Climatic variability within the Balquhidder catchments and its effect on Penman potential evaporation

Abstract

The Penman estimate of potential evaporation, E T, is widely used as a reference level for comparison and subsequent extrapolation of the results of catchment water use studies. The steep topography and 600 m altitude range of the Balquhidder catchments presented difficulties in determining representative values of E T and in the selection of the most appropriate sites for data collection. A network of three automatic weather stations, subsequently increased to four, was installed to sample climate and E T variability within the catchments. For comparison, an additional station was installed at a valley-bottom site typifying the probable location of a conventional meteorological site in a less intensive study. Despite reliability problems with the original logging equipment, the initial data led to well-defined between-site relationships which were used to infill gaps in the E T records. These synthesised records revealed that, contrary to expectations, the high-altitude exposed sites gave significantly higher E T values than the valley-bottom sites. Data obtained using more reliable logging equipment over a recent 12 month period have been analysed in some detail. The results confirm the higher E T values from the high-altitude sites, identify factors contributing to the trend and suggest that E T values obtained from valley-bottom meteorological stations, or from present methods of extrapolation from lowland sites, significantly underestimate potential evaporation in this type of mountain terrain.