Evaluation of regional acid sensitivity predictions using field data: issues of scale and heterogeneity

An acid waters sensitivity map has been widely used as a background resource for assessing potential impacts of forest management on stream water acidity within Wales. The validity of this approach has been explored by comparing predicted acid sensitivity with the observed acid neutralisation capacity (ANC) in stream runoff using data sets from two regional surveys of a major area of acidification concern in Wales. For catchments in excess of 20 ha, the acid sensitivity predictions are generally upheld, although biologically important extremes of acidity are not predicted. For smaller catchments of 2 to 5 ha area, the large observed variations in ANC are poorly predicted from the acid sensitivity map; there is no statistically significant relationship between predicted sensitivity and measured baseflow and stormflow ANC values for the catchments. This reflects the high degree of heterogeneity within catchments and the complexity of soil and groundwater interactions. The maps offer a starting point for assessing sensitivity. However, they provide no more than a very crude indication at the scale of interest to forestry managers. It is concluded that a more appropriate route for assessing sensitivity is via field measurement and the direct use of water quality and biological information.