Reconstructing 400 years of streamflow in the upper Llobregat basin (Catalonia, Spain) using stable isotopes in tree rings

The Ter-Llobregat basin system, located in Catalonia, supplies water to more than 100 municipalities, including Barcelona, serving a population of approximately 5 million. Recently, severe water shortages have prompted emergency measures to restrict consumption. Given the limited and fragmented nature of historical streamflow records, a deeper understanding of past hydrological dynamics is crucial. Tree-ring data has shown considerable potential for reconstructing streamflow series from limited instrumental data.

Here we present a four-century streamflow reconstruction for the upper course of the Llobregat River basin using streamflow data and stable oxygen (δ¹⁸O) and carbon isotope (δ¹³C) tree-ring chronology from a Pinus uncinata forest growing on the Pedraforca mountain (42°14′23.79″N, 1°42′10.58″E, 2,497 m.a.s.l.) in Catalonia, Spain. For the same period, we also examine historical flood events that occurred across the basin based on typology (fluvial or pluvial), intensity, and seasonality.

Despite the challenges posed by short, discontinuous instrumental records, we obtained robust mean reconstruction statistics with models built using 24 years of non-continuous streamflow data applying a leave-N-out calibration-validation procedure. This demonstrates the potential of the tree-ring isotopic chronologies. Indeed, the δ¹⁸O and δ¹³C records achieved significant correlation values up to –0.671 and –0.71, respectively, with mid-to-late summer total runoff streamflow records, while the ring-width chronology shows no-clear sensitivity to hydrological variability. The resulting streamflow reconstructions span the period 1600–2002. The δ¹⁸O-based model explains about 25% of the variance in the instrumental period, whereas the δ¹³C-based model explains up to 40%, providing valuable insight into wet and dry periods and extreme years over the past four centuries. Future work including older trees and additional tree-ring sites in the study region could further extend and improve the streamflow reconstruction beyond 400 years. 

This four-century streamflow reconstruction provides the first benchmark to characterize natural variability and extremes in the upper Llobregat basin. Placing recent hydrological variability in a centennial-scale long-term perspective is essential for improving water resources management approaches and supporting local-scale risk planning.