Introduction

Water is the most important component of life on earth. Living organisms are mostly water (e.g. a baby over 70%, lettuce over 90%). All must be able to maintain this content, against the constant pull of evapotranspiration, evaporation and secretion.

The water is replenished, ultimately, from precipitation. Depending on the species, it comes from rain (snow, ice) or dew, and surface, soil or ground water. Where there is life, there is water.

Water therefore passes from rain to river through the riverscape. That which does not, evaporates direct from the rain (varying with climate and surface cover). That which does, may be absorbed above the soil, by plants mostly, or pass above the soil, through it, or through lower levels in the ground (subsoil, aquifer). This last may be little (impermeable rock close to surface) or much (wet peat, alluvium, aquifer).

Sheet flow is usually only in very heavy storms (though these may be frequent, e.g. Chapter 3, R. Waccasassee, or extremely rare). The water may flow to the river or, and more so on steeper land, it may gather into watercourses first. Surface flow, just on or within the soil, is more usual. On slopes, rainsplash and overland flow is discontinuous in space and time, but is a single interacting phenomenon (Petts & Foster, 1985).