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Chapter 3 : Methodology
Description of the Study Area
Location and general characteristics
Bilate River is one of the inland rivers of Ethiopia whose source is located at Gurage Mountains in central Ethiopia. The river drains to the northern watershed of the Lake Abaya-Chamo Drainage Basin. The basin forms part of the Main Ethiopian Rift which is part of an active rift system of the Great Rift Valley. The Bilate River watershed (BRW) covers an area of about 5625 square kilometres and is located in the southern Ethiopian Rift Valley and partly in the western Ethiopian Highlands. The Bilate River catchment includes part of the SNNPRS regional zones which include: Hadiya, Kambata Tambaro, Gurage, Silte, Wolaita, Sidama, and Alaba special woreda and small parts of the south-central Oromiya regional states. The Bilate River Watershed stretches across different ecological zones ranging from the mid-south-west Ethiopian Highlands to the lowlands of the Rift Valley. The altitude of the watershed ranges from 1,146 at Lake Abaya to 3,393 meters above sea level at Mt. Ambaricho. Geographically, its location, extends from 6º 36’N 38º00’E at Lake Abaya Wolaita Zone SNNPR to 8º05’N 38 º12’E at Gurage and Silte Zones border, SNNPR; and from7º18’N 46’E at Kambata Zone to7º12’N38º22’E Sidama Zone.
The Bilate River is the longest river in the Abaya Chamo Basin, with a length of about 255 km. It is also the only river which flows into Lake Abaya from the north. The main river flows straight southwards to Lake Abaya. Most of the perennial tributaries come from the western side, while the eastern side has mainly intermittent streams, and hence the water contribution from the eastern side is relatively low. From the regional location point of view, the watershed covers mainly the north-eastern part of the SNNPR and some parts of the south central Oromiya Regional States. Although the upper course lies in Silte and Gurage Zones, most of its tributaries with large volumes of water come from Kambata, Wolaita and Hadiya mountains respectively.
The population distribution of the watershed has two characteristics. The first one is maximum rural population density in the upper and middle course areas of the western part of the basin, while the second is the eastern part that is dominantly known for agro-pastoralism and relatively sparse population distribution. The high population density in the western part of the basin is related to the suitability of agro-climatic conditions, soil type and availability of water resources. In these areas maximum rural population density is the highest in Ethiopia, which exceeds 500 persons per square km (CSA, 2013).
The ethnic and cultural distribution within the watershed is highly diversified. There are more than eight ethnic groups dwelling within the watershed. Their impact on the environment depends on their cultural agricultural and land management practices. For example, the ethnic groups living at the lowest elevation of the Bilate River or northern part of Lake Abaya are more of agro-pastoralists. On the other hand, the people living in the western part of the watershed are known for their intensive and mixed farming culture.
Topography, geology, land use and soils
Topography
The watershed and topographic characteristics of BRW has been extracted from a Digital Elevation Model (DEM) with 30m resolution which was acquired from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER). The topography of the BRW varies from the lowlands of altitude 1,146 metre above sea level (m.a.s.l.) near Lake Abaya to the highlands with peak elevation of 3,393 metre above sea level towards the northern realm of the watershed.
According to Wemmer, (2004) the relative relief of the Bilate area is divided into three parts: the first is a relatively steep profile with relative relief of 0.29, and is separated into several convex partitions from the headwaters to approximately 61 km downstream; the second part is a convex profile from approximately 61 km to 193 km with relative relief of 0.05, and the third is a smooth and straight profile from 193 km to Lake Abaya with relative relief of 0.04.
