Human activities may change various aspects of the hydrological cycle. Among other consequences this may contribute to climate modification over large areas of the Earth. Direct changes in in surface hydrology over large areas may occur as a result of changes in the vegetative cover or in the area covered by irrigation systems. Tropical deforestation results in significant modifications to surface hydrology, since any disruption in vegetative cover effects the water-storage capacity of the land and modifies the water supply. To the extent that runoff is increased, evapotranspiration is decreased, modifying the time and spatial characteristics of rainfall.