ABSTRACT: Hydrological reconstruction from tree rings and varved lake sediments.

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Abstract

The ARCHIVES project aims at reconstructing the annual hydro-climatic variability of the boreal region of the Quebec-Labrador Peninsula (Canada) over the past millennia. The project is based on tree-ring chronologies of more than one hundred years-old black spruce stands, several millennial tree-ring chronologies developed from sub-fossil trees and a network of lake sediment cores over a territory of more than 700 000 km2 (1400km in longitude x 500km in latitude). The dendrochronological network includes various tree-ring proxies such as ring width, ring density and ?13C and ?18O series. An extensive search for annually laminated lakes in the area permitted the identification of several sites with a strong potential for hydro-climatic reconstitution using annual varve thickness, grain size variability and sub-annual lamination as proxies for changes in river competency. We present here a 300 years-long reconstruction of hydrological variables at the watershed scale (annual water supply, spring and summer runoff) and some climate variables used in hydrologic forecast models, including an atmospheric index used by the modelling team of Hydro-Quebec (hydroelectric power supplier) to forecast spring flood volumes using both tree rings and annually laminated sediments. The sensitivity of the tree-ring chronologies and of varved series to hydrologic parameters were tested using statistical response functions.Our reconstruction methodology combines an analogue technique for the estimation of missing tree-ring data with an artificial neural network for optimal nonlinear calibration, including a bootstrap error assessment. Transfer functions were calibrated with water supply and meteorological data provided by Hydro-Quebec, and with Climate Research Unit (CRU) gridded climate data. The reconstructed series were validated using Reduction error (RE) and Root mean square error (RMSE) coefficients, standard cross-validation tests and verified with independent instrumental data. Statistical verification tests processed on the calibration period are very significant. However in order to consolidate our reconstruction and to compensate the few historical hydrological records available as independent data, we reprocessed the hydroclimatic reconstruction using the best analogue transfer function method. Both methods give similar results, validating our general approach. Finally, a direct relationship between clastic varves properties and regional river discharge variability allows us to demonstrate that hydroclimatic reconstructions can be developed from varved clastic sediments and compares very well with tree-ring reconstruction in the boreal region of eastern Canada.

Bibliography of Canadian Geomorphology