The diagram compares the water cycle of forested area to the water cycle of an urban area.
Overall, it demonstrates the continues movement of water above and below the Earth’s surface. It can be observed that both forested and urban hydrological cycles consist of six major steps, as precipitation, evaporation, transpiration, runoff, infiltration, and base flow. Among these steps, former three steps take place above ground, while latter two steps continue below ground.
With regards to the hydrological cycle in forests, the process begins with precipitation, which delivers water to the Earth. Consequently, a certain amount of water is absorbed by vegetation, permitting excess water to infiltrate through permeable topsoil and through less permeable subsoil on rock. At this stage, infiltrated water unites with the water table and flows towards rivers and sea, known as base flow. After the ground reaches its’ saturation capacity, remaining rainwater continues to run towards the open water bodies as an above ground stream, namely runoff. Thereafter, due to solar energy water is supplied back to the atmosphere by both evaporation and transpiration.
Conversely, in urban environment when rain falls, infiltration of water is lower than in forests due to lower penetration due to constructions and roads, which results in reduced base flow. Instead, considerable amount of water flows above the ground as runoff. However, due to urban waste this runoff is extremely polluted which ultimately contaminate water bodies. Moreover, owing to little or no vegetation in cities, erosion is high, resulting loss of top soil. Ultimately, water is circulated to the atmosphere via evaporation and transpiration, but because of reduced plant coverage and impervious surfaces evaporation and transpiration rates are significantly low in cities compared to forests, further aggravates erosion.