Correlation and causation in tree-ring based reconstruction of paleohydrology in cold semi-arid regions

This paper discusses ways in which the tree-ring based reconstruction of paleohydrology can be better understood and better utilized to support water resource management, especially in cold semi-arid regions. The relationships between tree growth, as represented by tree-ring chronologies (TRCs), runoff (Q), precipitation (P), and evapotranspiration (ET) are discussed and analyzed within both statistical and hydrological contexts. Data from the Oldman River Basin (OMRB), Alberta, Canada, are used to demonstrate the relevant issues. Instrumental records of Q and P data were available while actual ET was estimated using a lumped conceptual hydrological model developed in this study. Correlation analysis was conducted to explore the relationships between TRCs and each of Q, P, and ET over the entire historical record (globally) as well as locally in time within the wet and dry subperiods. Global and local correlation strengths and linear relationships appear to be substantially different. This outcome particularly affects tree-ring based inferences about the hydrology of wet and dry episodes when reconstructions are made using regression models. Important findings include: (i) reconstruction of paleoQ may not be as credible as paleoP and paleoET; (ii) a moving average window of P and ET larger than one year might be necessary for reconstruction of these variables; and (iii) the long term mean of reconstructed P, Q, and ET leads us to conclude that there is uncertainty about the past climate. And finally, we suggest using the topographic index to pre-judge side suitability for dendrohydrological analysis.