This study identified the importance of watershed attributes for water resource management using ArcGIS software, ASTER DEM and satellite images for the Chelekot micro-watershed, Tigray, Ethiopia. The study also evaluate the different hydrological parameters which are significant for the water resource management within the micro-watershed and finds the alternative solutions for water harvesting in the study area through the introduction of suitable soil and water conservation structures based on the finding. Principal watershed attributes including drainage pattern, topographic parameters, land use types, and soil types were evaluated and interpreted for the study micro-watershed. ArcGIS software was used for the computation, delineation of the boundary and morphometric analysis of the micro-watershed using topographical maps and ASTER DEM data. Results indicate that the micro-watershed has classified as a dendritic pattern with stream orders ranging from first to fifth order. The micro-watershed has homogeneity in texture and lack of structural control of surface flow. The drainage density is medium which indicates the area contains soils with medium infiltration rates and moderate relief. Drainage texture, stream frequency and the form factor of the micro-watershed are 4.1, 1.7 and 0.4 respectively. The bifurcation ratio of the micro-watershed ranges from 1 to 4.5 and the elongation ratio is 0.7 which reveals that the micro-watershed belongs to the less elongated shaped micro-watershed category. The mean bifurcation ratio of the whole micro-watershed is 3.3 indicating that the drainage pattern is not greatly influenced by geological structures. The micro-watershed land covers includes: cultivated land (75.8%), settlement and open land (10.5%), shrubs and plantation (13.2%), and water body (0.4%). The major soil types are Vertisol (58%), Camisole (32%), Regosol (9.5%) and Luvisol (0.7%). The textural classes are clay (5%), silty clay (22%), clay loam (17%), sandy loam (21%) and loam (35%) based on the soil textural map of the micro-watershed. Our results revealed that using GIS and ASTER DEM data based watershed morphometric analysis and hydrological evaluation at watershed scale is more applied and precise compared to other available techniques .
In semi-arid and arid areas of Africa, urban expansion, irrigation projects, and climate change, along with insufficient and unpredictable pattern of the rainfall pose pressure on existing water resources. Surface and ground water resources are inadequate to meet the crop water requirement and water for domestic consumption due to a rapid population growth and the demand for more water. The demand for water has increased over the years making the assessment of the quantity and the quality of water resources and its optimal utilization most critical. There is an urgent need for the evaluation of water resources as water plays a primary role in the sustainability of livelihoods and regional economy. Water management is the primary safeguard against drought and plays a central role in achieving food security at the local, national and global levels. Ever-growing population and urbanization are leading to over-utilization of water resources, thus exerting pressure on the limited civic amenities many of which are on the brink of collapse [
Identification of ground features such as geological structures, geomorphic features and their link to hydrological characteristics may serve as direct or indirect indicators of ground and surface water potential of an area. The geomorphic conditions are essential prerequisites in understanding the water bearing characteristics of hard rocks and drainages patterns. The function of rocks types and geologic structures in the development of stream networks can be better understood by studying the nature and type of drainage patterns and by a quantitative morphometric analysis. The watershed’s morphometric parameters are reflective of its hydrological response to a considerable extent and can be helpful in synthesizing its hydrological behaviour and water balance. A quantitative morphometric characterization and analysis of a watershed is considered to be the most satisfactory method for proper watershed management planning and implementation of soil and water conservation measures. The characterization of geomorphic attribute enables us to understand the relationship among different aspects of the basin’s drainage pattern and also enables a comparative evaluation of different drainage basins developed in various geologic and climatic regimes [
Remote sensing data, along with increased resolution from satellite imagery, makes these technologies appear poised to make a large impact on land resource management initiatives involved in monitoring of land use and land cove (LULC) mapping and change detection. These tools are enabling researcher to determine varying spatial ranges in semi-arid regions which are undergoing severe moisture stresses due to the combined effects of rainfall variability, climate change and growing population [
Surface hydrological indications are promising scientific tools for assessment and management of water resources. Drainage morphometric analyses are a prerequisite for selection of water recharge sites, watershed modeling, runoff modeling, watershed delineation, groundwater prospect mapping and geotechnical investigation [
Digital elevation models (DEMs), such as from the ASTER, GDEM and other types of models were used to extract diverse geomorphological parameters of drainage basins, including drainage networks, catchment divides, slope gradient and aspect, and upstream flow contributing areas [
The study was conducted in warm temperate and sub-humid area of Tigray regional state [
The study was conducted in Enderta woreda, South-East zone of Tigray Regional State, which lies between 13˚ - 14˚N latitude and 39˚ - 40˚30" East longitudes. Enderta woreda consists of two main towns such as Mekelle and Kwiha. In addition there are several small towns including: Aynalem, May Keyah, May Mekden, and Aragure. Prominent villages in this district are: Chelekot, Debri, Kokolo, Adi Negoda and Alem. The study was carried out in Chelekot micro-watershed, where the lift irrigation project was proposed. This study site was selected to investigate the various characteristics of the micro-watershed and to understand the water resource potential of the area. In addition, the main aim of the project was to improve the livelihood of the farmers by introducing irrigation projects.
