Energy Budget At Lake Taihu And Its Response To Climate Change

Lakes are a main component of the Earth’s climate system. Their surface energy budget and energy partitioning processes are dominant drivers of biophysical and biogeochemical cycles in the lake ecosystem. Long-term contimuous eddy convariance observations of energy fluxes not only provide prerequisite dataset for lake model parameterization and validation, but also bring advandage to weather forcasting and air pollution modeling. Being effective sentinels of climate change, lakes’ reponse to global warming should advance our understanding of global water cycle variability, also provide timely information to aid the ongoing lake remediation efforts and inland water resource managements.In this study, the radiation and energy components were observed across Lake Taihu, China within an eddy covariance mesonent consisting of three lake sites, reflecting different biological attributes and wind-wave regimes, and one land site. The observation data, reanalysis and climate model products will be used to address four-fold issues:(1) to analyze the energy balance closure for eddy covariance observations at Lake Taihu on multi-time scales and to indentify the main reasons for energy imbalance;(2) to investigate the temporal and spatial variations in radiation and energy balance across the large and shallow freshwater Lake Taihu and elucidate the underlying mechanisms;(3) to evaluate the performance of19classic evaporation models of variaring complexities by long-term latent heat observations;(4) to exploring the response of energy budget and partitioning at Lake Taihu to climate change under historical and future scenarios. The major results are as follows:(1) Mechanical turbulence is the main constraint to energy balance at Lake Taihu. The energy balance closure was observed at only0.59for the smooth lake with half-hour averages, but increased to0.73using daily averages. Throughout the year, energy balance showed obvious deficit during warming months but perfect closure during winter months. Our results suggest that the poorer closure for very unstable conditions than for less unstable conditions was due to reduced friction velocity. Although lake-breeze deteriorated the energy balance closure by0.1on hourly scale, which was indiscernible on daily scale. (2) On the monthly to annual scales, the radiation and energy fluxes showed little spatial variations (within16W m-2on monthly scale, and1W m-2on annual scale) across Lake Taihu, indicating a lack of sensitivity to wind speed, water depth, water quality and the presence of submergd macrophytes. From the modeling perspective, Lake Taihu can be parameterized as one grid cell in climate models at least on the monthly and annual scales. Nocturnal evaporation, fueled by heat release from the water column, was a major component of the local hydrologic cycle, contributing to48%of the annual evaporation. Owing to the high temperature, the annual Bowen ratio (0.12-0.13) of Lake Taihu was lower than those found in the literature for subtropical and northern lakes and also much lower than that observed at the adjacent land site (0.58). On the annual cycle, the lake-land thermal contrasts were larger at Lake Taihu than that at Lake Superior in the temperature latitudes.(3) The most influential input is the water heat storage for lake evaporation simulations at Lake Taihu, followed by the radiation parameter. The Bowen ratio energy balance model, the Priestley-Taylor model and the deBruin-Keijman model, which have incorporated the available energy constraints, agreed most closely with the eddy covariance observations. The model evaluation results have emphasized the importance of energy budget observation for lake evaporation prediction. Other alternate methods performance was highly sensitive to empirical coefficients, which were locally determined and hardly to be transferred. Althogh profile observation of water temperature is labor-consuming, temperature probes are suggested to be deploied in several locations in a lake to reflecting variations in water depth. Several modified evaporation models reduced bias both on monthly and annual scales, the most cost effective combination methods:the Priestley-Taylor model and the deBruin-Keijman model are still highly recommended to be chosen for evaporation estimation at Lake Taihu or other lakes with similar physical and climatic settings.(4) The sensitivity of energy partitioning to air temperature rising predicted by climate model is surprisingly lower than the historical simulations at the Lake Taihu, especially under high emission scenarios. Evaporation at Lake Taihu were predicted to increase both in historic time series (1979-2012) and future simulations (2013-2100), the rising rate increased as emission becoming more intensive (from RCP2.6to RCP8.5). Variations in Bowen ratio relative to air temperature at Lake Taihu can be well captured by the Priestley-Taylor model with the observed coefficient of1.36. As predicted by the Priestley-Taylor model, annual Bowen ratio has decreased substantially since the satellite era at subtropical Lake Taihu in response to global warming, while evaporation fraction significantly increased with scaling temperature. However, the climate model predictions surprisingly underestimated the Bowen ratio and evaporation fraction sensitivity to air temperature rising, in particular under high emission scenarios. As compared to the Great Lakes, Taihu is projected to response more quickly to climate change. However, it is expected that global warming has more intensive effect on energy partitioning process at northern Great Lakes by shortening ice duration and modulating ice phenology.