Natural and anthropogenic impacts on a freshwater wetland, Lake Bogoria, Kenya
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Location
Continent: Africa
Country: Kenya
State/Province:
City/Town: Loboi Swamp, Lake Bogoria
UTM coordinates and datum: none
Setting
Climate Setting: Semi-Arid
Tectonic setting: Continental Rift
Type: Process
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Description
Introduction
Wetlands are an important water resource in arid regions (<400 mm annual precipitation) and may also provide grazing opportunities for farm animals. Permanent freshwater wetlands are also important habitats for a wide range of other animals (insects, mollusks, birds, reptiles, and mammals) and aquatic plants, but humans are having increasingly negative impact on these delicate natural systems. Negative impacts include pumping or draining water for irrigation of farmland, as well as poor land use practices that increase influx of sediment, fertilizers, pesticides and animal waste to the wetland (Homewood and Lewis 1987).
A wetland is an area of the land surface that is permanently or seasonally saturated (Mitsch and Gosselink 2000). Localized wetlands (ranging in size from a tens of m2 to a few km2) can occur even in arid regions as long as there is a sustained natural supply of water (Thompson and Hamilton 1983). Long-lived wetlands in arid areas are usually linked to geological structures such as faults or bedrock fracture systems that provide conduits for groundwater flow (Ashley et al. 2004). Water in the subsurface (groundwater) is inherently protected from evaporation and thus buffered against changes in annual rainfall. Water moves slowly (m/yr) through bedrock under the influence of gravity. The local geology (structure and lithology) determines the path and locations where flow discharges onto the surface.
Geology
Loboi Swamp is a 1.5 km2 freshwater wetland (Figure 1a) situated north of the equator (0°22'N/36°03'E) in the East African (Gregory) Rift Valley just north of Lake Bogoria (Figure 1b). The rift valley is an elongate depression created by extensional tectonics within the African plate. Local relief is 1000 meters between valley bottom and top of bordering high relief fault blocks.
Geomorphology
Loboi Swamp is located at ~998 m elevation on the low-gradient Loboi Plain that is ~ 22 km long by ~20 km wide. The swamp lies in the N-S axial depression of the rift that today separates Lake Bogoria (~970 m elevation) from Lake Baringo (~990 m elevation) (Figure 2). Loboi Swamp, just northwest of the drainage divide (~999 m), acts as a large 'sponge' by retaining water and moderating its flow. All water originates as rainfall, but may take different paths, thus the wetland may have a number of water sources. Loboi is fed by spring water that flows from a fault on swamp's west side and it is also sourced by water from the north-flowing Loboi River (S). The Loboi River (S) flows adjacent to but not into the swamp and terminates near the Loboi Plain drainage divide. Water draining the swamp is named Loboi River (N) and it flows northward into Lake Baringo (Figure 3).
Hydrology
The region is semi-arid receiving 700 mm year-1 precipitation (25-year average) on the rift valley floor and ~1300 mm year-1 on the local highlands; potential evaporation exceeds 2500 mm year-1 and is 3 to 4 times the rainfall (Figure 4) (LaVigne and Ashley 2002). With this negative hydrological budget, groundwater sources are very important to people living there. The long term record of rainfall shows a relatively regular variation (5-7 years) in annual rainfall that corresponds to globally recognized events of El Niño (wet periods) and La Nina (dry periods)(NOAA 2003)(Figure 4A). High precipitation levels during 1977-1979, 1988-1990, and 1997-1998 in the Bogoria area coincided with El Niño events worldwide. The annual rainfall pattern is dominated by monsoons during spring and fall whereas local storms in the late afternoon occur in the summer. Extended dry periods are common in the winter (Nicholson 1996)(Figure 4B).
Anthropogenic impacts
Archaeological records in the area indicate that human occupation extends back several thousand years (Farrand et al. 1976). The most recent immigration of people to the region was ~300-250 years ago (Thom and Martyn 1983). These early settlers came from adjacent highlands and lived mainly by herding goats and sheep. More recently land use changes, such as the use of wood for building material and fuel, as well as browsing and grazing by goats, cows and donkeys have led to a general degradation of the landscape (Snelder and Bryan 1995). With growth in population during last 50 years there has been an increase in farming and a drainage ditch was dug in 1970 to siphon off water to agricultural fields to the northwest of the wetland (Figure 3). Comparison of the areas of the Loboi Swamp between 1969 (aerial photographs) and 2002 reveals a dramatic reduction of about 60% (Figure 3) (Ashley et al. 2004).
