According to the United States Department of Agriculture, “Vegetation and plant debris slow surface runoff, encouraging sediment and sediment-bound contaminants to settle before entering surface water. Once in the soil, contaminants can be immobilized and transformed by soil microbes or taken up by vegetation. Groundwater flowing through the root zone is also filtered by these processes. Additionally, trees can trap windblown dust before it enters stream and lakes.”
Trees and forest areas also help filter water used for drinking water purposes in New York’s water supply system – the largest unfiltered water system in the United States. The water system delivers 1.2 billion gallons of water each day to 9 million residents. New York manages this because it gets water from watersheds where forests, swamps and soils act as natural filters, removing pollutants.
New York’s water supply is drawn from three watersheds, the Delaware, Catskill and Croton rivers. In 1997, the city made a revolutionary decision to preserve its natural water sources rather than spending 8-10 billion dollars on a new filtration plant. In comparison, maintaining the natural ecosystem of the watersheds was estimated to cost only 1.5 billion dollars. Among various initiatives, one aspect was that the city paid landowners in the watersheds to keep the forests as they were.
However, of the three watersheds, the Croton river region was already much more populated and degraded, with more paved surfaces and increased stormwater runoff and pollution, compared to the other two river areas. This necessitated the construction of a filtration plant for the water coming from the Croton river area, but not for the other two. This illustrates the difference in water quality and subsequent cost to the exchequer when forests are lost or degraded.
Most states in the United States encourage the planting of trees along rivers and streams, and several states have incentives for farmers doing so. For example, streamside tree planting has been used in Maryland and other US states bordering Chesapeake Bay to reduce pollution from agricultural and pesticide runoff. The states have planted over 8000 miles of trees along riversides, and studies showed reductions of up to 88% of nitrate and 76% of phosphorus from fertilizers used in adjacent fields because of filtration by the trees. Plans include planting 15,000 more miles of streamside buffers.
Besides directly filtering the water before it enters streams from fields, trees also help indirectly. Pollution entering rivers is especially troublesome when rivers have insufficient water. When water volumes in a river are high, pollution and sewage are diluted to some extent. While treatment of pollution is essential, a river in full flow can have a mitigating effect on pollution, according to the Central Pollution Control Board of India.
Depending on the species, trees that shade crops can reduce evapotranspiration. The water use efficiency of crops also increased in the presence of trees, which act as shelters that create a conducive microclimate around crops. Increased water use efficiency means more growth per unit of water.