Emissivity Difference And Thermal Infrared Temperature Retrieval Of Taihu Lake Eutrophic Water Body

Water temperature is an significance indicator for water body and water environment.It affects water and energy transfer balance of between water and atmosphere,regulates the local climate,and controls the growth and reproduction of aquatic plants,planktonic plants and other microorganisms,thereby affects the ecological chain.Water body's skin temperature can be quickly and precisely retrieved from thermal infrared remote sensing data,which requires water body's emissivity.Measurement data show that water emissivity is affected by water composition and changes with it.However,the literature on the effect of water composition on the water emissivity is scarce.The vast majority of water temperature remote sensing studies take this parameter as a constant value.And there will be a large error in the temperature inversion of the case ? water body where water quality is complex with high density of algae and suspended solid.Aiming to solve this problem,we studied the effects of blue-green algae and suspended solid on the retrieval of water emissivity and water temperature in Taihu Lake,which is characterized by high density of algae and suspended solid.Using Nicolet iS50 Fourier transform infrared spectrometer and PIKE integrating sphere,we measured the water emissivity under different concentrations of blue-green algae and suspended solid.An emissivity calculating method for water body based on concentration of blue-green algae and suspend solid was proposed.Spatial analysis and accuracy evaluation of the Taihu Lake water emissivity and surface temperature retrieval on specific dates are given using the calculating method.The results have shown that the presence of high density of blue-green algae bluegreen algae and suspended solid has a significant effect on water emissivity and water temperature retrieval.The emissivity of water body decreases with the increase of suspended solid concentration in MODIS 31 and 32 band,slightly decrease in MODIS 31 band and slightly increase in MODIS 32 band with the increase of blue-green algae concentration.The temperature retrieval results increase with the increase of both blue-green algae and suspended solid's concentration.When taking both chlorophyll a and suspended matters into consideration,the maximum deviation of retrieved temperature reaches as large as 1.21�C.Under the condition of low concentration of suspended matter,the maximum effect of chlorophyll a on temperature retrieval is about 0.86�C.Under the condition of low concentration of chlorophyll a,the maximum effect of suspended solid on temperature retrieval is about 0.86�C.The average temperature retrieval accuracy of all validation points was improved by 0.41�C.And the temperature retrieval results of 87.5% points among all validation points were improved,the errors were reduced as well,and the temperature retrieval accuracy was improved by 0.57�C.Experimental results showed the introduction of emissivity function can effectively suppress the estimation error of emissivity caused by blue-green algae and suspended solid,and improve the retrieval accuracy of the water body surface temperature.Analyzation on temperature response of high-density blue-green algae water under the same sunlight conditions was carried out.For the experimental thermometers with inconsistent measurement accuracy and different temperature responses,the response relationships were calibrated.Based on this,a synchronous temperature observation experiment of both the water surface and the lower layer of the water under the same ilumination conditions was designed.As the experimental results showed,for the Taihu Lake water sample containing high-density blue-green algae,the temperature of surface water was 3~4�C higher than that of the clean water sample during the day and 1~2�C lower at night,and the temperature of lower level water was 1~2�C lower than that of the clean water sample during the day and 1~2�C higher at night.Experiments have shown that the coverage of blue-green algae has a significant effect on the emissivity response of water body.Under sunlight irradiation,accumulation of high-density bluegreen algae on the water surface can effectively increase the temperature response efficiency of the surface water and has a significant effect of increasing temperature,which will greatly promote the vernal growth and development of blue-green algae blooms and is critical to the early warning of blue-green algae blooms.