Natural impacts
The shrinking of the wetland has been assumed to be due mainly to the use of water for irrigation (Fig. 3). Although a measurement of water volume (effluent discharge) flowing out of the swamp in the irrigation ditch (0.035 m s-1) is only 10% (0.35 m s-1) of the spring water (influent discharge) flowing into the swamp. Thus, there must be other factors affecting the water budget of the swamp. There has been an overall decrease in rainfall since 1960 and the water loss by evapotranspiration exceeds local precipitation by a factor of four. With no change in the current hydrological budget, the swamp is likely to continue to decrease in size. However, another factor in this fragile system is the fluvial contribution (Figure 5). The drainage divide between the lakes is located just ~ 3km north of Lake Bogoria. Surface water north of the divide flows into Lake Baringo and drainage south of the divide flows into Lake Bogoria. During a period of elevated discharge during an El Niño period (2002) similar to the periods shown in Figure 4, the Loboi River (S) avulsed (catastrophically changed position) and flowed south into Lake Bogoria. This avulsion reduced the recharge to the swamp even more (Harper et al. 2003). However, the channel was rerouted northward again by a government-sponsored engineering project in 2004.
The Loboi Swamp is a fragile environment and can easily be destroyed by both natural and anthropogenic factors. The Loboi community is currently struggling to balance their deeply rooted cultural tradition of pastoralism, a rapidly increasing population, increasing agriculture that requires irrigation, and a deteriorating landscape.
Wetlands are an important water resource in arid regions (<400 mm annual precipitation) and may also provide grazing opportunities for farm animals. Permanent freshwater wetlands are also important habitats for a wide range of other animals (insects, mollusks, birds, reptiles, and mammals) and aquatic plants, but humans are having increasingly negative impact on these delicate natural systems. Negative impacts include pumping or draining water for irrigation of farmland, as well as poor land use practices that increase influx of sediment, fertilizers, pesticides and animal waste to the wetland (Homewood and Lewis 1987).
A wetland is an area of the land surface that is permanently or seasonally saturated (Mitsch and Gosselink 2000). Localized wetlands (ranging in size from a tens of m2 to a few km2) can occur even in arid regions as long as there is a sustained natural supply of water (Thompson and Hamilton 1983). Long-lived wetlands in arid areas are usually linked to geological structures such as faults or bedrock fracture systems that provide conduits for groundwater flow (Ashley et al. 2004). Water in the subsurface (groundwater) is inherently protected from evaporation and thus buffered against changes in annual rainfall. Water moves slowly (m/yr) through bedrock under the influence of gravity. The local geology (structure and lithology) determines the path and locations where flow discharges onto the surface.
Geology
Loboi Swamp is a 1.5 km2 freshwater wetland (Figure 1a) situated north of the equator (0°22'N/36°03'E) in the East African (Gregory) Rift Valley just north of Lake Bogoria (Figure 1b). The rift valley is an elongate depression created by extensional tectonics within the African plate. Local relief is 1000 meters between valley bottom and top of bordering high relief fault blocks.
Geomorphology
Loboi Swamp is located at ~998 m elevation on the low-gradient Loboi Plain that is ~ 22 km long by ~20 km wide. The swamp lies in the N-S axial depression of the rift that today separates Lake Bogoria (~970 m elevation) from Lake Baringo (~990 m elevation) (Figure 2). Loboi Swamp, just northwest of the drainage divide (~999 m), acts as a large 'sponge' by retaining water and moderating its flow. All water originates as rainfall, but may take different paths, thus the wetland may have a number of water sources. Loboi is fed by spring water that flows from a fault on swamp's west side and it is also sourced by water from the north-flowing Loboi River (S). The Loboi River (S) flows adjacent to but not into the swamp and terminates near the Loboi Plain drainage divide. Water draining the swamp is named Loboi River (N) and it flows northward into Lake Baringo (Figure 3).
Hydrology
The region is semi-arid receiving 700 mm year-1 precipitation (25-year average) on the rift valley floor and ~1300 mm year-1 on the local highlands; potential evaporation exceeds 2500 mm year-1 and is 3 to 4 times the rainfall (Figure 4) (LaVigne and Ashley 2002). With this negative hydrological budget, groundwater sources are very important to people living there. The long term record of rainfall shows a relatively regular variation (5-7 years) in annual rainfall that corresponds to globally recognized events of El Niño (wet periods) and La Nina (dry periods)(NOAA 2003)(Figure 4A). High precipitation levels during 1977-1979, 1988-1990, and 1997-1998 in the Bogoria area coincided with El Niño events worldwide. The annual rainfall pattern is dominated by monsoons during spring and fall whereas local storms in the late afternoon occur in the summer. Extended dry periods are common in the winter (Nicholson 1996)(Figure 4B).
