The water cycle has been taking place for billions of years before humans existed. Today, humans influence the cycle in different ways.
Figure 1: An overview of human influences that impact the water cycle
Forests are an integral part of the water cycle as they intercept water and allow it to slowly infiltrate the soil via stem flow. Humans cause large-scale deforestation through development, logging and agriculture. Deforestation means less water is able to infiltrate the soil causing excessive runoff, which can lead to flooding and landslides.
Urban areas replace forests, grasslands, wetlands, etc. As roads, pavements and buildings are impenetrable, water cannot infiltrate the soil which increases surface runoff, in this environment, this is referred to as urban runoff. Water is also directed to rivers via drains, which decreases the water table and increases flooding.
Groundwater can be used for irrigation systems. If the depletion of groundwater exceeds the rate at which it is replenished, this can lower the water table and cause rivers to dry up.
Pollution in the water cycle
Pollution from agriculture and industry, such as synthetic fertilizer, sewage and industrial waste, seeps into lakes and rivers through surface runoff and soil leaching (or deliberate dumping). The burning of fossil fuels releases pollutants into the atmosphere which can produce acid rain and contaminate water sources.