Lakes & Reservoirs: Research and Management 2012 17: 131–141 Analysing the extent (size and shape) of Lake Basaka expansion (Main Ethiopian Rift Valley) using remote sensing and GIS M. O. Dinka* School of Water Resources and Environmental Engineering, Institute of Technology, Haramaya University, Dire Dawa, Ethiopia Abstract This study attempted to estimate the extent of the expansion of the area of Lake Basaka over the past 50 years (1960 to 2010), using LANDSAT images, field observations, local information and topographic maps. The analysis revealed that the lake has exhibited a dramatic expansion over the past five decades. An increase in the lake water level by 7.6 m over this period resulted in flooding about 45.8 km2 of surrounding areas and an incremental lake volume of about 280 Mm3. About 70% of the lake expansion was observed in the period between the 1970s and 1990s. This phenomenon coincides with the periods of remarkable land use ⁄ cover changes in the region. Furthermore, the beginning of the lake expansion coincided with the introduction of irrigated agriculture in the region, and construction of Koka Dam in the mid-1960s in the upper Awash River Basin. This study suggests that the observed expansion trend, if it continues unabated, could result in certain socio-economic and environmental consequences in the region in particular, and to Ethiopia in general. Groundwater inundation or salty water intrusion, for example, might occur in the area in the near future, thereby affect- ing the sustainability of regional irrigated agriculture. The lake has the potential to inundate the surrounding region (Matahara Sugar Estate; towns of Fantalle and Matahara) and might connect with the Awash River during the next 10–15 years. This would affect downstream irrigation developments in the Awash Basin and the livelihoods of people that depend on the basin’s water resources. The overall findings of this study emphasize the need to adopt mitigation mea- sures before the lake expansion results in irreversible damage to the region or the basin. Key words Awash Basin, Basaka Lake, ERDAS Imagine, expansion extent, indices, Landsat image. INTRODUCTION The expansion of Lake Basaka is a problem because The surface area of Lake Basaka, unlike most of the of its poor water quality. The lake water is saline ) other closed basin Main Ethiopian Rift (MER) Valley (EC6.3 dS m 1), sodic (SAR300) or alkaline (pH9.6), Lakes, is expanding at a rapid rate (Tessema 1998; Abebe therefore being not usable for irrigation and drink- 2000; Alemayehu et al. 2006; Ayenew 2007; Belay 2009; ing ⁄ consumption (Olumana 2010). The drastic expansion Goerner et al. 2009; Olumana et al. 2009; Olumana 2010). of the lake with its poor quality is expected to affect the The total surface area of the lake was about 3 km2 in surface- and groundwater dynamics and soil properties of 1957 (Abebe 2000; Alemayehu et al. 2006), but is esti- the region. This is especially serious in regard to the mated to be 43 km2 in 2008 (Olumana et al. 2009). The sustainability of MSE and nearby villages (Matahara lake was essentially a small (surface) pond before the and Fantalle) and the Awash basin irrigation development establishment of Matahara Sugar Estate (MSE) in the in general. The expansion of the lake has already 1960s (Oral communication, May 2008). seriously affected the socio-economics of the indigenous (Karayyuu) people (Gebre 2004, 2009; Elias 2008) and might threaten the economic welfare of the society, *Corresponding author. Email: [email protected] depending on the water resources of the downstream Accepted for publication 9 February 2012. basin. Figure 1 illustrates some of the effects of the lake Ó 2012 The Author Doi: 10.1111/j.1440-1770.2012.00500.x Journal compilation Ó 2012 Blackwell Publishing Asia Pty Ltd 132 M. O. Dinka (a) (b) (c) (d) Fig. 1. Effects of the Basaka Lake expansion on: (a) irrigation development (MSE); (b) structures (railway and road); (c) pastoralism (Karrayyu people); and (d) ecosystem and grazing lands. expansion on the environment, socio-economic situation, images and ancillary data (topomaps, field observations, productivity, etc. local information). The specific objectives of this study Identification of the differential vulnerability of the were to: (i) estimate the spatio-temporal expansion extent nearby areas to future inundation hazards because of the of Lake Basaka; and (ii) predict possible future expansion lake expansion is extremely important, in order that of the lake, assuming that past trends will continue. The timely actions can be taken by the sugar estates, and study results provide valuable information for national other responsible institutions and decision-makers. Thus, and international stakeholders (beneficiaries, researchers, quantification of the spatio-temporal expansion of the lake irrigation managers, decision-makers, etc.). is