The Chelekot valley is located about 16 km west of Mekelle town. The valley is faulted and the rock beds are disturbed with a lithological composition of shale, marl dolerite intercalation and alluvium sediments. The mean annual temperature of this area ranges between 16˚C - 20˚C. The average annual rainfall of the area ranges between 500 - 1000 mm. This area is characterized by erratic rainfall and frequent droughts. The rainy season is occurring between June and September and the subsistence agricultural production is almost entirely dependent on this timing of the rainfall [
The study area consists of different types of lithology which varies with the morphology. Morphologically, the study area is divided into three major land systems: the Mekelle “Plateau”, the Ethiopian Rift Escarpment and the Giba River Tributaries. In general, the most common soils of the study woreda (Enderta) are: Calcisols, Cambisols, Kastanozems, Leptosols, Luvisols, Phaozems, Regosols, Vertisols and Fluvisols. The Fluvisols are mainly confined to the alluvial deposits along the river valley [
In this study an integrated use of multispectral satellite data, the digital elevation model (DEM) and Ethiopia topographical sheets were utilized for database generation and extraction of various drainage parameters. The details of data type, software and sources used are discussed below:
The micro-watershed area was delineated from rectified; mosaiced Ethiopian Mapping Authority (EMA) topographic maps at 1:50,000 scale printed by the EMA [
The soil type and texture analysis was carried out at Mekelle University Soil Laboratory, Mekelle. The USDA particle size classes, viz. Sand (2.0 - 0.05 mm), Silt (0.05 - 0.002 mm) and Clay (<0.002 mm), were followed when assigning textural classes. The soil texture was analysed by using hydrometric method [
To obtain the land unit map and update the basin’s drainage map of the micro-watershed, a set of 4 Landsat-7 ETM and 2 Landsat-5 TM images (Landsat ETH+ data of the year 2005) which is already orthorectified, were downloaded from Global Land Cover Facility (GLCF). Two sets of Landsat-7 images (acquired on 27 January,
Parameters | Formulae | References |
---|---|---|
Stream order (U) | Hierarchical rank of streams | [ |
Stream length (Lu) | Length of the stream | [ |
Mean stream length (Lsm) | Lsm = Lu/Nu where: Lsm = mean stream length, Lu = total stream length of order “U”, Nu = total no. of stream segments of order “U” | [ |
Stream length ratio (RL) | RL = Lu/(Lu − 1) where, RL = stream length ratio, Lu = the total stream length of the order “U”, Lu − 1 = the total stream length of its next lower order | [ |
Bifurcation ratio (Rb) | Rb = Nu/Nu + 1 where, Rb = bifurcation ratio, Nu = total number of stream segments of order “U”, Nu + 1 = number of segments of the next higher order | [ |
Mean bifurcation ratio (Rbm) | Rbm = average of bifurcation ratios of all order | [ |
Stream frequency (Fs) | Fs = Nu/A where, Fs = stream frequency, Nu = total no. of streams of all orders, A = area of basin | [ |
Drainage texture (T) | T = Dd × Fs where, T = Drainage texture, Dd = drainage density, Fs = stream frequency | [ |
Drainage density (Dd) | Dd = Lu/A where, Dd = drainage density, Lu = total stream length of all orders, A = area of basin | [ |
Circularity ratio (Rc) | Rc = 4πA/P2 where, Rc = circularity ratio, pi = pi value, A = area of basin, P2 = square of the perimeter | [ |
Elongation ratio (Re) | [ | |
Form factor (Ff) | Ff = A/L2 where, Ff = form factor, A = area of basin (km), | [ |
Relief | R = H − h where, R = relief, H = Vertical distance of the lowest highest points = Vertical distance of the lowest points of watershed in (m) | [ |
Relief ratio | Rh = H/Lb where, Rh = relief ratio, H = total relief of the basin in km, Lb = basin length | [ |
Length of overland flow (Lg) | Lg = 1 Dd × 2 where, Lg = length of overland flow, Dd = drainage density | [ |
2000 and 05 February, 2000) were merged in a mosaic. ASTER DEM, with a 15 meters spatial resolution, were also purchased, but needed to be orthorectified and georeferenced. Google Earth images were a complementary tool during the photo-interpretation because of their high resolution of most of the study area. This allowed a good determination of the land use and land cover pattern of the micro-watershed. This collection of different satellite images and photos was also useful for multi-temporal analysis. DEM of the micro-watershed was extracted from ASTER data obtained during February 2005 with resolution of 90 m, downloaded from the GLCF. The ASTER DEM was utilized to prepare topographic, slope and delineation of drainage map of the micro-watershed using the ArcGIS 9.3 spatial analyst tool. The field data and topographic maps were analyzed using geographic information system (ArcGIS) and ERDAS imagine softwares. In addition, different techniques were used for conducting LULC mapping such as image restoration, enhancement and classification of the satellite imageries.
