Diel aspects of the thermal structure and energy budget of a small English lake

Abstract

SUMMARY. The did response of the thermal structure of Esthwaite Water to prevailing meteorological conditions was analysed for seven selected days, covering different phases of stratification. Meteorological measurements were combined with published empirical relations to compute 24‐h thermal energy budgets for 2 days. Amplitudes of diel changes were controlled by the seasonal phase of stratification and the magnitude of incident solar radiation. Heat storage usually followed a sinusoidal curve with a minimum during the night and early morning and a maximum during the late afternoon. Wind‐induced turbulent mixing, and vertical circulation induced by nocturnal cooling of the surface water through evaporative and conductive processes, also modified depth‐time trends in thermal structure.The budget accounted for almost all the thermal energy exchanges of the lake. Possible sources of a slight imbalance are considered. Although there was a positive daily surplus of net radiation, it may not always imply heat gain by the lake as it may be counter‐balanced by heat losses through fluxes of latent and sensible heat, Net back‐radiation was a major component of daily heat loss. The combined latent and sensible heat fluxes also accounted fora high proportion of heat loss to the atmosphere. Net flow of sensible heat to the lake was small but not always inappreciable. Net daily heat storage represented 26% (9 May) and 13% (17 June) of solar radiation Input. Advected energy due to lake inflow and outflow was negligible. Qualitative comparisons are made with long‐term (seasonal and annual) energy budgets and heat fluxes.