Geology
BRW forms part of the geological foundations of East Africa known to be formed from a complex of metamorphic and volcanic rocks, which can be assigned to the era of the Precambrian and the Palaeozoic (Forch and Althoff, 2009). The relief of today‘s Ethiopia as well as the one of the Bilate River catchment is strongly influenced by the geological conditions and therefore structurally strongly dependent. Only limited amounts of information specific to the geology of the Bilate watershed itself are available. Geological information published by FAO (1998) provides the Oligocene-Miocene basalts dominating the Bilate River Catchment. These basalts can be found in the central area between Alaba Kulito and Bilate Tena, accompanied by Quaternary rhyolites and trachytes in the north and Holocene lacustrine sequences in the south of the catchment. Furthermore, on the southwest border, many subordinated Oligocene and Miocene volcanics can be found. Geophysical studies by Thiemann et al., (2004) show that the entire Main Ethiopian Rift is situated in a hot zone with a width of around 1000km, which displays low density and thickness while the Western Ethiopian Highlands are characterised by quaternary Rhyolites and Trachytes. Lake Abaya, which is in close proximity to the watershed, is typified by Holocene lake and swamp deposits. Physiographically the Bilate River basin is a tectonic valley. Along its length much of the valley is bounded by fault scarps or steep slopes on either side, as described in Tenalem et al. (2008) and therein cited references. The floor of the valley is mostly flat plain and appears to be in part a remnant of the depositions floor of an ancient large water basin. The study area is part of the western rift margin which is characterized by chain ridge, hills, deep and wide valleys of small and large streams, and narrow flat lands between the valleys having gentle slopes. It is due to the uplift and subsequent rifting phenomena that created localized and regional fractures and faults.
Soil
The soil types within the watershed can only be roughly estimated, due to the inadequate scale of the available soil data. The soil data used in this research was obtained from the Food and Agriculture Organization of the United Nations data base (FAO, 2003). According to the FAO Soil Map the soil depths in the study area is between 1.00 and 2.00m and the dominant soil types are Eutric Nitosols, Plinthic Ferralsols, Eutric Cambisols, Ochric Andosols and Haplic Xerosols. The occurrence of different soil types is related to geology, although the relief has a significant influence on the development of soil types in some areas.
The distribution of land-use systems, in the BRW, is linked to the prevailing climatic gradient and anthropogenic land use activities. Two main agricultural systems can be distinguished within the BRW. The land use system in the Western Ethiopian Highlands is dominated by small-scale subsistence agriculture while the Rift Valley has several different systems such as pasture and commercial farms. The northern part of the Rift valley is used for large-scale maize farming, which operates commercially; also, the private farmers in this area have larger fields. In the semi-arid part of the Rift Valley, vegetation is generally less dense than in the western highlands of the watershed, and trees only grow in riparian areas. Towards the south, the communal lands are predominantly utilized by pastoralists for extensive livestock production, mainly cattle. A few irrigated mechanized farms are found near the mouth of Bilate River around Bilate Tena (Dimtu), of which the state owned tobacco farm is the major one.
Land use data with 500 x 500 m spatial resolutions were obtained from the Ministry of Agriculture (MoA) which is derived from FAO 98 land use classification for Ethiopia and further reclassification was performed in the model used for simulation of hydrological processes. The land cover in the BRW is predominated by different types of agricultural land (87%), grass and rangeland 0.8%and the remaining mixed land cover, including plantation forest, shrub land and wetland, accounts for about12.2%.The forests are transformed to croplands and/or grazing areas. The change from forests to crop and pasture land is directly related to increasing human population especially in the rural areas.
Hydro-meteorological characteristics of the BRW
Rainfall in the BRW shows high spatial variability. The illustration of spatial variation in annual precipitation is shown in the contour map in Figure 3.5. The mean annual rainfall at the stations with complete records was summarized and then spatial interpolation was performed over the entire watershed. Ordinary Kriging interpolation with exponential variogram show the spatial variation of rainfall The mean annual rainfall in the BRW ranges between 721 and 1353mm which shows large spatial variability with a maximum rainfall as large as 1.87 times the minimum rainfall. Areas that belong to part of the Western Ethiopian Highlands show higher rainfall on an annual basis while the part of the watershed that belongs to the Ethiopian Rift Valley shows lower rainfall.
Data source and analyses
Rainfall
There are more than 18 rainfall observation stations in and around the Bilate River watershed (Fig 3.6). Time series rainfall data of these stations was obtained from the National Meteorological Agency (NMA) of Ethiopia. For the time period Jan/01/1984 to Dec/31/2013 rainfall stations with an amount of daily data above 75% were selected. From the available stations, only 11 stations satisfied the criteria. The selected stations with their mean annual value and the percent of daily missing rainfall data for the 30 years period under study are summarized in Table 3-1.
Some of the stations are located outside the boundary of the study area selected for hydrological simulation but still the area is located within the same hydro-meteorological setting, thus the stations that satisfy the criteria were used to fill the missing data by interpolation technique.
