Soil moisture controls on water vapor and carbon fluxes in semi-arid regions

Abstract

In this chapter we focus on savannas as a representative semi-arid, or dry-land, cover type. Savannas cover some 20% of the global land mass, including 40% of Africa (Scholes and Walker 1993). The term Savanna is defined by a co-existence of herbaceous and woody vegetation (often referred to as grass and trees, for simplicity) that is largely due to the limited nature of water availability (but also reflecting the imprints of fire and herbivory). In fact, as a type, the savanna is largely unique in the strength of contrast in the important characteristics between its two dominant vegetation types (Scholes and Walker, 1993). Savannas have a significant role in global net primary productivity, second only to tropical forests (Atjay et al. 1987). In contrast to tropical forests, however, these water limited systems exhibit dramatic changes in their structure (e.g. grass and tree densities) and function (e.g. water and carbon fluxes) in response to typical seasonal, inter-annual, ENSO, and decadal precipitation variability (Scanlon and Albertson 2003a). As for the southern African savannas, as an example, the anticipated decreases in regional rainfall (10-20%) and continued increases in population put the global and regional services of these savannas in jeopardy (IPCC 1992; Moleele and Mainah, 2003).