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.