Predicting the onset of thermal stratification in shallow inland waterbodies

Abstract.

Diurnal thermal stratification in shallow lakes and ponds is well documented and affects biogeochemical gradients. The onset of thermal stratification should occur when the stratification parameter I _S  =−HB/u* rises above a critical value, I _crit . Previous applications of the stratification parameter lack a common definition for the onset stratification and include assumptions inappropriate for shallow inland waterbodies. Here, the onset of stratification is defined relative to a practical threshold given by the Rohr-Stillinger criterion, N² = ε/(20v). Using this definition, the utility of I _S for predicting stratification in two shallow ponds was evaluated using select periods from over 600 days of temperature profiles, standard weather data and measured light attenuation coefficients. Though uncertainties in B and u* can produce errors in I _S on the order of 40–60%, the parameter is robust for predicting the onset of stratification. Accuracy can be improved by sitespecific determinations of wind sheltering. The critical value for the onset of stratification was found to be approximately 1. The stratification parameter can be an effective screening tool for 1) predicting the presence of significant chemical gradients, and 2) determining the minimum wind speed required to prevent diurnal thermal stratification from developing.