Effects of urban vegetation on the hydrologic cycle: runoff point of view Introduction Increased urbanization results in increased impervious surface area and thus increased runoff leading to impaired water quality, threatened water supplies, and reduced groundwater recharge . Urban forest canopies are effective at reducing runoff due to rainfall storage on leaves until removal by evaporation, and channelization of water down stems and trunks into the soil and away from impervious surface . This study aims to investigate the effects of urban vegetation on the hydrologic cycle from runoff collection point of view. Some concepts regarding to the evapotranspiration and runoff collection will be conducted. These resources could be used as references in urban vegetation design in order to reduce production of runoff and also to improve the environmental quality. Factors Affecting Evapotranspiration Availability of soil moisture: As the soil dries down from field capacity, the rate of absorption of water is reduced because of increased resistance to water movement in the soil and within the tree as well as loss of soil-root contact. Soil aeration: Poor soil aeration in compacted or flooded soils decreases water absorption by inhibiting root growth, inducing decay of roots, and suppressing development of mycorrhiza. Soil temperature: As the soil temperature decreases from 25 to 5 degree Celsius, the resistance to water flow through the roots is approximately doubled. Concentration of salts in the soil water: High concentrations of deicing salts and fertilizers in the soil solution may reduce absorption of water by osmotic effects. Root length distribution: the roots function primarily as passive absorbing organs and the rate of absorption of water is controlled by transpiration . Root Distribution
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