7.2.3 Observational Basis for the Effects of Land Surface on Climate

7.2.3.1 Vegetative Controls on Soil Water and its Return Flux to the Atmosphere

Scanlon et al. (2005) provide an example of how soil moisture can depend on vegetation. They monitored soil moisture in the Nevada desert with lysimeters either including or excluding vegetation and for a multi-year period that included times of anomalously strong precipitation. Without vegetation, much of the moisture penetrated deeply, had a long lifetime and became available for recharge of deep groundwater, whereas for the vegetated plot, the soil moisture was all transpired. In the absence of leaves, forests in early spring also appear as especially dry surfaces with consequent large sensible fluxes that mix the atmosphere to a great depth (e.g., Betts et al., 2001). Increased water fluxes with spring green-up are observed in terms of a reduction in temperature. Trees in the Amazon can have the largest water fluxes in the dry season by development of deep roots (Da Rocha et al., 2004; Quesada et al., 2004). Forests can also retard fluxes through control by their leaves. Such control by vegetation of water fluxes is most pronounced for taller or sparser vegetation in cooler or drier climates, and from leaves that are sparse or exert the strongest resistance to water movement. The boreal forest, in particular, has been characterised as a ‘green desert’ because of its small release of water to the atmosphere (Gamon et al., 2003).