The basin area, basin perimeter, and basin length of the Chelekot micro-watershed were found to be 106 km2, 44 km, and 16 km respectively. Streams are classified based upon their form and patterns or networks they create in the landscape. The term “stream network” refers to the connectivity of the stream tributaries and is becoming an increasingly important concept because it is used in GIS hydrologic distributed modeling [
The total length of stream segments decreases as the stream order increases (
No. | Parameters | Stream order (W) | Total | ||||
---|---|---|---|---|---|---|---|
I | II | III | IV | V | |||
1 | Number of stream (Nu) | 130 | 38 | 9 | 2 | 1 | 180 |
2 | Bifurcation ratio (Rb) | 3.42 | 4.22 | 4.5 | 1 | ||
3 | Mean bifurcation ratio (Rbm) | 3.29 | |||||
4 | Total length of streams (km) | 97.32 | 45.53 | 19.89 | 5.71 | 8.63 | 177.08 |
5 | Mean length of streams (km) | 0.98 | |||||
6 | Stream length ratio (RL) | 3.71 |
the hydrological characteristics of the river basin because they characterize the permeability of the rock formations in a basin. It also indicates if there is a major change in the hydrological characteristics of the underlying rock surfaces within the basin [
The results showed that the bifurcation ratio (Rb) values ranging between 1 and 4.5 and the mean bifurcation ratio of the micro-watershed is 3.3. This indicates that the drainage pattern has not been affected by structural disturbances and the observed Rb is not the same throughout stream orders of the micro-watershed. These irregularities depend upon the watershed geological and lithological development (
The results of this study indicated that the drainage density of the Chelekot micro-watershed is 1.7. Lower drainage density of the micro-watershed indicates towards coarse drainage pattern and sub-humid climate of the study area. The coarse texture gives more time for the infiltration of overland flow and hence to groundwater recharge. A low value of the drainage density indicates a relatively low density of streams and thus a slow stream response to runoff [
In the present study, the relief ratio (Rh) value of the micro-watershed is 25.2 which shows that the major portion of the micro-watershed is having moderately steep slopes (
The aspect map is a very important parameter for understanding the impact of sun on the area’s local climate. In most cases a west-facing slope will be warmer than an east-facing slope, especially in the afternoon. Aspect has major effects on vegetation distribution. The aspect map of the Chelekot micro-watershed was derived from ASTER DEM and represents the compass direction of the aspect. 0_ is true north; a 90_ aspect is to the east (
The highest elevation in the micro-watershed is 2424 m a.s.l. and the lowest is 2019 m a.s.l. exists in the western and northwestern parts of the micro-watershed which induces highest runoff and thus, less possibility for rainfall water infiltration. The slope map of the study micro-watershed is grouped in to six classes in percents. 0% - 3% (flat or almost flat), 3% - 8% (gentle slopping), 8% - 15% (sloping), 15% - 30% (moderately steep), 30% - 50% (steep) and >50% (very steep) (
Height of basin mouth (z) m | Maximum height of the basin (Z) m | Total basin relief (H) m | Relief ratio |
---|---|---|---|
2019 | 2424 | 405 | 25.15 |
of stop dams for water harvesting or infiltration ponds to recharge the groundwater. Slope is a crucial parameter which directly controls the balance between runoff response and soil infiltration rates of a terrain. High runoff production in higher slope regions results in less soil infiltration. This factor significantly controls the development of aquifers.
