Quantitative Analysis and Implications of Drainage Morphometry of the Agula Watershed in the Semi-Arid Northern Ethiopia
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Appl Water Sci (2017) 7:3825–3840 DOI 10.1007/s13201-017-0534-4 ORIGINAL ARTICLE Quantitative analysis and implications of drainage morphometry of the Agula watershed in the semi-arid northern Ethiopia 1,2 1 3 4 Ayele Almaw Fenta • Hiroshi Yasuda • Katsuyuki Shimizu • Nigussie Haregeweyn • Kifle Woldearegay5 Received: 31 October 2016 / Accepted: 17 January 2017 / Published online: 7 February 2017 Ó The Author(s) 2017. This article is published with open access at Springerlink.com Abstract This study aimed at quantitative analysis of analysis revealed fine drainage which implies the domi- morphometric parameters of Agula watershed and its sub- nance of impermeable soft rock with low resistance against watersheds using remote sensing data, geographic infor- erosion. High relief and steep slopes dominates, by which mation system, and statistical methods. Morphometric rough landforms (hills, breaks, and low mountains) make up parameters were evaluated from four perspectives: drainage 76% of the watershed. The S-shaped hypsometric curve network, watershed geometry, drainage texture, and relief with hypsometric integral of 0.4 suggests that Agula characteristics. A sixth-order river drains Agula watershed watershed is in equilibrium or mature stage of geomorphic and the drainage network is mainly dendritic type. The evolution. At sub-watershed scale, the derived morphome- mean bifurcation ratio (Rb) was 4.46 and at sub-watershed tric parameters were grouped into three clusters (low, scale, high Rb values (Rb [ 5) were observed which might moderate, and high) and considerable spatial variability was be expected in regions of steeply sloping terrain. The observed. The results of this study provide information on longest flow path of Agula watershed is 48.5 km, with drainage morphometry that can help better understand the knickpoints along the main river which could be attributed watershed characteristics and serve as a basis for improved to change of lithology and major faults which are common planning, management, and decision making to ensure along the rift escarpments. The watershed has elongated sustainable use of watershed resources. shape suggesting low peak flows for longer duration and hence easier flood management. The drainage texture Keywords Morphometric analysis Á Watershed characteristics Á Remote sensing Á Geographic information system Á Semi-arid Á Ethiopia Electronic supplementary material The online version of this article (doi:10.1007/s13201-017-0534-4) contains supplementary material, which is available to authorized users. Introduction & Ayele Almaw Fenta [email protected] Morphometric analysis is an important aspect of charac- 1 terization of watersheds. It involves computation of quan- Arid Land Research Center, Tottori University, 1390 Hamasaka, Tottori 680-0001, Japan titative attributes of the landscape related to linear, aerial and relief aspects from elevation surface and drainage 2 Department of Land Resources Management and Environmental Protection, Mekelle University, P.O. Box 231, networks within a watershed. Over the past several dec- Mekelle, Ethiopia ades, morphometric analysis to evaluate watersheds and to 3 Faculty of Agriculture, Tottori University, 4-101 Koyama- describe the characteristics of surface drainage networks cho Minami, Tottori 680-8553, Japan with reference to land and water management has been a 4 International Platform for Dryland Research and Education, major emphasis in geomorphology. Pioneer studies by Tottori University, 1390 Hamasaka, Tottori 680-0001, Japan Horton (1932, 1945) demonstrated the significance of 5 Department of Earth Sciences, Mekelle University, quantitative morphometric analysis to better understand the P.O. Box 231, Mekelle, Ethiopia hydrologic and geomorphic properties of watersheds. Since 123 3826 Appl Water Sci (2017) 7:3825–3840 then, several methods of watershed morphometry were Integrating remote sensing data and GIS tools, therefore, further developed (e.g., Miller 1953; Strahler allow automated computation of morphometric parameters 1954, 1957, 1964; Schumm 1956; Melton 1957; Faniran and have been successfully employed by several researchers 1968) that enabled morphometric characterization at (e.g., Magesh et al. 2011; Thomas et al. 2012; Kaliraj et al. watershed scale to extract pertinent information on the 2014; Banerjee et al. 2015; Roy and Sahu 2016) for gener- formation and development of land surface processes. ating updated and reliable information to characterize Morphometric analysis represents a relatively simple watershed physiographic attributes. approach to describe the hydro-geological behavior, land- Significant advances in remote sensing