The technical potential for hydropower worldwide is between 31 petawatt-hours a year (PWh/y) to 127 PWh/y, while the economic potential is between 9 PWh/y to 15 PWh/y, the report notes. Hydropower, which is the dominant component of renewable energy, is also under the threat of climate change. Climate change has a large impact on water resources and thus on hydropower. Hydropower generation is closely linked to the regional hydrological conditions of a watershed and reacts sensitively to seasonal changes in water quantity. Hydropower is one of the oldest sources of mechanical power and the largest source of renewable electricity generation in use. Global capacity is around 1300 GW. Hydropower is site-specific and so each project will be unique. Hydropower plants are classified according to their size into micro, mini, small, and large hydropower. In terms of generating the capacity the large plants are the most important. These can be either dam and reservoir plants or run-of-river stations. The latter are the easiest to construct and least disruptive, but the former stores energy and is therefore much more flexible in the way it can be used. Energy is taken from hydropower plants through turbines and a number of designs such as Pelton, Francis and Propeller turbine exist to exploit different head heights of water. Most hydropower developments have environmental effects which must be taken into account before constructionHydropower potential is a function of available water and suitable terrain The main advantages of hydropower are low unit cost, low emissions, ease of control and storage.  The challenges   of hydropower are huge land requirements for storage, soil erosion, ecosystem disturbance and seasonal variability. About 75,000 hectares of land and 14 trillion liters of water are required on average basis to produce 1 billion kWh annually    Hydropower plants have a reservoir developed behind a dam to supply water to the hydraulic turbine for generation of a highly flexible, dispatchable electricity supply. Hydropower can be combined with wind, solar and other sources to supply reliable steady and affordable grid electricity. Hydropower can also be exploited from run-of-the-river resources which have less environmental impact but with overreliance on steady supply of rainwater whose supply is unsteady and unpredictable. Apart from power generation, reservoirs can control floods, supply water, and power from stored water even during drought. Hydroelectric electric power plants are useful for grid electricity sustainability particularly during peak hours where plants that generate flexible and cheaper electricity are on high demand