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 resourc e for assessing potential impacts of forest management on stream water acid ity within Wales. The validity of this approach has been explored by compar ing predicted acid sensitivity with the observed acid neutralisation capaci ty (ANC) in stream runoff using data sets from two regional surveys of a ma jor area of acidification concern in Wales. For catchments in excess of 20 ha, the acid sensitivity predictions are generally upheld, although biologi cally important extremes of acidity are not predicted. For smaller catchmen ts of 2 to 5 ha area, the large observed variations in ANC are poorly predi cted from the acid sensitivity map; there is no statistically significant r elationship between predicted sensitivity and measured baseflow and stormfl ow ANC values for the catchments. This reflects the high degree of heteroge neity within catchments and the complexity of soil and groundwater interact ions. The maps offer a starting point for assessing sensitivity. However, t hey provide no more than a very crude indication at the scale of interest t o forestry managers. It is concluded that a more appropriate route for asse ssing sensitivity is via field measurement and the direct use of water qual ity and biological information.