Introduction

Forest ecosystems strongly affect the water cycle through their impact on evapotranspiration, precipitation, infiltration, runoff, and, consequently, soil erosion and stream-flow (see following sections). The removal of forest vegetation leads to an increase in water yields (e.g., Bosch and Hewlett, 1982; Section 2.4) and a shift in the predominant mechanism of runoff generation (Dunne and Black, 1970a; Dunne, 1978; Section 2.3). It also enhances snowpack accumulation and shortens the snowmelt season (Section 2.4). Moreover, in deforested watersheds, evapotranspiration is strongly reduced (Section 2.7). The impact on precipitation is more complex: Large-scale (i.e., >105 km2) deforestation is expected to reduce regional precipitation (e.g., Bonan, 2008a), though the effect of forest removal also depends on synoptic patterns of atmospheric circulation and geographic setting (e.g., latitude, location with respect to mountain ranges and oceans). The deforestation of small watersheds (<10 km2) is not expected to have a substantial impact on precipitation, whereas the clearing of intermediate sized areas (15,000–50,000 km2 [Lawrence and Vandecar, 2015]) could increase local precipitation (see also Chapter 4). Deforestation can also affect the hydrologic conditions and microclimate of nearby (downwind) ecosystems (Ray et al., 2006).

Landmasses receive water as precipitation and lose it either as water vapor fluxes into the atmosphere (evapotranspiration) or as surface and subsurface flows in the liquid phase (runoff). In recent years, these two fluxes have been named green water and blue water flows, respectively, to stress the fact that evapotranspiration receives a strong contribution from vegetation (transpiration) (Falkenmark and Rockstrom, 2004; Figure 2.1). As explained in the following sections, the overall effect of deforestation on the hydrologic cycle is a decrease in water vapor fluxes and increase in runoff. Thus, green water flows decrease and blue water flows increase. This means that more water is likely to become available for societal withdrawals (but also for environmental uses) in areas located downstream from the cleared watershed. In turn, forest management can strongly impact the water resources of a watershed, and sometimes the thinning of woody vegetation has been proposed as an option to increase water availability in semiarid areas (e.g., Ingebo, 1971; Griffin and McCarl, 1989). Such an approach, however, has only limited applicability because forest removal and the consequent increase in overland flow have the effect of increasing sediment yields and soil erosion rates, thereby damaging the landscape, often irreversibly within human timescales.