Non-climatic growth of the saline Lake Beseka, Main Ethiopian Rift
Introduction
Past lake levels are used as proxy for palaeoclimate in numerous studies (e.g., Kiage and Liu, 2006, Stager et al., 2005, Verschuren et al., 2000). For a meaningful interpretation, signals derived from bio- and lithostratigraphic data need to be corrected for influences other than climate variations. Our study aims to determine these factors taking Lake Beseka as an example for a rift lake in a tectonically active environment. The endorheic Lake Beseka, in the centre of the semi-arid rift valley of the northern Main Ethiopian Rift (nMER, cf. Fig. 1), has rapidly expanded over recent decades (e.g. Williams et al., 1981).
East Africa is a region of extremely complex meteorological and climatological phenomena and coupling-mechanisms, possibly one of the most complicated of the continent. The topography of the rift exerts a strong influence on microclimate, drainage systems, and local ecosystems (e.g. Seleshi and Zanke, 2004, Gissila et al., 2004). The lakes occupying the rift valley are either dominated by highland runoff and riverine delivery or by groundwater and local runoff (Brandt, 1982). In arid areas like the one exemplified in this study, a high fraction of lakes will be endorheic and thus more susceptible to slight perturbations of the water balance. According to Mercier et al. (2002) height changes of a lake's water column may be caused by
- (i)
Changes in surface pressure;
- (ii)
Wind-driven events (i.e. seiches) and tides;
- (iii)
Fluctuations in the volume of this column due to an alternating temperature or composition;
- (iv)
Modifications of the hydrologic cycle (i.e. recharge, outflow, evaporation).
Items (iii), (iv) will ultimately result in lake water volume changes.
The first three factors may be relevant for larger rift lakes, but cannot explain a water level increase of more than 4 m within less than three decades (Ayenew, 2004) of the comparatively small Lake Beseka. Cause (iv) can be directly triggered by non-climatic factors, for example tectonically induced alteration of flow patterns, changes in the heat budget due to geothermal effects, and artificial regulation. Anthropogenic influence on rift lake levels can also be exerted indirectly through water withdrawal (Ayenew, 2004) or an increased sediment input as a result of inadequate agricultural practices (Geremew, 2000; WWDSE, 2001). Lakes also fluctuate in accordance to climatic variations (Street and Grove, 1976; Chernet, 1982; Chalié and Gasse, 2002), especially when they are terminal (Ayenew, 2004). All these potential contributors to lake level change must be quantified in order to understand the hydrological setting of a drainage area.
Changes in the level of continental lakes can be assessed by measuring the absolute or relative elevations of the water level by continuous gauge records. However, in many settings it might be difficult to use standard hydrological survey data. A historical data record might not exist or not be accessible. Archives of satellite data covering up to 35 years can provide an inexpensive alternative or addition to incomplete ground measurements. RADAR altimeters were shown to yield reasonably accurate measurements of variations in water surface elevation (e.g., Mercier et al., 2002). However, they are only applicable for lakes situated along the satellite altimeter's ground track. In addition, the lakes need to be wider than 500 m and need to have a surface area of more than 100 km2 (Crétaux and Birkett, 2006). In this study we quantify the expansion of Lake Beseka over the past decades with multitemporal optical remote sensing data, a bathymetric model, and lake level records. Moreover, we apply hydrochemical analyses and temperature measurements to contribute to the study of Beseka's rapid lake level change.
Section snippets
Study area
The East African Rift System (EARS) cuts through the Ethiopian highlands (Fig. 1). One of its subsystems, the nMER, is among the few sites worldwide featuring a transition from continental to oceanic rifting (Ebinger and Casey, 2001). Present-day volcanic and seismic activity and faulting (Keir et al., 2006, Keranen et al., 2004, Kurz et al., 2007) are phenomena linked to this geologic process. Located within an active tectono-magmatic segment (e.g. Kurz et al., 2007), Lake Beseka's basin is
Quantification of the growth of Lake Beseka
Data from Landsat (180-054, 168-054: 1973, 1986, 1989, 2000, 2001, 2002) and ASTER (2000, 2001) were investigated in order to describe temporal variations in the surface area of Lake Beseka. To avoid obscuration by clouds, these data were acquired in the dry season. Due to a similar spectral signature of the littoral and recent basalt flows on the northern shore, the precise shoreline of Lake Beseka is difficult to map. Hence, two methods were applied on the satellite data to measure the
Results
The time series of satellite data documents that Lake Beseka's surface area quadrupled from 11.1 km2 in January 1973 to 39.5 km2 in February 2002 (Fig. 5). Aerial photographs from 1957 and 1964 show a lake area of about 3 km2 (Tessema, 1998). Putting aside seasonal fluctuations the lake has been steadily rising at an average rate of approximately 18 cm/yr (Fig. 6). In total the water level increased by about 8 m from 1976 to reach 955 m asl in September 2004.
The striking similarity between the shapes
Discussion and conclusions
We used several methods to examine the growth of Lake Beseka and establish the contribution of various potential water sources. The influence of neotectonic activity has rarely been taken into account. Previous approaches did consider the basin morphology only to a very limited extent (Tessema, 1998, MWR, 1998, Engida and Russom, 2004). In this study, we present the most detailed model of the lake floor yet considered. In combination with measurements from optical satellite data and digital
Acknowledgements
The realisation of this project would never have been possible without the logistical support of the staff of the Department of Biology (Addis Ababa University) and the kind assistance of K. Adem, T. Ayenew, H. Hagos, Z. Shiferaw and M. Umer (all Department of Geology, AAU). T. Eshete, M. Adem and L. Gurmu (Metehara Sugar Plantation) helped us with detailed information on the sugar plantations near Beseka. M. Buchwitz (TU Bergakademie Freiberg) supported us with detailed geologic knowledge of
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