Visualizing Water – Slide 3

We also extract a significant amount of blue water for our use, both from surface water (rivers and lakes) and groundwater. This is what hydrologists mean by water WITHDRAWALS, i.e., the act of withdrawing water from the annual flows of blue water. If you look to the left side of the diagram, where the pink flows are pulled out from the blue water flows, you can see that we withdraw almost 3,000 BCM of surface water and a little more than 1,000 BCM of groundwater each year. This is equivalent to 4% of the total volume of rain and snow falling each year.
We use that blue water for all kinds of purposes. For irrigation. For cooling our electrical plants. For drinking and sanitation. For industrial processes. Once we’re done using it, one of two things happens to the water. It either evaporates to the atmosphere or it flows back into rivers and lakes, where it is potentially available for other uses before it finds its way to the ocean and evaporates, eventually falling back to earth as rain or snow.
If you look on the right of this diagram, you can see that hydrologists refer to these two different paths that blue water can take after we use it as CONSUMPTION or RETURN flow. Consumption means that we consumed the water by evaporating it to the atmosphere. As a result, it’s no longer available to us in rivers and lakes. RETURNED water goes back to rivers and lakes, so it is still potentially available to us for other uses before it ends up in the ocean. Think of it this way: WITHDRAWALS – RETURNS = CONSUMPTION.
That’s the global water picture. All in all, we make direct use of 9% of annual rain and snow, about half of it in the form of green water used in rainfed agriculture, and about half in the form of blue water that we withdraw from rivers, lakes, and aquifers. Of all the blue water that we withdraw, about one quarter is CONSUMED through evaporation to the atmosphere, and the remaining three quarters or so is RETURNED to rivers and lakes. 36% of all the water falling from the sky onto the land surface eventually flows back to the ocean.
It’s worth noting that we usually don’t return water in the same condition, and sometimes not even at the same location, that we withdrew it. Sometimes we return it at a higher temperature, or with added chemicals or nutrients. All of these changes can affect the health of the rivers and lakes to which this water is returned, and its potential to be used again downstream.