Vegetation-mediated impacts of trends in global radiation on land hydrology: A global sensitivity study

Incident solar radiation has changed in the last 50 years, as an initial dimming trend from 1960 to approximately 1990 was followed by an ongoing brightening period, with concomitant changes in the partitioning between direct and diffuse fractions. Such radiation changes are expected to affect the global water cycle. In this study, we use the Community Land Model (CLM) to perform global offline simulations for the period 1948-2004 and study the effects of solar forcing changes on trends in evapotranspiration and runoff. The modeled components of the hydrologic cycle respond strongly to the imposed radiation changes in several regions, especially in the tropics. Exceptions are regions with soil moisture-limited evapotranspiration regime, such as the U.S. Great Plains. In Europe and the Eastern US, the imposed 7 W m ^-2 solar dimming for 1960-1990 leads to an evapotranspiration reduction of 1.5 W m ^-2 or approximately 5% of the mean and an enhancement of runoff by equal percentage. In these regions, the imposed 6 W m ^-2 solar brightening leads to a 3 W m ^-2 increase of evapotranspiration in 1990-2004, and a runoff reduction of between 7 and 10% of the mean. Additional simulations investigating the impact of higher diffuse radiation fraction during 1960-1990 suggest mostly an increase of evapotranspiration in the tropics of 2.5 W m ^-2 (3% of mean) due to increased photosynthesis from shaded leaves, but with smaller opposite effects elsewhere because of lower ground evaporation. The runoff trend resulting from the imposed radiation/aerosols effect is of the same sign and approximate relative magnitude (but larger absolute magnitude) as those calculated, in various studies, for other potential drivers of runoff change such as climate, CO ₂, or land use. These results thus strengthen the claim that radiation effects on runoff are not to be neglected. Understanding the impacts of radiation on the water cycle will affect projections of river flow and freshwater availability for human consumption.