INTRODUCTION

The previous chapter on natural waters concluded with a discussion of natural ecosystem changes on the earth's surface. This chapter extends that analysis to encompass both intentional and inadvertent modifications of evapotranspiration, plant–soil moisture relations, and related ecosystem processes for human ends. These human ends range from use of forest, grass, and shrubland watersheds to irrigation and drainage schemes in large river basins, to stormwater management and non-point-source pollution control in urbanizing environments.

Every human activity affecting land use and land cover not only affects the hydrologic processes of interception, infiltration, evapotranspiration, aquifer storage, and runoff discussed in the previous chapter, but by altering water quality it also modifies plant–soil–water relationships. In addition to their feedback effects on land use productivity, these hydrologic processes affect natural ecosystems through erosion, sedimentation, and changes in soil organisms and habitats. A cubic meter of soil contains millions of bacteria and fungi, as well as hundreds of springtails, nematodes, and earthworms, all of which are directly affected by everyday soil–water management. In addition to these local effects, this chapter discusses recent research on effects of atmosphere–land surface fluxes on the global hydrologic cycle.

Table 5.1 indicates the extraordinary diversity and plenitude of life in the soil. While algae are primary producers that photosynthesize on wet soil surfaces, most soil organisms are secondary and tertiary consumers, converting dead plants and animals to humus. Through these processes and their movement through the soil, they also affect the porosity and moisture holding capacity.