extremely important to optimize and balance the inter- ests of economical and ecological land uses. Different METHODOLOGY studies (local and international) were conducted on the Study area expansion of Lake Basaka. Nevertheless, no profound sci- Lake Basaka is located in the Middle Awash Basin, Fant- entific investigation has yet been made regarding the alle Woreda of Oromiya Region (8.5°1.5¢N, 39°51.5¢E, geometry (size and shape) of the lake from its formation 950 m a.s.l.), about 200 km south-east of the Ethiopia’s (1950s) to the present time (2010). The lake is now get- capital city, Addis Ababa, along the Addis–Dire Dawa– ting more attention because of its dramatic change in size Djibouti or Addis–Harar–Jigjjiga highway road (Fig. 2). and shape, and the accompanying effects on the region’s Matahara is situated on a nearly level plain stretching economy and environment. along the course of Awash River and surrounded by This study was initiated to assess the expansion extent mountain chains with variable elevations in the south, (size, shape) of the lake from time series of LANDSAT south-east, south-west and north-west directions (Fig. 2), Fig. 2. The study area: location (left) and topographic view (TIN) extracted from digital elevation model (right). Ó 2012 The Author Journal compilation Ó 2012 Blackwell Publishing Asia Pty Ltd Analysing the extent (size and shape) of Lake Basaka expansion 133 being extensions of the Chercher (Hararghe) Highlands Table 1. Details of satellite data used in the study (Abejehu 1993). The area is delineated by Mount Fentalle (volcanic crater) and undulating plateaus in the north Cloud Resolution Acquisition and north-east directions, respectively. Awash National Sensor Path ⁄ Row Cover (%) (pixel size) Time Park also is situated in the flat plain of Matahara area Landsat–MSS 180 ⁄ 54 0 57 * 57 m Jan, 1973 (east of Lake Basaka). The area can be viewed physio- Landsat–TM 168 ⁄ 54 0 30 * 30 m Jan, 1986 graphically as a topographic depression, where run-off + Landsat–ETM 168 ⁄ 54 0 28.5 * 28.5 m Dec, 2000 and alluvial sediments overflow from the surrounding + Landsat–ETM 186 ⁄ 54 0 28.5 * 28.5 m Feb, 2008 catchment areas (Mohr 1971; cit. Olumana 2010). + Landsat–ETM 186 ⁄ 54 9 28.5 * 28.5 m Oct, 2010 Being located in the upper part of MER, in the central rift valley region, Matahara is vulnerable to occurrences of tectonic and volcanic activities. As the result, the area Mapping Authority, and detailed topographics for some is characterized by features of past and recent volcanic parts of the lake catchment were obtained from MSE. events, examples being vast lava extrusions at the foot The DEM (90 m resolution) was downloaded from the slope of Fentalle Mountain (volcanic crater), dots of NASA Shuttle Radar Topography Mission (SRTM), via extensive scoriaceous hills in the locality (Mohr 1971), the National Map Seamless Data Distribution System site lava flows produced by the eruption in the northern part (http://www2.jpl.nasa.gov/srtm/). of the lake in the 1980s (Halcrow 1978), and the presence Supplemental information on lake conditions, land of a number of hot springs flowing into the lake (Olumana use ⁄ cover (LUC) conditions, soil conditions and topo- 2010). The area is bordered by older volcanoes and a rift graphic features was obtained from field observation. Fur- margin in the east, by young Quaternary complexes of thermore, information on the present and past conditions Fantalle Crater in the north and by Kone on the west of the lake was obtained from interviews with the elders side. The formation of common rock types in the area is of the local indigenous peoples. The DEM resolution was the product of the recent volcano complexes, along with increased to 15 m by the resembling technique available fissural basalt, rhyolite, alluvial and lacustrine sediments in GIS (ArcInfo v 9.2: ESRI (2006), ArcGIS 9.2. Redlands, (Mohr 1971; Belay 2009). California, USA) and subsequently processed for the The area is characterized by a bimodal and erratic study area and the surrounding features. rainfall distribution pattern, with the major rainy season being between July and September. Occasional minor Satellite image processing rains can occur between March and May. Analysis of the The LANDSAT images were registered with the topo- area’s long-year-average (LYA) weather data (1966–2010) graphic map of the project area by matching some identi- indicates that the mean annual rainfall and temperature fiable features (e.g. road crossings, railways, rivers, were about 543.7 mm and 26.5 °C, respectively. The LYA irrigation canals, bridges, etc.) on both the base map and pan evaporation for the area was 6.92 mm day)1, with the the satellite images. The images were then imported and reference ET being about 4.65 mm day)1. The general processed with ERDAS Imagine 9.1 (ERDAS 2006) and climate of the area is semi-arid.