New terms for the small water cycle

green rain, green rain cycle, aquifer rain, basin loops, and potential precipitation recycling

One of the earliest uses of the word precipitation recycling, for water that evapotranspires from the land, and then falls back onto the land, was made in 1991 by Kaye Brubaker, Entekhabi, and Dara Entekhabi, and P.S. Eagleson, who were professors in the deparment of civil and environmental engineering at MIT. At that time, a lot of the hydrology field was housed in engineering departments, because traditionally hydrology had been about human-made dams and aqueducts. But in 1980s a breakaway branch of hydrology also began to look at how nature shaped the water cycle. Then these MIT professors became intrigued with how hydrology connected not just ecology, but also with atmospheric science. They became interested in how water from the land could go up into the atmosphere and then come back down as rain.

Brubaker, Entekhabi, and Eagleson wrote in their paper “Estimation of Continental Precipitation Recycling” :

“The total amount of water that precipitates on large continental regions is supplied by two mechanisms: 1) advection from the surrounding areas external to the region and 2) evaporation and transpiration from the land surface within the region. The latter supply mechanism is tantamount to the recycling of precipitation over the continental area. The degree to which regional precipitation is supplied by recycled moisture is a potentially significant climate feedback mechanism and land surface-atmosphere interaction, which may contribute to the persistence and intensification of droughts.

The results indicate that the contribution of regional evaporation to regional precipitation varies substantially with location and season. For the regions studied, the ratio of locally contributed to total monthly precipitation generally lies between 0.10 and 0.30 but is as high as 0.40 in several cases.”

They were interested in how much evapotranspiration went up, and then came back down in the same region.

Another hydrology professor, Hubert Savenije, proposed in the late 1990s another way to look at precipitation recycling, he suggested that what was key was how much evapotranspiration that went up, and then came back down on the same continent. That’s because its important to know, for instance, how much evapotranspiration from California, comes back down anywhere in the US, because that means the land-use in California affected rain elsewhere in the US. His student Ruud Van der Ent in 2010, would calculate how much evapotranspiration from one part of a continent would come back down somewhere else. They found that about 50-60% of the evapotranspiration from the US west coast would come back down as rain somewhere in the US. So if we drain aquifers, pave over lands, chop down forests on the west coast of the US, it will lessen rain inland. It will lessen rain to the Colorado river, which provides water for 7 of the US southwest states.

In the year 2000, Michal Kravick made the phrase the small water cycle more popular in his co-authored booklet New Water Paradigm. Small water cycle meant the same thing as precipitation recycling. Small water cycle became more the term of choice in the eco-crowd.

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As fields develop, they often create new terms for different parts of their objects of study, as a more complex and nuanced understanding of the field unfolds.

In this vein, I would like to suggest some terms that might be useful to distinguish different parts of the hydrological cycle, and in particular the small water cycle. Let me know which of the terms below you like, or find useful, or maybe have an even better term for in the comments below.

Green rain : Green water is water associated with vegetation and soil. So green rain is rain that comes from vegetation or soil.

Green water cycle / Green rain cycle : The small water cycle that comes from evapotranspiration from vegetation and soil.

We can distinguish between a local small water cycle (related to Brubaker’s idea of precipitation recycling) and a continental small water cycle (related to Savenije’s idea of precipitation recycling). I like the idea of describing the rain that comes back down in its own watershed. This could be called the Watershed Cycle. I had a brainstorming session with one group, where we came up with another name for the Watershed Cycle - Basin Loop. Pat Keys, who worked with Ruud Van der Ent, showed that 19 of the 29 major cities of the world, could get a lot of extra water for their cities if they restored land in their watersheds, i.e. increased their basin loop.

Aquifer rain: Trees bring up a lot of groundwater in the summer and then evapotranspire it up into the air. Some of that then becomes rain. Rain that originates from groundwater I’ve given the term aquifer rain. The groundwater-atmosphere water cycle is where rain oscillates between the aquifer and atmosphere.

Pat Keys also came up with the phrase precipitationshed for the area of land where water evapotranspired from to create rain for a particular place. So the precipitationshed for Cairo would be all the land whose evapotranspiration contributed to rain in Cairo.

His term makes me think about another possible term. Aquifershed, would be the area of aquifer that contributed to the rain in a particular place.

There is an interesting concept called potential evapotranspiration, which is the maximum amount of evapotranspiration that can come from the land, given enough moisture in the soil. (Its not infinite, because there is only so much of the sun's energy that can evaporate the water). This concept helps form one of the few unified laws of rain and evapotranspiration - the Budkyo’s curve In the tropical rainforests, there is more than enough water, so potential evapotranspiration is pretty close to actual evapotranspiration. In the desert though there is not enough water to evaporate, so actual evapotranspiration is much less than potential evapotranspiration.

[Budkyo Curve from Creed and Spargo ]

The potential evapotranspiration idea made me think, we could also define a concept called potential precipitation recycling (or potential small water cycle, maximum small water cycle, maximum rain). This would help clarify the maximum rain we could get if we restored the land so that it was able to absorb the rain fully, and if we replenished the aquifers. It gives us a goal to get to in our eco-restoration projects.

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This is a reader supported publication, I am thankful for those who contribute financially. It helps support this writing.

REFERENCES

KL Brubaker, D Entekhabi, PS Eagleson in 1991 discussed recycled precipitation as that from precipitation of terrestial origin vs ocean origin. Calling it precipitation recycling, and also using the term continental precipitation recycling

The Precipitationshed: Concepts, Methods, and Applications, Keys, Patrick W. Stockholm University, Faculty of Science, Stockholm Resilience Centre.ORCID iD: 0000-0002-7250-1563

Savenije, Hubert HG. "New definitions for moisture recycling and the relationship with land-use changes in the Sahel." Journal of Hydrology 167, no. 1-4 (1995): 57-78

Van der Ent, R. J., and H. H. G. Savenije. "Length and time scales of atmospheric moisture recycling." Atmospheric Chemistry and Physics 11, no. 5 (2011): 1853-1863.

Van der Ent, Rudi J., Hubert HG Savenije, Bettina Schaefli, and Susan C. Steele‐Dunne. "Origin and fate of atmospheric moisture over continents." Water Resources Research 46, no. 9 (2010).