Color of Water: Is it old wine in a new bottle?

In recent literature color has been added to water.  Blue water is defined as water which can be collected, pumped or transported.  It essentially refers to water in surface and groundwater reservoirs.  Green water on the other hand refers to water held by matric forces in an unsaturated soil, held against gravity.  A few decades ago, we referred to agricultural production based on water in the unsaturated zone as ‘rain-fed agriculture’, and agriculture based on water in reservoirs as ‘irrigated agriculture’.

Color of Water
Photo Credit: McKay Savage on Flickr

 

Hydrologists are fully aware that the water held in the soil matrix will percolate to groundwater and recharge the aquifer if additional water infiltrates.  Otherwise, soil water held within an evaporating plane (the depth of this plane will depend on the depth of the root-zone, soil texture and structure) will move to the soil surface against gravity, consumed by roots if present, and evaporate or transpire.  Water held below this plane will move very slowly towards the aquifer and become groundwater.  Studies conducted in the Mallee Region of Australia show that it could take decades for this water to reach groundwater depending again on soil type and depth to groundwater.  Therefore, most of the water in the unsaturated zone will one day become (blue?) water that can be pumped or transported.

So, by adding ‘color’ to water, have we advanced our ability to manage agricultural water demand better?  The key challenge in rain-fed agriculture is to conserve water in the root-zone by minimizing soil evaporation—but we don’t yet have a color for this type of water!  Adding color to water will not to minimize soil evaporation.  Instead, it will give an incorrect (and over-simplified?) understanding of water as a resource to non-hydrologists.  These non-hydrologists could be key decision makers!
Additional Resources:

Virtual water and water footprints: Compelling notions, but notably flawed by Dennis Wichelns

Virtual water: A helpful perspective, but not a sufficient policy criterion by Dennis Wichelns