Dinâmica dos processos evaporativos: variabilidade climática e mitigação de impactos

Abstract

Water losses from evaporation and evapotranspiration are considered the main natural sources of water loss in surface reservoirs. In the Brazilian semi-arid region, evaporation could consume up to 50% of the water inflow into reservoirs. Therefore, the study of evaporative processes is of great relevance to water resource management, whether in semi-arid climatic conditions, as is the case in most of the state of Ceará, or in temperate climates. In light of this, this thesis is divided into three chapters, two of which were conducted in the state of Ceará (Brazil) and one in Germany. The first chapter assessed the long-term trend in potential evaporation, as estimated using the Penman method, based on meteorological data measured at eight specialised weather stations in the Jaguaribe River Watershed. The Mann-Kendall test and Sen’s slope were applied to the potential evaporation data and the meteorological variables to determine the statistical significance and magnitude of the temporal variation, with the aim of identifying the factors responsible for the observed changes. In the second chapter, the impact of cloud cover and cloud types on incident solar radiation was assessed. Historical time series of cloud cover, cloud types and midday solar radiation were used, obtained from six meteorological stations across Germany. Incident radiation on clear days and under cloud cover conditions was compared in order to understand the effect of cloud cover and cloud types on radiation. In the third chapter, the effect of installing a floating photovoltaic system (FPS) on the main meteorological variables that govern the evaporation process was assessed, as well as its impact on the water balance of a large reservoir. The influence of the FPS installation on the evaporation process was investigated using a sensitivity analysis of the Penman model, which was parameterised to estimate evaporation taking into account the PFS coverage. The impact of the FPS on the water balance was estimated by extrapolating experimental data to the reservoir scale, enabling the estimation of evaporated flow and the effects on water availability under dynamic hydrological conditions. Based on the findings of this thesis, the main conclusions are: (i) There is statistically significant evidence of a change in the pattern of potential evaporation in the Jaguaribe River catchment, but with non-uniform occurrence (increases and decreases), with increases in air temperature and insolation being responsible for the increase in evaporation, and reductions in wind speed and increases in relative humidity being the causes of the decrease in evaporation. (ii) As the cloud cover fraction increases, a trend towards a reduction in radiation incident on the Earth’s surface is observed. However, for medium and high clouds, it is found that, under conditions of cloud cover between 1 and 2 octas, there is a slight increase in radiation incidencewhen compared to the radiation recorded on clear days. (iii) It was found that global radiation is the meteorological variable to which the model is most sensitive. The presence of the FPS tends to bring the reservoir to a new equilibrium level, with an increase in the flooded area, partially offsetting the reduction in the evaporation rate. The installation of the FPS shows high potential as a strategy for increasing the water availability of reservoirs.