Water temperatures and heat budgets in Dorset chalk water courses

Field measurements have established the heat budgets for stations on two water courses which drain catchment areas dominated by the upper chalk in Dorset. UK. Information on the main advective and non-advective heat fluxes were collected during winter and summer periods in 1994 at a site near to a spring source on a tributary of the River Piddle, and at a station situated downstream on the River Bere. Inputs of heat energy were dominated by radiative fluxes, with net radiation receipt accounting on average for around 90% of the non-advective total in both summer and winter. Sensible heat (convective/conductive) transfer from the atmosphere was enhanced in the summer by the lower water temperatu- re of these spring-fed streams, while relatively warm water temperatures during winter enhanced heat losses by evaporation, which was the dominant non-advective heat loss component in this season. Significant heat losses also occurred by sensible transfer and by back radiation. Summer measurements revealed that conduction of heat into the non-vegetated gravel bed of the River Piddle tributary was the dominant form of loss, but this output was strongly reduced in summer by weed cover on the bed of the River Bere. Considerable daily and diel variability was evident in non-advective heat flux components. Heat advected into the study reaches via precipitation was negligible, but groundwater inflows added to the heat storage of the water courses in both winter and summer. This effect was most marked at the headwater site and in the summer season because of lower streamflow discharge.