Rising atmospheric carbon dioxide concentrations may increase streamflow

Abstract

Historically, studies of the greenhouse effect of carbon dioxide (CO2) have dealt primarily with temperature and only secondarily with precipitation. In the latest report on this topic1, however, the subject of streamflow is broached with an analysis2 which suggests that watersheds in the western United States will suffer 40–75% reductions in streamflow for a doubling of the atmospheric CO2 content, leading to a 2 °C rise in air temperature and a 10% drop in precipitation. A shortcoming of that study is that it does not include the direct antitranspirant effect of atmospheric CO2 enrichment that would accompany any CO2-induced climatic change, whereby increasing CO2 content of the air tends to induce partial stomatal closure, so reducing plant transpiration and thereby conserving soil moisture and increasing runoff to streams. Inclusion of this latter effect in a simple model of watershed runoff applied to 12 drainage basins in Arizona indicates that 40–60% increases in streamflow may well be the more likely consequences of a CO2 concentration doubling, even in the face of adverse changes in temperature and precipitation.