 0e20
20e40
40e60
Sand Silt Clay Sand Silt Clay Sand Silt Clay Cultivated land Open-grazing Controlled grazing
39 42 40
42 37 42
19 21 18
41 43 53
37 38 36
22 19 11
44 43 43
41 41 36
15 16 21
The Abernosa ranch was established in 1954 occupying 5000 ha. Currently, however, due to high human and livestock population pressure, its size has dwindled to an area of about 2200 ha. Agricultural practices are mix of farming with rain-fed cultivation and cattle rearing for subsistence. Due to the erratic nature of rainfall and poor fertility of the soils, agricultural production is very low (Eshete, 1999). Controlled grazing has been carried out within the paddocks that are close to the Head Quarters of the ranch for controlled grazing while the open-grazing is on communal grazing fields and on cultivated lands after harvest. There is considerable over stocking in the open/communal grazing areas and hence the limited grazing fields have less capacity to maintain the everincreasing livestock population of the surrounding community. While open-grazing land commonly maintains unlimited herd of cattle and small ruminants, the stocking in the controlled grazing is usually kept at light to moderate stocking with 1250 cattle and 650 goats in the 2200 ha land and with an average stocking rate of less than one animal per hectare. 2.2. Soil sampling and analysis The experimental site was divided into three area blocks, which served as replicates and each containing: controlled grazing area; open/communal grazing area and cultivated land as treatments. The controlled grazing area with its intact acacia vegetation was fenced and managed with rotational grazing for more than 50 years. Both the open-grazing and cultivated lands were under the ranch up until 1994 and later abandoned because of increased human and livestock intervention (Eshete, 1999). The locations of soil profiles in each of the treatments were randomly chosen within similar physiographic conditions such as landscape position and slope percents. A total of 81 soil samples (3 replications 3 treatments 3 profiles 3 soil depth layers e 0e0.2, 0.2e0.4 and 0.4e1.0 m) were collected during the cropping period for laboratory analyses. The soil samples were collected in January, 2007 from three excavated profiles randomly located in each of the three land use types at the above indicated varying depths for determination of dry bulk density, soil texture, organic carbon content and gravimetric soil moisture content. Gravimetric soil moisture content at sampling (in undisturbed soil cores) was determined using the standard procedures with oven drying to a constant weight at 105 C for 24 h. Dry bulk density was calculated from the oven-dry weight of identical core samples as used for soil moisture determination. Volumetric soil moisture content was determined from gravimetric moisture content and dry bulk density. Soil texture was determined using the hydrometer method (Landon, 1991).
rate was determined using double-ring infiltrometer (Bertrannd, 1965). One set of three double-rings was used for a total of three measurements at a time in each location. The obtained values were averaged to represent each location. The inner rings had diameters of 28, 30 and 32 cm and the outer rings 53, 55 and 57 cm. The rings were driven about 5 cm into the soil using a metal plate and sledgehammer. Water was filled to a level 20 cm above the soil surface. The rings were refilled to the 20 cm head level each time when the head approached 5 cm above the soil surface. Changes in water levels were recorded at the time increments 0, 1, 2, 5, 10, 15, 20, 30, 40 and 60 min for calculation of infiltration rates and cumulative infiltration values. Statistical differences were tested using two-way analysis of variance (ANOVA) following the general linear model (GLM) procedure of SPSS version 12.0.1 for Windows (Julie, 2001). Tukey’s Honest Significance Difference (HSD) test was used for mean separation when the analysis of variance showed statistically significant differences (p < 0.05). 3. Results and discussion Many of the considered soil properties showed significant differences with respect to land use type and soil depth. The soil textural analysis showed that the soil under all land use practices was loam (Table 1). This similarity in property indicates that the texture of soils was not the main reason for differences in soil water properties. Therefore, the differences in infiltration capacity and other soil water properties could be attributed singularly to the increased organic matter content and better land management under the controlled grazing. The soil organic carbon (Fig. 3) showed significantly higher in the controlled grazing than in either the cultivated or openly grazed lands (Yimer and Abdelkadir, 2010). Dry bulk density was consistently smaller in the controlled grazing than in the other land use types (Table 2). This may be related to the differences in surface soil compaction especially in moist soil conditions, discontinuity in macro pores, destruction of structure and aggregate stability that has resulted from over stocking and continued tillage in both the open-grazing and cultivated lands, respectively (Abdul-Megid et al., 1987; Broersma et al., 1995; Veldkamp, 1994). Moreover, the smaller organic carbon contents (1.24% and 1.38%), which are very low levels according to Landon (1991) may have also been responsible for the higher bulk
3.5
3.0
Soil organic carbon (%)
Table 1 Soil textural fractions (%) with respect to types of land use practice and soil depths.
