Test: Introduction to Engineering Hydrology

The percentage of total quantity of fresh water in the world available in the liquid form is about 3%.

The vast majority of the Earth's water is in the form of oceans, which are saltwater and not suitable for drinking or irrigation. Freshwater, which is water that is free of salt and other minerals and is suitable for drinking and irrigation, makes up a very small percentage of the Earth's total water supply. According to the United Nations, about 2.5% of the Earth's water is freshwater, and of that, only about 0.3% is available as surface water in lakes, rivers, and streams, while about 2.2% is found in groundwater reserves. In other words, about 3% of the total quantity of water on Earth is available in the liquid form as freshwater.

The average stream flow

Annual  run off

Whenever precipitation occurs, a part of rainfall is infiltrated into the soil and other part is surface runoff meeting in the rivers. 
Moreover, not all the water evaporated from the sea is transported towards land and most of it fall back as precipitation in oceans itself.
Therefore, it is found that about 9% more, water evaporates from the ocean than what falls back them as precipitation.

Most of the earth’s water sources such as rivers, fakes, oceans, ground water, etc, get their supplies 
from rain, while the rain water in itself is derived from the evaporation from these sources. 
Water is intact lost to the atmosphere as vapour from the earth, which is then precipitated back in the form of rain, snow, hail, dew, sleet or frost, etc. This evaporation and precipitation continues forever and thereby, a balance is maintained between the two. This process is known as hydrologic cycle.