A 963-year reconstruction of summer (JJA) stream flow in the White River,Arkansas,USA,from tree-rings

An average of three baldcypress (Taxodium distichum) ring-width chronologies was used to recon struct total summer (JJA) mean daily flow of the White River at Clarendon, Arkansas, for years 1023–1985. A quadratic transformation of the tree-ring data accounted for 68% (R² adjusted for degrees of freedom lost) when regressed against total summer flow 1931–1985. The distribution of the quadratic reconstruction matched the gauged distribution much better than a simple linear model which only accounted for 62% of the variance. The model was validated by comparing regression estimates against independent data. Years with summer flow below the 25th percentile occur nonrandomly, i.e., they tend to cluster, in both the reconstructed and gauged data. Hydrologic regimes have apparently varied considerably in the past on annual to century time-scales, with extended dry and wet periods that exceeded anything in the modern record. The frequency of both wet and dry extremes has varied considerably over the last millennium. The eleventh through thirteenth cen turies were not analysed due to reduced replication, but the well-replicated fourteenth and twentieth centuries both have large numbers of extremes. The twentieth century appears to have more extreme low flows than the previous centuries and also to have a large number of high flows. The practical consequences for society of variation in extremes and persistence of low flows may be considerable. Climatic change or anthropogenic changes to the watershed (e.g., widespread upland clearing for agriculture and logging of bottomland forests) may be responsible for the change in hydrologic regime during the twentieth century.