1069
0-0.2m depth 0.2-0.4m depth 0.4-1.0m depth Overall average for each land use
c
2.5
c c
B
2.0
b b
b
A
1.5
a 1.0
b
A
a
a a
a
0.5
2.3. Infiltration measurements 0.0
In each of the land use types, three representative locations for infiltration measurements were randomly distributed (3 replicates 3 treatments 3 measurement points). The weather during all the measurements was sunny and dry. The infiltration
Cultivation
Open-grazing
Cont-grazing Overall (depth)
Land use/soil depth (m) Fig. 3. Soil organic carbon for different land use types and depths.
1070
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Table 2 Soil bulk density (B.d, g cm3) and gravimetric soil moisture (GSM) as influenced by land use and soil depths (mean SE). Overall means followed by the same letter(s) are not significantly different (p ¼ 0.05) as influenced by land use and soil depths. Variables
Depth (m)
Land use types Cultivation
Open-grazing
Cont. grazing
Overall
Bd
0.0e0.2 0.2e0.4 0.4e1.0 Overall 0.0e0.2 0.2e0.4 0.4e1.0 Overall
0.93 (0.03) 0.89 (0.01) 0.91 (0.02) 0.91 (0.01) b 7.87 (0.74) 4.29(0.72) 4.88 (0.74) 5.68 (0.47) a
0.96 (0.02) 0.89 (0.01) 0.90 (0.02) 0.91 (0.01) b 4.54 (0.71) 4.98 (0.74) 6.49 (0.70) 5.34 (0.35) a
0.84 (0.01) 0.82 (0.03) 0.87 (0.03) 0.85 (0.01) a 8.14 (0.70) 6.70 (0.69) 7.16 (0.72) 7.33 (0.55) b
0.91 (0. 02) b 0.87 (0. 01) a 0.90 (0.01) ab
Table 3 Summary of two-way ANOVA results for water infiltration rates (cm min-1) and cumulative infiltration (cm) under different land use types. Source of variations
Land use (LU) Time (T) LU T Error
Infiltration rates
6.86 (0.53) b 5.32 (0.45) a 6.17 (0.45) ab
compaction in the soil surface, especially grazing on moist soils, as is the case in this study(Taboada and Lavado, 1988) influencing the size, amount and distribution of pores, and thereby the rate of water infiltration and percolation (Abdul-Megid et al., 1987). The bulk density is a commonly used parameter to relate to soil porosity, giving an effective indication of the compaction level and infiltration capacity of soils (Taboada and Lavado, 1988; Tollner et al., 1990). In our study, the continued cultivation and unmanaged communal grazing also seem to have altered the organic carbon content. The smaller organic carbon content, possibly resulting in poorer soil structure, could partly be responsible for the higher dry bulk density and poor infiltration observed in cultivation and open-grazing. We physically observed, right after the infiltration measurements, that the maximum depth the water moved down the soil profiles under cultivation and grazing on average ranged from 0.10 to 0.15 m, whereas it penetrated more than 0.40 m under the controlled grazing. The extremely low amount and slow movement of water in the studied soils are good indicators of the effect of land changes from native woodlands coupled with poor land management practices on water movement in the soil system. The fact that the soil moisture content in the soil profiles was higher in the controlled grazing than in the other land use types could be attributed to the lower dry bulk density and higher soil organic carbon contents resulting in higher infiltration and water holding capacities, and thereby lower surface runoff of water. It appears that open-grazing and cultivation had negative effects on the soil structural stability and macropore continuity and thus on the rate of water entry and percolation in the soil system.
