Dr. Heidi Peterson mentions that about 39% of the rainfall over land is available as water for human use. However there is much less than that in many areas. What are some potential reasons for that?

1.) High rates of Evaporation.

2.)uneven distribution of water geographically

Explanation:

When there is warming atmosphere there will be more evaporation, that's more water is available for precipitation. For instance, every 1°F increase in temperature, the atmosphere can hold around 4 percent more water vapor, which leads to heavier rain and increases the risk of flooding of rivers and streams.

Higher Evaporation

Availability of water at the location

Presence of Pollutants

Explanation: Rainfall is one of the main ways that the water system replenishes water that is available to human use. Another form is snow.

There is an uneven distribution of rainfall that is available for human use in different locations. One of the factors that can contribute to this is a higher rate of evaporation such as is the case in tropical regions.

Another is low availability of water at a location. Arid locations, for instance require much more rainfall to have enough to replenish surface water, this will in turn reduce the amount available for human consumption.

Presence of pollutants, natural such as clay in water, and artificial such as human/industrial waste is another factor that reduces the percentage of rainfall water available for human use.