A probabilistic approach to disentangling climate change & land use change effects on river flows

Determining the respective attribution proportions of climate change and land use change to streamflow changes in river systems is of increasing interest to researchers and practitioners tasked with managing river basins. We propose an extension to established techniques of attributing the relative proportions of climate change (CC) and land use change (LUC) drivers to streamflow change by instead considering the proportions as distributed through a probability density function rather than as a point value. The novel method is demonstrated for parent catchments (catchments not nested within any other and not sharing any water flows with any other catchment) across Scotland. Results are determined by the flow, temperature and precipitation data, and by analysis of the change in these vectors. The ratio of the LUC and CC attribution proportions (LCAP) is then more appropriately expressed as a vector of values, each associated with its own probability value within a probability density function (pdf).  Results demonstrate that the LCAP ratio pdf can vary considerably through time, can be expressed as a probabilistic estimate within confidence limits and that it is possible to track physical changes in the catchment in the evolution of the probability density function.  Particular metrics for the LCAP ratio, such as the median value through time, can be derived from the pdf.  The LCAP ratio resulting from analysis is also a function of the flow metric chosen – change in high flows (e.g. Q05) is generally more driven by CC whereas low flows (e.g. Q95) are more driven by LUC.  It can also be concluded, with a high degree of confidence, that for Scottish catchments in general, and for much of the time, both CC and LUC are significant drivers of streamflow change, but it can also be shown that there is a relationship between the magnitude of the LCAP ratio and certain physical catchment descriptors such as area, median catchment height or shape compactness. Hence, these findings may have implications for future catchment flood management utilising nature-based solutions to reverse landscape degradation and mitigate effects of climate change, provided that the economic and social costs are outweighed by the benefits.