Improved methods to assess water yield changes from paired-catchment studies: application to the Maroondah catchments

The statistical methods underpinning analysis of streamflow data from paire d-catchment studies have not changed much since the 1960s. Whilst such anal yses are widespread in hydrologic practice and research, attention is rarel y given to the problems of heteroscedacity, seasonality, serial correlation , and non-normally distributed variates. Each of these problems can potenti ally invalidate the basic assumptions upon which traditional statistical me thods are based. We describe methods to contend with some of these problems and apply them to mountain ash (Eucalyptus regnans) forested catchments in the Maroondah Basin, south-eastern Australia. A seasonal regression model with lag-one auto-regressive (ARI) error was developed to predict monthly s treamflow at treated catchments based on streamflow data from a control cat chment. It is particularly well-suited to situations where little pre-treat ment data is available. Differences between observed and predicted streamfl ow were used to quantify the effect of forest treatment on streamflow. Resu lts from two catchment groups broadly matched the trend predicted by a prev ious regional model, with 2-3 year increases in streamflow, followed by dec reases over the following one or two decades. A third group of catchments a lso showed initial increases, but expected subsequent decreases in streamfl ow were offset by the flow-increasing effects of insect infestations. (C) 2 001 Elsevier Science B.V. All rights reserved.