HOW CLIMATE AND LAND USE DETERMINE THE HYDROLOGY OF LAKE NAIVASHA BASIN
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 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 upstream 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 years 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 significantly improve understanding of the hydrological fluxes in the basin and constrain model simulations. The flux measurements at the heterogeneous site also allowed for the characterization of the seasonal and interannual variability of the energy and water fluxes of the ecosystem covering two wet years (2012 and 2013) and one drought year (2014). On an annual scale, more than 60% of Rn was partitioned as LE and H, with the latter being the largest consumer of Rn (~34%) and dominant for most months. The transition from H to LE dominance occurred from early noon to late afternoon in the wettest months of April and May. The residual energy balance closure term (C) accounted for between 25-40% of the Rn with the imbalance tending to be highest during periods of high insolation. Annual evapotranspiration accounted for at least 80% of the annual precipitation received at the site. The measurements showed that, during wet seasons, the Rn was the dominant control of energy partition into LE while during the dry seasons, VPD and gs strongly dominated the energy partitioning into LE. This cyclic pattern in dominance of controlling factors between wet and dry seasons provide insights towards formulating models that quantify evapotranspiration for ecosystems that experience seasonal shifts in controlling factors.
While land use and climatic controls on eco-hydrological processes are well understood and easily quantified using physical models in hydrological sciences, socio-economic controls on eco-hydrological processes have received little attention. Socio-economic developments often change the water balance substantially and are highly relevant in understanding changes in hydrological responses. Lake Naivasha basin has experienced substantial land use and land cover changes predominantly caused by socio-economic drivers. Accounting for the implications of these socio-economic drivers of is vital to the understanding of hydrological and ecological functioning of the basin. Using a statistical cascade-modeling approach, coupling socio-economic factors to eco-hydrological processes it was shown that socio-economic factors have intensified land use changes leading to increased flow regimes over the last 25 years. The population in the upstream part of the basin contributed over 60% to the land use/cover transformations in that part of the basin. Water abstractions from the lake and its conjunctive aquifer influenced the lake storage changes less than the contribution from upstream runoff volume. Upstream runoff volumes directly affected the lake storage changes by over 70% whereas water abstractions affected the lake storage changes by only 10%. The downstream population and cut-flower export volume accounted for at least 70% of water abstraction. Finally, the large aggregation of high biomass density of large herbivore mammal species in the lake fringe was largely due to land use changes than precipitation. The low variance explained by the annual precipitation on the herbivores biomass contradicts earlier observations that ungulates density is dependent on annual precipitation (Coe et al., 1976; East, 1984; Georgiadis et al., 2003; Ogutu and Owen-Smith, 2003).
Overall, the cascade modelling approach offered insight on the contribution of socio-economic factors on the eco-hydrological regime of the Lake Naivasha basin. Thus, our method could potentially be used to show the influence of eco-hydrological processes on socio-economic developments. Trends in the water balance components, evapotranspiration and its climatic controls were investigated and a modelling framework integrating socio-economic factors of land use change to basin wide eco-hydrological processes were presented.
