Quantification of evaporation from a small subtropical water storage using Eddy covariance, scintillometry and modelling techniques

Abstract

A significant source of water loss in Australia occurs through evaporative losses from small storage reservoirs. The predicted increase in water scarcity as a result of climate change and increase in water demand due to major population growth, means that water is becoming an increasingly valuable commodity in many regions. A small storage reservoir known as Logan’s Dam was selected to research the ability of a variety of methods to accurately measure and predict evaporation from small water bodies and to gain a better understanding of the physical controls on the energy balance of a small reservoir in a subtropical environment. The reservoir is located within the Lockyer catchment in southeast Queensland, Australia, a region that has experienced water shortages in the recent past. Reliable and representative evaporation rates are essential for effective water resource management. However, studies of open water evaporation have been infrequent and have generally been performed at large water bodies. In order to test suitable methods for direct measurement of evaporation from small water bodies, this thesis presents a comparison of the Eddy Covariance (EC) and scintillometry methods. Analysis was undertaken to ascertain whether the theoretical assumptions required for both techniques are valid in the complex environment of a small reservoir. Footprint analysis suggested that in the majority of conditions the footprints for both EC and scintillometry measurements were confined to the water surface. On average scintillometer estimates of evaporation were approximately 21 % greater than EC measurements. An analysis of two years of EC evaporation measurements made at Logan’s Dam is presented in this thesis. Diurnal cycles in evaporation were found to be largely controlled by diurnal variations in the function of the horizontal wind speed and the lake-air vapour pressure difference. Occasional intraseasonal peaks in evaporation were associated with the passage of cold fronts over southern Queensland, which brought strong dry westerly winds. It was found that seasonal variations in evaporation closely followed the seasonal changes in net radiation. The predicted total evaporative water loss from the reservoir during the 2-year measurement campaign was 1,982 mm. This total evaporative water loss is equivalent to almost half of the reservoir storage capacity. In order to reliably estimate evaporation using commonly available hydrometeorological data, several methods for modelling sub-daily evaporation were tested against EC measurements. Results from a simple mass transfer model and the 1-dimensional Dynamic Reservoir Simulation Model (DYRESM) compared reasonably well with direct EC measurements. However, an evaporation model that was recently developed specifically for use in small reservoir environments performed poorly at Logan’s Dam. This was largely due to the model overestimating the influence that atmospheric stability had on evaporation. The potential impact of anthropogenic climate change on future evaporation rates and water availability within the Lockyer catchment was also investigated in this thesis. Future projections of evaporation were made using several future greenhouse emissions scenarios. Results showed that for the most likely future emissions scenario, evaporation is expected to increase by approximately 5 % by 2050. In addition, rainfall is expected to decrease by approximately 8 %. These projected reductions in rainfall and increases in evaporation, combined with the knowledge that changes in annual rainfall are known to be amplified in annual runoff, mean that the availability of water resources in the Lockyer catchment region may be greatly diminished in the future. This thesis presents new knowledge about the ability of a variety of evaporation measurement and modelling techniques to accurately quantify evaporation rates in the complex environment of a small reservoir and discusses the limitations of these techniques in such an environment. The thesis makes a significant contribution to our understanding of evaporative water losses from small reservoirs in subtropical climates. New insights are presented about how the environmental processes that drive evaporation are influenced by variability in the weather and climate at a range of temporal scales. Importantly, this thesis also estimates how evaporation rates may be affected by projected changes in future climate.