technology have form processes, soil physical properties and erosion char- led to availability of higher quality digital elevation models acteristics and, hence, provides a holistic insight into the (DEMs). For instance, availability of Shuttle Radar hydrologic behavior of watersheds (Strahler 1964). The Topography Mission (SRTM) and Advanced Space-borne hydrological response of watersheds is interrelated with their Thermal Emission and Reflection Radiometer (ASTER) physiographic characteristics, such as size, shape, slope, DEMs free of charge via http://earthexplorer.usgs.gov/ has drainage density, and length of the streams (Gregory and provided new potentials in watershed scale quantitative Walling 1973). Recent studies demonstrated that quantita- morphometric analysis. In the past decade, several studies tive morphometric analysis has several practical applications used SRTM (90 m resolution) and/or ASTER (30 m reso- that include land surface form characterization (Reddy et al. lution) DEMs in a GIS environment to derive morphome- 2004; Thomas et al. 2012; Magesh et al. 2013; Kaliraj et al. tric characteristics of watersheds with different geological 2014; Banerjee et al. 2015), watershed prioritization for soil and hydrological settings (e.g., Romshoo et al. 2012; and water conservation (Gajbhiye et al. 2014; Meshram and Thomas et al. 2012; Gajbhiye et al. 2014). These studies Sharma 2015), environmental assessment (Magesh et al. demonstrated that SRTM and ASTER DEMs provide 2011; Al-Rowaily et al. 2012; Rai et al. 2014; Babu et al. reliable datasets with global coverage that enabled evalu- 2016), and evaluation and management of watershed ation of morphometric properties and various relief fea- resources (Pandey et al. 2004). Furthermore, comparison of tures. A recent comparative study by Thomas et al. (2014) the quantitative morphometric parameters helps understand showed that topographic attributes extracted from the the geomorphological effects on the spatial variation of space-borne (SRTM and ASTER) DEMs are in agreement hydrological functions (Romshoo et al. 2012; Sreedevi et al. with those derived from topographic maps. Their study also 2013). Understanding drainage morphometry is also a pre- revealed that despite the coarser resolution (i.e., 90 m), requisite for runoff modeling, geotechnical investigation, SRTM DEM shows relatively higher vertical accuracy and identification of water recharge sites and groundwater pro- better spatial relationship of topographic attributes than the spect mapping (Sreedevi et al. 2005; Fenta et al. 2015; Roy finer resolution (i.e., 30 m) ASTER DEM when compared and Sahu 2016). As such, morphometric analysis is an with topographic maps. important procedure for quantitative description of the Turcotte et al. (2001) demonstrated that morphometric drainage system; thus enabling improved understanding and analysis solely based on automated DEM-based approaches better characterization of watersheds. has limitations in representing the actual drainage structure Earlier studies successfully applied conventional methods of a watershed, and ways to recondition DEMs for of morphometric characterization based on map measure- improved performance have been suggested (e.g., Hell- ments or field surveys (e.g., Horton 1932, 1945; Strahler weger 1996; Soille et al. 2003). Studies by Turcotte et al. 1952, 1954, 1957, 1964); however, it has been recognized (2001) and Callow et al. (2007) showed that DEM recon- that such methods of generating information especially for ditioning using stream networks digitized from topographic large watersheds are expensive, time-consuming, labor maps greatly improves replication of stream positions and intensive and tedious. Recently, increasing availability of reduces error introduction in the form of spurious parallel remote sensing datasets with improved spatial accuracy, streams. Therefore, in the present study, a more rational advances in computational power and geographical infor- approach of morphometric analysis has been employed mation system (GIS), enable evaluation of morphometric using SRTM DEM with relatively fine spatial resolution parameters with ease and better accuracy (Grohmann 2004). (i.e., 30 m) and natural stream networks digitized from On the one hand, remote sensing enables acquisition of topographic maps (scale 1:50,000). The natural stream synoptic data of large inaccessible areas and is very useful in networks were used to recondition the DEM prior to the analyzing drainage morphometry. On the other hand, GIS computation of flow direction and flow accumulation grids. provides a powerful tool and a flexible environment and as The objective of this study was to derive morphometric such the information extraction techniques are less time- parameters related to drainage network, watershed geom- consuming than ground surveys for