Revealing the impact of forest exotic plantations on water yield in large scale watersheds in South-Central Chile

Land-use and forest cover change play important roles in socio-economic processes and have been linked with water supply and other ecosystem services in various regions of the world. Water yield from watersheds is a major ecosystem service for human activities but has been dramatically altered by landscape management superimposed on climatic variability and change. Many studies from different regions of the world have documented that in small watersheds (<100 ha) fast-growing forest plantations reduce water yield. Nevertheless, these effects have not been adequately documented in large watersheds >10,000 ha. In this paper, we examine the temporal variation of the residuals between best-fit precipitation-runoff relationships and instrumental streamflow records for two large watersheds (Purapel en Nirivilo, PNN and Cauquenes en el Arrayan, CQA) located in the Mediterranean-climate coastal range of South-Central Chile. In these watersheds, high resolution satellite imagery shows a decline in native forest cover from 52.3% to 14.2% for PPN and 36.1% to 8.1% in CQA, between 1975 and 2000. Conversely, in the same period the percentage area covered by forest plantations, mainly Pinus radiata, increased from 12% to 55% in PPN, and 4.7% to 42% in CQA. We observed a decreasing trend in summer runoff residuals regressed against annual precipitation in the same period, with slopes significantly different from zero for PPN (p = 0.035) and CQA (p = 0.008). We interpreted this pattern as an evidence of change in the hydrological regime in these watersheds as a consequence of forest cover and land-use changes. From a reanalysis of the observed data we estimate a decrease in runoff from 13.1 to 7.5 mm/summer for PPN and from 7.3 to 5 mm/summer for CQA, refer to the period 1991-2000 compared to 1981-1990. Multiple regression analyses of annual and seasonal flows show that besides precipitation, percentage-cover of forest plantations is a statistically significant predictor of summer flow with a partial negative correlation of -0.45 and -0.44 for PPN and CQA, respectively, p < 0.05. This study clearly shows the important effect that land-use change can have on water yield and to our knowledge this is the first study documenting the decrease in summer runoff in a landscape where native forest cover has dramatically declined and forest exotic plantations have expanded. Similar methods could be used elsewhere to inform policy and decision-making regarding forest and land-use planning. (C) 2009 Elsevier B.V. All rights reserved.