Recession constants are non-stationary: Impacts of multi-annual drought on catchment recession behaviour and storage dynamics

Abstract

Recession analysis is commonly adopted to characterise catchment behaviour, particularly in relation to the role of catchment storage in streamflow production. Recession techniques are data hungry and characterise average behaviour over long periods of recorded data, making them generally unsuitable in transient conditions. Nevertheless, in the context of multi-annual drought, sufficient data may be available to characterise temporal change by examining different subperiods. In this work, we assessed changes in recession behaviour of catchments affected by multi-annual drought in south-eastern Australia, focusing on how changes in recession patterns over time correlate to drought-induced shifts in annual rainfall-runoff relationships. We applied two methods, drawn from the vast methodological literature on recession analysis. They consider different information and entail different assumptions; one is based on average daily recession rates and the other on hourly recession slope plots. Despite the differences, their results are consistent: catchments that exhibited significant changes in rainfall-runoff relationship during the drought also display significantly steeper recessions during and after the drought. Subsequent investigation suggests that only a portion of the increases in recession speed are attributable to increased evaporative demand. Rather, they are likely caused by decreased connectivity between subsurface storage and catchment surface water and increased transmission losses through the streambed. With this work, we demonstrate the usefulness of recession analysis to investigate sources and effects of non-stationary behaviour, providing insights into the impacts of long-term drought on streamflow production and catchment storage dynamics.