Changing streamflows: how much is due to climate?

Posted on January 5, 2013

By affecting evaporation, transpiration and the amount of water stored as snow, climate change is likely to have significant impacts on streamflow.  Although this topic has been the subject of several scientific studies, it seems that there’s scope for more nuanced analyses of climate – flow relationships.  It’s commonly assumed that observed historical trends in hydrology, such as increasingly severe summer droughts and the earlier timing of snowmelt and spring flows, are evidence of climate warming, but the complicating effects of other factors have tended to be overlooked.  In a recent commentary paper, Prof. Julia Jones of OregonStateUniversity noted that observed trends in streamflow can also reflect the responses of vegetation to past disturbances.  For example, the harvesting of forests can bring forward the timing of snowmelt and peak streamflow by several weeks, and changes in plant composition during the process of forest succession after fire, insect outbreaks or volcanic activity can lead to gradual increases or decreases in streamflow.   Another complicating factor is the capacity of vegetation to adapt to climate change (for example, by restricting transpiration in warmer conditions), which can result in no detectable change in streamflow even though climatic warming is occurring.   A third consideration is that gradual changes in human uses of land and water, such as the development of infrastructure for flood control & water supply, can generate historical trends in streamflow that are correlated with, but not caused by, climate change.  For instance, in many parts of the USA, the construction of dams and reservoirs has reduced maximum flows and increased minimum flows.   Therefore, since streamflow records are affected by a number of factors, it’s important to recognise the geographical context and consider how ecological and social processes affect our interpretations of the effects of climate on hydrology.

Reference:  Jones, J.A.  2011.  Hydrologic responses to climate change: considering geographic context and alternative hypotheses.  Hydrological Processes 25, 1996–2000.