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The Monin‐Obukhov Similarity Theory (MOST) links turbulent statistics to surface fluxes through universal functions. Here, we investigate its performance over a large lake, where none of its assumptions (flat homogeneous surface) are obviously violated. We probe the connection between the variance budget terms and departure from the nondimensional flux‐variance function for CO₂, water vapor, and temperature. Our results indicate that both the variance storage and its vertical transport affect MOST, and these terms are most significant when small fluxes and near neutral conditions were prevalent. Such conditions are common over lakes and oceans, especially for CO₂, underlining the limitation of using any MOST‐based methods to compute small fluxes. We further show that the relaxed eddy accumulation (REA) method is more robust and less sensitive to storage and transport, adequately reproducing the eddy‐covariance fluxes even for the smallest flux magnitudes. Therefore, we recommend REA over MOST methods for trace‐gas flux estimation.

During and after rainfall events, the interaction of precipitation with hot urban pavements leads to hot runoff, and its merger with urban streams can result in an abrupt change in water temperature that can harm aquatic ecosystems. To understand this phenomenon and its relation to land cover and hydrometeorological parameters, we analyzed data spanning two years from 100 sites in the eastern United States. To identify surges, we first isolated temperature jumps of at least 0.5°C over 15 min occurring simultaneously with water flow increase. Surge magnitude was defined as the difference between peak stream temperature and baseflow temperature right before the jump. At least 10 surges were observed in 53 of the studied streams, with some surges exceeding 10°C. Our results demonstrate that the watershed developed area and vegetation fraction are the best descriptors of surge frequency (Spearman correlation of 0.76 and 0.77, respectively). On the other hand, for surge magnitude and peak temperature, the primary drivers are stream discharge and stream temperature immediately before the surge. In general, the more urbanized streams were found to be already warmer than their more “vegetated” counterparts during baseflow conditions, and were also the most affected by temperature surges. Together, these findings suggest the existence of a hydrological urban heat island, here defined as the increase in stream temperature (chronic and/or acute), caused by increased urbanization.