1.0
Cultivated land Open grazing Controlled grazing
0.8 -1
density observed in the open-grazing and cultivation since the correlation between the soil organic carbon and bulk density was highly significant in both land use types (0.88, p < 0.001) (Fig. 3). The bulk density was significantly higher in the top surface soil than in the sub surfaces (p < 0.05, Table 2) indicating the impact of surface soil compaction by animal trampling. The gravimetrical moisture content showed a significant variation with land use type (p ¼ 0.003); high in the controlled grazing than in the other land use types (Table 2). There was also a significant difference in soil moisture across all depths (p ¼ 0.046); higher in the top surface soil than in the layers below. This may have been due to the increased water holding capacity of soil that has resulted from increased soil organic matter in the upper soil layers. The infiltration rates, through out the measurement period, were significantly higher (p < 0.001,) for controlled grazing (0.45 cm h1 0.04) than for cultivation (0.20 cm h1 0.02) and open-grazing (0.19 cm h1 0.03) (Table 3 & Fig. 4). The cumulative infiltration also paralleled the infiltration rate (Fig. 5). Taking the hydraulic gradient into account, saturated hydraulic conductivity can roughly be estimated at <1 cm h1 across all land use types. According to rating classification of Landon (1991), soils under the control grazing and lash grass vegetation are high in their infiltration rate (15.6e52.2 cm h1). While soils under cultivation and open-grazing are in many cases slow in their infiltration rate. Slow and/or low infiltration rate may mean high surface water losses, limited deep percolation, and depressed yield (Landon, 1991). These observations would corroborate the view that the reduction in infiltration, rate caused by a change in land use type from woodland to cultivation or grazing, may be attributed to degradation of the surface horizon structure and compaction (Jiménez et al., 2006). Cultivation, for example, may shear and pulverize the soil, reduce the macropore space and produce a discontinuity in pore space between the cultivated surface and the subsurface soil (Abdul-Megid et al., 1987; Broersma et al., 1995), thereby reducing the infiltration rates and water percolation in the soil profile and, consequently, increasing potential for surface water runoff (Broersma et al., 1995). Trampling by cattle is reported to cause increases in bulk density by
Infiltration rate (cm min )
GSM
0.6
0.4
0.2
Cumulative infiltration
d.f.
MS
p
MS
p
2 8 16 54
0.585 0.171 0.009 0.003
<0.001 <0.001 0.003
20.774 3.671 0.377 0.026
<0.001 <0.001 <0.001
0.0 0
10
20
30
40
Time (min) Fig. 4. Infiltration rate curves for land use types.
50
60
A. Abdelkadir, F. Yimer / Journal of Arid Environments 75 (2011) 1067e1071
References
25 Cultivated land Open grazing Controlled grazing
Cumulative infiltration (cm)
20
15
10
5
0 0
10
20
1071
30
40
50
60
Time (min) Fig. 5. Cumulative infiltration curves for land use types.
4. Conclusion Soils under the three land use types in the study area showed differences in their organic carbon contents, soil texture, bulk densities, moisture contents and infiltration capacities. Assuming that the controlled grazing locations are appropriate ecological references, uncontrolled and open access grazing and cultivation practices had to a large extent decreased the infiltration rate (approximately by 56 and 58%, respectively). Surface soil compaction (shown in 13% increase in bulk densities under cultivation and open-grazing), due to tillage and animal trampling coupled with lower contents of soil organic carbon in cultivation, and opengrazing appeared to be the principal factors for the decline of infiltration and available soil moisture causing higher surface water runoff. Under such circumstances, occasional heavy rain storm events may easily give rise to destructive surface runoff forces resulting in loss of humans, animals and crops, as has been the case, in many instances, in the Rift Valley areas. Therefore, dry land management, aimed at rehabilitating the land through long term tree cover and well regulated grazing system to enhance the recovery of the physical and chemical soil qualities, is very crucial for the sustainable ecosystem functioning of this environmentally fragile area. Acknowledgments This study was financed by Wondo Genet College of Forestry and Natural Resources, Ethiopia. We would like to thank Dr. Enrique Vivoni, Associate Editor of the Journal of Arid Environments and the two anonymous reviewers for their valuable comments. We are grateful to the late Sileshi Kifle and Estifanos Mathewos for their assistance in the field, Tigneh Eshete for producing the location map of the study area; and to the staff members of the Abernosa Ranch Head Quarters and the surrounding communities for allowing the excavation of soil pits in their fields.
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