## 3 Experimental Measurement of Water Temperature Field
### Test Content  
The experimental campaign was conducted from May to June 2024 at a geographical location of \(114\)\({}^{\circ}\)5\({}^{\prime}\)44\({}^{\prime\prime}\) East longitude and \(30\)\({}^{\circ}\)18\({}^{\prime}\)9\({}^{\prime\prime}\) North latitude, with an elevation of 48 m above sea level. The observation target is a freshwater lake. The study period was characterized by consistently clear weather conditions, providing optimal environmental circumstances for the observations. During the experimental process, systematic measurements of atmospheric parameters were performed at a height of 5 m above the water surface, establishing the upper boundary condition for the thermal boundary layer investigation. We conducted tests on the vertical profile temperature of the water body, wind speed, and solar shortwave radiation to understand the diurnal variations in the vertical profile of the water body. We analyzed the influence of changes in wind speed and solar radiation on the vertical profile temperature of the water body, and compared the results with theoretical calculations to verify the accuracy of the vertical profile temperature calculation model. This part of the testing was primarily concentrated in three distinct time periods during the day and night: 08:00-12:00, 13:00-17:00, and 18:00-22:00. The water profile temperature was measured every 10 min within each time period, as illustrated in Figure 4a. Consequently, 24 data chains were obtained for each time period, resulting in a total of 72 profile data per day. Based on these data, the spatiotemporal distribution of the vertical profile temperature of the water body was reconstructed. During the measurement process, environmental parameters such as wind speed and solar radiation were simultaneously collected, as depicted in Figure 4b, to serve as driving data for analyzing the changes in the vertical profile temperature of the water body. While measuring the vertical profile temperature of the water body, we conducted measurements of the skin layer properties of the water body during the day and night to understand the variations in the skin layer properties and verify the accuracy of the cold skin calculation model for the water body. Due to the thinness of the water body's skin layer, typically in the order of millimeters, the CTD thermometer, with a length of 0.44 m and a diameter of 63.25 mm, and its relatively large temperature probe aperture, did not possess the capability to detect the fine temperature profile of the water body's skin layer. This is because the probe was not fixed at the very top of the instrument, and the instrument body entered the water before the temperature probe during immersion, which to some extent disrupted the temperature distribution of the skin layer. Therefore, considering the thickness range of the skin layer, a K-type ultra-fine temperature measurement wire was employed to test the skin layer temperature. This type of temperature sensor has a weld point diameter of \(<\)0.3 mm and a wire diameter of 0.2 mm, meeting the measurement requirements. One end of the probe was fixed to a digital waterproof micrometer, and the other end was connected to a single-channel handheld thermometer. The distance the probe extended into the water was controlled by moving the micrometer, and the temperatures at several discrete points within the skin layer were collected, as shown in Figure 3(c). During the testing process, meteorological and hydrological parameters, including wind speed, surface water temperature, high-altitude air temperature, solar shortwave radiation, and downward longwave radiation, were simultaneously collected. Based on the interface heat flux calculation model, the heat flux components at the time of measurement were evaluated, and the intrinsic relationship between heat flux and the skin layer was established.