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University of Twente (2016)

How climate and land use determine the hydrology of Lake Naivasha Basin

Odongo Vincent Omondi

Titre : How climate and land use determine the hydrology of Lake Naivasha Basin.

Auteur : Odongo Vincent Omondi

Etablissement de soutenance : University of Twente

Grade : Doctor University of Twente 2016

Résumé partiel
Recent theoretical advances recognize the importance of stochastic processes and probabilistic representations that couple vegetation, climate and hydrology (Jackson et al., 2009) . Vegetation exerts strong controls over key hydrologic variables and therefore changes in land cover affect the water balance, quantity and quality of water resources. As a consequence, growing human population exacerbates changes in land cover and land use leading to increased pressure on water resources and hydrologic regulation. It is important to understand the hydrological processes occurring at a catchment scale and the controlling factors within the catchment. Understanding the hydrological response to changes in land use and land cover as well as climate variability give valuable insights on prediction of water yields and quality across landscapes and forecasting consequences of land use conversations on stream flows and lake levels. The main objective of this study was to quantify the impacts of climate and land use on the hydrological response of Lake Naivasha Basin. The study focused on identifying climatic and human- induced factors that determine the responses prevailing at different observable scales. In Chapter 2, a comprehensive analysis of hydro- climatological trends and variability characteristics were investigated for the Lake Naivasha basin with the aim of understanding changes in water balance components and their evolution over the past 50 years. The results showed that ups tream flows and precipitation in the basin were fairly homogenous with most stations showing little abrupt or gradual changes. Downstream flows however, experienced increasing trends in one part of the basin and decreasing trends in other parts. The lake experienced a significant decline over the period at a mean rate of 0.06 m year-1 . This decline was about 50% lower than reconstructed lake volumes, thereby suggesting net losses attributable to measurement errors, ground water seepage or lake water abstractions. Homogeneity of precipitation in the basin over the period of study was indicative of no evidence of climatic change impacting the hydrological regime of the basin but other factors than precipitation were the likely cause of the observed changes. In chapter 3 the SEBS model was applied to quantify evapotranspiration of the different land uses/cover in the basin for the period 2003 to 2012. Using MODIS and ECMWF ERA- Interim as input data to compensate for lack of data in the basin, Evergreen forest and closed shrublands, released the least amount of water back to the atmosphere because they covered the smallest area in the basin compared to grasslands, cropland/natural vegetation mosaic and savannas. Overall, annual evapotranspiration over the 10 yea rs showed a declining trend ( 10%) which was likely due to reduced net radiation combined with increases in both actual vapor pressure and decreases in the air- surface temperature difference. These factors accounted for at least 90% of the estimated decline. Validation of SEBS results against 2-year flux measurements at a heterogeneous site (See chapter 4) further indicated that, heterogeneity of the measurement site, uncertainty in the input data, scale mismatch between input and measured data at the site and uncertainties inherent in the formulation of the SEBS model were the likely causes of mismatch between measured and modelled fluxes. It was recommended that inclusion of additional local flux measurements at different land use/cover types would signifi cantly improve understanding of the hydrological fluxes in the basin and constrain model simulations.

Version intégrale (ITC)

Page publiée le 2 février 2018