| Local Experience With Micro-Hydro Technology |
|B. Development of hydropower resources|
The graphical presentation of continent-wise potentials in fig. 2 does of course not show how distribution is within the regions and over time. There are two main factors that determine the generating potential at any specific site: the amount of water flow per time unit and the vertical height that water can be made to fall (head). Head may be natural due to the topographical situation or may be created artificially by means of dams. Once developed, it remains fairly constant. Water flow on the other hand is a direct result of the intensity, distribution and duration of rainfall, but is also a function of direct evaporation, transpiration, infiltration into the ground, the area of the particular drainage basin, and the field-moisture capacity of the soil. Runoff in rivers is a part of the hydrologic cycle in which -powered by the sun - water evaporates from the sea and moves through the atmosphere to land were it precipates, and thence returns back to the sea by overland and subterranean routes.
Source: AMBIO, Vol. 3, No. 3-4, 1974: The Global Freshwater Circulation
Area-wise distribution of river runoff (in mm/year) in fig. 3 gives an indication of the geographical situation of hydro resources in the various parts of the world. It appears that regions around the aequator, Central America and parts of South-East Asia, northern Europe and North America have higher than average runoffs. In large parts of northern Africa (Sahel, Sahara), Arabia, Central Asia, Australia and western North America, as well as southern Africa and America, runoff is far below average. These areas are of little or no interest in the context of hydropower potential. For areas with average and higher runoff, the short-, medium-and long-term variations of flow are of prime interest. It is this local pattern that determines the availability of water to generate power in relation to time and duration. Such variations are subject to the weather regime, i.e. seasons, and a multitude of other factors such as those already mentioned. Generally speaking, perennial rivers with slight flow variations are the most suitable for hydropower development. High runoff variations, on the other hand, make harnessing more difficult, and extremes such as only seasonal runoff and floods impose serious economic and technical constraints on possible utilisation.