from different activities such as agricultural use, domestic needs and industrialization. It can also used to understand rain water infiltration in the micro-watershed, recharge to the groundwater and surface runoff rates. Land use pattern changes become an important component in hydrological monitoring, modeling and natural resources management in general [
The results indicated that the soil types of the micro-watershed were Pellic Vertisol, Vertic Cambisol, Profoundic Luvisol, Calcaric Regosol, and Haplic Cambisol (
The majority of the soils of this region are reported to be shallow with low soil fertility, high runoff, and low infiltration capacity [
The soil texture was the only physical property that received laboratory analysis. Texture influences the porosity and the degree of soil compaction, which in turn, influences the movement and availability of water in the soil. Sandy soils contain mostly large pores. They hold little water, and excess water drains through them easily. A loam is a soil that contains a roughly balanced mixture of sand, silt, and clay. Soils which are mostly silt or clay have mostly small pores that do not drain water readily. Loamy soils have more chemical activity than sandy soils, and hold more water [
Land Use Type | Area (Sq.m) | Area (Sq.km) | Percentage (%) |
---|---|---|---|
Water body | 464565.6 | 0.5 | 0.4 |
Shrubs and plantation | 14056567.2 | 14.1 | 13.2 |
Settlement and open land | 11153642.4 | 11.2 | 10.5 |
Cultivated land | 80663151.6 | 80.7 | 75.8 |
Total | 106350130.8 | 106.3 | 100 |
Soil type | Area (Sq.m) | Area (Sq.km) | Percentage (%) |
---|---|---|---|
Vertic Cambisol | 33807163.3 | 33.8 | 31.8 |
Haplic Cambisol | 77297.5 | 0.1 | 0.1 |
Pellic Vertisol | 61461421.0 | 61.5 | 57.9 |
Profoundic Luvisol | 774780.5 | 0.8 | 0.7 |
Calcaric Regosol | 10066943.2 | 10.2 | 9.5 |
Total | 106187605.4 | 106.3 | 100.0 |
(22%), clay loam (17%), sandy loam (21%) and loam (35%) (
Application of GIS and DEM for analysis of a micro-watershed’s morphological attributes plays a significant role for proper hydrological study of any terrain which indirectly maintains the hydrogeological condition of the watershed. The quantitative analysis of watershed attributes is found to be of great utility in watershed delineation, soil and water conservation and watershed management. The analysis of watershed attributes conducted in
Soil texture class | Area (Sq.m) | Area (Sq.km) | Percentage (%) |
---|---|---|---|
Clay | 5578980.6 | 5.6 | 5.2 |
Silty clay | 23672742.0 | 23.7 | 22.3 |
Clay loam | 18296053.7 | 18.3 | 17.2 |
Sandy loam | 21851973.3 | 21.9 | 20.5 |
Loam | 36988445.5 | 37.0 | 34.8 |
Total | 106388195.1 | 106.3 | 100.0 |
the Chelekot micro-watershed confirmed that the watershed has moderate relief and less elongated shape. Artificial recharge and runoff harvesting for groundwater development are selected based on small-scale topographic maps. Drainage analysis makes a constructive input with the application of RS and GIS based tools in selecting artificial recharge sites in the area. These analyzed drainage parameters provide comparative indices of the permeability of rock surfaces. If this information is integrated with the other hydrological attributes, the strategy of sitting recharge and water harvesting measures provides better groundwater development and management plan for the area. The drainage pattern classification of the study micro-watershed is dendritic in nature. This may be due to more or less homogeneous lithology and structural controls. Moderate drainage density is observed over the hilly terrain with semi-permeable hard rock substratum, and moderate drainage density over the moderately semi-permeable sub-soils and moderate relief areas. Moderate drainage density areas are favorable for identification of groundwater potential areas. Slope of the micro-watershed plays a key role in determining infiltration and runoff production. Infiltration is inversely related to slope (i.e. the gentler the slope, the higher the infiltration) [
The results of analysis of the micro-watershed attributes show that the micro-watershed has a moderate relief and less elongated shape. The micro-watershed drainage network is dendritic type, indicating homogeneity in texture and requiring less structural controls. This type of basin structure helps explain various terrain parameters such as the nature of the bedrock, infiltration capacity, groundwater recharge, runoff production and soil erosion. A moderate drainage density and stream frequency indicate a moderate subsurface formation permeability rate. The observed parameters reveal recharge related measures, and areas where surface water augmentation measures can be undertaken for water resource management and soil conservation structures. A large scale watershed analysis using GIS, remote sensing data and Digital elevation Model (DEM), would be efficient for understanding terrain parameters such as the nature of bedrock, infiltration capacity, surface runoff. The resulting information would help in understanding the status of land form and their processes, drainage management, and groundwater potential for watershed planning and management. This study will be useful for water resource management at the micro level of any terrain, by planners and decision makers for sustainable watershed development programs.
The results of this study can be used for site suitability analysis of soil and water conservation structures. Subsequently, these parameters were integrated with other hydrological information, land use land cover, land forms, geology, water level and soil in the GIS domain to arrive at decisions regarding suitable sites for soil and water conservation structures (bund, check-dam, and percolation ponds, recharge shaft, etc.) for groundwater development and management. The study recommended that the micro-watershed needs detail and further hydrogeological and geophysical investigations for more proper water management and selection of artificial groundwater recharge structures.
We would like to thank Mekelle University and Tigray Bureau of Water Resources and Energy for providing financial support for undertaking the Watershed Feasibility study and Detail Design of Head Works of Chelekot irrigation schemes. We also appreciate Bob Sturtevant from Colorado State University, Amare Sisay from Hawassa University and Etefa Guyassa from University of Ghent, Belgium for their comments and suggestions.