Abstract:
A Watershed is the natural base for studying and modelling
the terrestrial system, because the inputs and outputs are
defined and quantified, and second the integrated system
responses are determinant (Swift and Cunningham, 1986). Watershed
, which is the locus of those points from which runoff, reaches the outlet of the str=!am, is a natural geographical unit with a certain extent of homogeneity and uniformity. This natural unit is easily visualised in a mountain setting whose boundaries for precipitation, evaporation, and subsurface flow are clearly defined by topography. Also, it is an open physical system in terms of inputs of precipitation and solar radiation and outputs of discharge, evaporation and re-radiation. The inter-dependent nature of land and water resources thus necessitates the consideration of watershed as the basic unit in developing planning and the reasons for that include, (i) from the ridge lines everything runs downhill, (ii) this not only include water, soil and pollutants, but also most generally includes transportation route like roads, railroads, and, of course,
rivers, (iii) one of the few thing which generally moves uphill
is the population. As population density increases and as land
resources become scarce the population expand into the upper
parts of the watershed (but remains within the watershed
boundaries), (iv) historically cities have grown up 'around
intersection of rivers and river mouths where they spill into bays and oceans, (v) watershed are contiguous and therefore will aggregate to larger units.
