Inter-annual variability of the global terrestrial water cycle

Variability of the terrestrial water cycle, i.e. precipitation (P), evapotranspiration (E), runoff (Q) and water storage change (1 Δ) is the key to understanding hydroclimate extremes. However, a comprehensive global assessment for the partitioning of variability in P between E, Q and 1 Δ is still not available. In this study, we use the recently released global monthly hydrologic reanalysis product known as the Climate Data Record (CDR) to conduct an initial investigation of the inter-annual variability of the global terrestrial water cycle. We first examine global patterns in partitioning the long-term mean P̄ between the various sinks Ē, Q̄ and 1Δ̄ and confirm the well-known patterns with P̄ partitioned between Ē and Q̄ according to the aridity index. In a new analysis based on the concept of variability source and sinks we then examine how variability in the precipitation σ²_P (the source) is partitioned between the three variability sinks σ²_E σ²_Q and σ²_Δsalong with the three relevant covariance terms, and how that partitioning varies with the aridity index.We find that the partitioning of inter-annual variability does not simply follow the mean state partitioning. Instead we find that σ²_Pis mostly partitioned between σ²_Q²_Δsand the associated covariances with limited partitioning to σ²_E. We also find that the magnitude of the covariance components can be large and often negative, indicating that variability in the sinks (e.g.σ²_Q,;²_Δs)can, and regularly does, exceed variability in the source (σ²_P). Further investigations under extreme conditions revealed that in extremely dry environments the variance partitioning is closely related to the water storage capacity.With limited storage capacity the partitioning of σ²_P is mostly to σ²_E, but as the storage capacity increases the partitioning of σ²_P is increasingly shared between σ²_E²_Δsand the covariance between those variables. In other environments (i.e. extremely wet and semi-arid semi-humid) the variance partitioning proved to be extremely complex and a synthesis has not been developed. We anticipate that a major scientific effort will be needed to develop a synthesis of hydrologic variability.