Anthropogenic impacts
Archaeological records in the area indicate that human occupation extends back several thousand years (Farrand et al. 1976). The most recent immigration of people to the region was ~300-250 years ago (Thom and Martyn 1983). These early settlers came from adjacent highlands and lived mainly by herding goats and sheep. More recently land use changes, such as the use of wood for building material and fuel, as well as browsing and grazing by goats, cows and donkeys have led to a general degradation of the landscape (Snelder and Bryan 1995). With growth in population during last 50 years there has been an increase in farming and a drainage ditch was dug in 1970 to siphon off water to agricultural fields to the northwest of the wetland (Figure 3). Comparison of the areas of the Loboi Swamp between 1969 (aerial photographs) and 2002 reveals a dramatic reduction of about 60% (Figure 3) (Ashley et al. 2004).
Natural impacts
The shrinking of the wetland has been assumed to be due mainly to the use of water for irrigation (Fig. 3). Although a measurement of water volume (effluent discharge) flowing out of the swamp in the irrigation ditch (0.035 m s-1) is only 10% (0.35 m s-1) of the spring water (influent discharge) flowing into the swamp. Thus, there must be other factors affecting the water budget of the swamp. There has been an overall decrease in rainfall since 1960 and the water loss by evapotranspiration exceeds local precipitation by a factor of four. With no change in the current hydrological budget, the swamp is likely to continue to decrease in size. However, another factor in this fragile system is the fluvial contribution (Figure 5). The drainage divide between the lakes is located just ~ 3km north of Lake Bogoria. Surface water north of the divide flows into Lake Baringo and drainage south of the divide flows into Lake Bogoria. During a period of elevated discharge during an El Niño period (2002) similar to the periods shown in Figure 4, the Loboi River (S) avulsed (catastrophically changed position) and flowed south into Lake Bogoria. This avulsion reduced the recharge to the swamp even more (Harper et al. 2003). However, the channel was rerouted northward again by a government-sponsored engineering project in 2004.
The Loboi Swamp is a fragile environment and can easily be destroyed by both natural and anthropogenic factors. The Loboi community is currently struggling to balance their deeply rooted cultural tradition of pastoralism, a rapidly increasing population, increasing agriculture that requires irrigation, and a deteriorating landscape.
Associated References
- ASHLEY, G.M., MAITIMA MWORIA, J.M., MUASYA, A.M., OWEN, R.B., DRIESE, S.G., HOVER, V.C., RENAUT, R.W., GOMAN, M.F., MATHAI, S., and BLATT, S.H., 2004, Sedimentation and evolution of a freshwater wetland in a semi-arid environment, Loboi Swamp, Kenya, East Africa: Sedimentology, v. 51, p. 1-21.
- FARRAND, W.R., REDDING, R.W., WOLPOFF, M.H., and WRIGHT, H.T., 1976, An archeological investigation on the Loboi Plain, Baringo District, Kenya, Technical Report: Ann Arbor, Museum of Anthropology, University of Michigan, p. 88.
- HARPER, D.M., BROOKS CHILDRESS, R., HARPER, M.M., BOAR, R.R., and OTHERS, A., 2003, Aquatic biodiversity and saline lakes: Lake Bogoria National Reserve, Kenya: Hydrobiologia, v. 500, p. 259–276.
- HOMEWOOD, K., and LEWIS, J., 1987, Impact of drought on pastoral livestock in Baringo, Kenya 1983-85: Journal of Applied Ecology, v. 24, p. 615-631.
- LAVIGNE, M., and ASHLEY, G.M., 2002, Climatology and rainfall patterns: Lake Bogoria National Reserve (1976-2001), in Sciences, G., ed.: Piscataway, NJ, Rutgers University, p. 48.
- MITSCH, W.J., and GOSSELINK, J.G., 2000, Wetlands: New York, John Wiley & Sons, Inc.
- NICHOLSON, S., 1996, A review of climate dynamics and climate variability in eastern Africa in Johnson, T.C., and Odada, E.O., eds., The limnology, climatology and paleoclimatology of the East African lakes: Amsterdam, Gordon and Breach Publishers, p. 25-56.
- NOAA, 2003, http://www.cpc.ncep.NOAA.gov.
- SNELDER, D.J., and BRYAN, R.B., 1995, The use of rainfall simulation tests to assess the Influence of vegetation density on soil loss in degraded rangelands in the Baringo District, Kenya: Catena, v. 25, p. 105-116.
- THOM, D.J., and MARTYN, J.L., 1983, Ecology and production in Baringo-Kerio Valley, Kenya: Geographical Review, v. 73, p. 15-29.
- THOMPSON, K., and HAMILTON, A.C., 1983, Peatlands and swamps of the African continent, in Gore, A.J.P., ed., Ecosystems of the World, Mires: Swamp, Bog, Fen and Moor, Regional Studies: Amsterdam, Elsevier, p. 331-373.