Mean annual rainfall in the Margaret River catchment has declined by 11% for the period 2001-2014 compared to the long-term record (1975-2000) and mean annual flow has decreased by 50% over the same period (Greening 2015). Prior to 1985 flow was continuous throughout the year. The continuous flow period (days with flows greater than 0 ML/day annually) has declined by 22% for this period; possibly due to a reduction in groundwater inflow (Greening 2015).
This follows a general decline in annual stream flow for river systems across the Whicher plan area of between 25% and 46% for the same period (DoW 2013). A concomitant decline in mean annual rainfall of between 5 and 24% has also been recorded, and although it was not possible to separate the influence of drying climate and abstraction on the reduction in flows, drying climate was considered the dominant factor because reductions in flow had been observed at gauges affected by water use and also in catchments where there was no water use (DoW 2013).