The appropriate daily rainfall, minimum and maximum temperature data was arranged by the day of a year (DOY) entry format. Data quality control was done by careful inspection of the completeness, and the spatial and temporal consistence of the records in the study area. The missing values of daily data were calculated and simulated by using INSTAT +v.3.36 first and second order Markov-chain simulation models (Stern et al., 2006). A Markov-based random model was established to generate simulated time series of daily precipitation, and the simulated statistic parameters demonstrated good consistency with their observational equivalents (Yuguo et al., 2010).
The inbuilt Markov chain model of InStat software performs the simulation of the missing data in two steps. First, it determines the probability of dry and wet weather from the input weather data of the recorded dates, the model depicts rainfall or no rainfall dates. If there is rainfall, then it comes to the second step which is simulating the precipitation amounts.
Some features of the observed daily rainfall at five selected stations in the BRW are shown in Figure 3.7. These stations were chosen based on their completeness, which have time series data of more than 95% for the period January 1984 to December 2013 and they all are located within BRW.
The box plots shown in Figure 3.7 were built for the rainy days of the corresponding months; these consider only days with rainfall amount of more than zero. This was done because if days with zero rainfall amounts are included, almost all the quartiles of the box plot will become zero except for the higher quartiles. As we can see from the legend of the box plot, the top horizontal line of the box plot indicates the 90% quartile while the bottom horizontal line indicates the 10% quartile. The edges of the box represents the inter quartile range (IQR) which is the difference between the 75% quartile from the top and the 25% quartile from the bottom. The median line is represented by the broken horizontal line inside the box. In all box plots the median value is closer to the 25% quartile than to the 75% quartile, which shows the skewed distribution of the rainfall.
Temperature data
Compared to rainfall data, there is small amount of time series data for minimum and maximum temperatures (Tmin and Tmax) in the watershed due to the large number of stations that have many missing values and uneven spatial and temporal distributions. There are 18 gauging stations in and around the BRW. However, only a few stations have a data record for acceptable limit of time series for minimum and maximum daily temperature. From the 18 gauging stations of the NMA, only six of them have recorded data above 70%. The summary of the stations and their daily data availability is shown in Table 3.2 and their spatial distribution is shown in Figure 3.6 along with the rainfall gauging stations.
The inter-annual surface temperature analysis in the BRW is made by using time series data of five stations for the period of Jan/01/1984 to Dec/31/2031. The five stations were selected based on the completeness of data record. These stations are Alaba Kulito, Angacha, Bilate, Hosana and Wulbareg.
The inter-annual variability of daily maximum temperature is shown in figure 3.8 with the 95% confidence of the mean values. The pattern of daily maximum temperature is more or less the same in all the stations, where the highest values of the daily maximum temperature are observed in February and March which is the dry period of the area, with the exception of Wulbareg station. The lowest value of maximum temperature is recorded in months of July and August.
Figure 3.9 shows the inter-annual variability of the daily minimum temperature averaged over each month. Unlike the daily maximum temperature, it is not easy to draw a trend of daily minimum temperature for the selected stations. Relatively, Hosana station shows the smallest minimum temperature value in average, whereas Bilate shows the highest value.
DEDICATION
DECLARATION
ACKNOWLEDGEMENTS
ABBREVIATIONS
ABSTRACT
Chapter 1: Introduction
1.1 General background
1.2 Statement of the problem
1.3 Objectives of the research
1.4 Research questions
1.5 Outline of the thesis
Chapter 2 : Review of Literature
2.1 Climate Change
2.2 Climate variability
2.3 Evapotranspiration
2.4 Climate models in climate change studies
2.5 Downscaling of climate models
2.6 Watershed modeling
2.7 Local adaptations to climate change
Chapter 3 : Methodology
3.1 Description of the Study Area
3.2 Data source and analyses
3.3 Methods
Chapter 4 : Result and Discussion
4.1 Temporal and Spatial Variability of Rainfall and Evapotranspiration in the Bilate River Watershed, Southern Ethiopia
4.2 Statistical Downscaling (Delta Method) of Precipitation and Temperature in Bilate
Watershed
4.3 Response of the stream flow level of the Bilate watershed to climate model outputs
4.4 Local Perceptions and Adaptation to Climate Variability and Change in the Bilate River
Watershed
Chapter 5 : Conclusions and Recommendations
5.1 Conclusions
5.2 Recommendations
References
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Local Adaptation Practices in Response to Climate Change in the Bilate River Basin, Southern Ethiopia
