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University of Twente (2019)

Effect of land cover change on water balance components in Gilgel Abay catchment using SWAT model.

AGA HADILAWIT TADESSE

Titre : Effect of land cover change on water balance components in Gilgel Abay catchment using SWAT model.

Auteur : AGA HADILAWIT TADESSE

Etablissement de soutenance : University of Twente

Grade : Master of Science in Geo-Information Science and Earth Observation

Résumé
Understanding effect of land cover (LC) change on water balance components is important for water resources planning and management. This study examines the effect of the land cover change on water balance components that include streamflow and actual evapotranspiration in Gilgel Abay catchment, Ethiopia. Five land cover maps of 1986, 2001, 2008, 2013 and 2018 are prepared and are used as input for Soil Water Assessment Tool (SWAT) model approach. Model simulation periods cover five windows that are a baseline (BL) and four altered periods (AP) : 1986-1994 (BL), and altered periods including 1995-2001 (AP1), 2002 – 2008 (AP2), 2009- 2013 (AP3) and 2014 -2016 (AP4). SWAT is calibrated for the baseline period and optimized SWAT-model parameter set served for simulation for the subsequent four altered periods under two LC scenarios, with and with LC update. Land cover classification relied on supervised classification. Classification results are satisfying with Kappa coefficient that ranges between 0.75-0.81. The land cover change analysis shows that for the assessment period 1986-2018, that agricultural and residential area increased by 10.74% and 4.1% respectively ; bare land and grassland decreased by 19.3% and 2.9% respectively. In the same period, forest and wetland values do not show clear increasing or decreasing trend : forest and wetland covered 8.44 % and 0.45% in 1986, 12.86% and 0.24% in 2001, 7.57% and 0.47% in 2008, 17.33% and 0.35% in 2013 and 15.88% and 0.40% in 2018 respectively. The SWAT model was calibrated using monthly streamflow at Wetet Abay gauging station. The model shows good performance with Nash Sutcliffe Efficiency (NSE) of 0.83. The model simulation assessment at Wetet Abay gauging station in AP1 to AP4 show good performance with NSE of 0.78 to 0.69 with LC update and deteriorated performance with NSE of 0.75 to 0.28 without LC update. The effect of LC changes on water balance components at the Gilgel Abay outlet to Lake Tana shows the runoff coefficient at annual base decreased from 56% in BL to 49% in AP4 ; while such coefficient calculated for evapotranspiration (i.e. ∑evapotranspiration/∑ precipitation) increased from 40% in BL to 49% in AP4 ; surface runoff/total discharge increased from 38% in BL to 49% in BL ; and base flow/total discharge decreased from 62% in BL to 0.51% in AP4. 86% of the change in streamflow is attributed to LC change while the remaining 14% is attributed to climate change. This study also shows that for the assessment period effects of climate change on the hydrology of the Gilgel Abay basin are less prominent than effects by land use changes. Regardless of the limitation the study is relevant for sustainable water and environmental planning whereby planners and decision-makers can use.

Mots clés : Gilgel Abay catchment, Land Cover, water balance components, SWAT Model

Version intégrale (ITC)

Page publiée le 5 octobre 2019