The influence of multiyear drought on the annual rainfall‐runoff relationship: An<scp>A</scp>ustralian perspective

TLDR

Hydrological predictions often assume stationarity, yet changing climate may alter runoff generation and the rainfall‑runoff relationship, making accurate streamflow projections and drought adaptation strategies critical. The study examines whether decade‑long rainfall shifts alter catchment‑scale rainfall‑runoff relationships. Using annual rainfall and runoff records from south‑eastern Australia, the authors statistically compare rainfall‑runoff relationships during extended dry periods to the historical norm and investigate how catchment properties and dry‑period characteristics explain observed changes. Protracted drought caused a significant shift in the rainfall‑runoff relationship in about 46% of catchments, reducing runoff for a given rainfall, especially in drier, flatter, and less forested areas.

Abstract

Abstract Most current long‐term (decadal and longer) hydrological predictions implicitly assume that hydrological processes are stationary even under changing climate. However, in practice, we suspect that changing climatic conditions may affect runoff generation processes and cause changes in the rainfall‐runoff relationship. In this article, we investigate whether temporary but prolonged (i.e., of the order of a decade) shifts in rainfall result in changes in rainfall‐runoff relationships at the catchment scale. Annual rainfall and runoff records from south‐eastern Australia are used to examine whether interdecadal climate variability induces changes in hydrological behavior. We test statistically whether annual rainfall‐runoff relationships are significantly different during extended dry periods, compared with the historical norm. The results demonstrate that protracted drought led to a significant shift in the rainfall‐runoff relationship in ∼46% of the catchment‐dry periods studied. The shift led to less annual runoff for a given annual rainfall, compared with the historical relationship. We explore linkages between cases where statistically significant changes occurred and potential explanatory factors, including catchment properties and characteristics of the dry period (e.g., length, precipitation anomalies). We find that long‐term drought is more likely to affect transformation of rainfall to runoff in drier, flatter, and less forested catchments. Understanding changes in the rainfall‐runoff relationship is important for accurate streamflow projections and to help develop adaptation strategies to deal with multiyear droughts.