Moisture feedback loops

The small water cycle

A third of Netherlands is under sea level, so the Dutch take the study of hydrology very seriously, with many academics in the field. Hubert Savenije is one of their professors in this field. He is the head of the water resources division at Delft University and the President of the International Association of Hydrological Sciences.

He has penned a well-written paper in a scientific journal that sounds like a manifesto for the small water cycle - the feedback cycle of moisture from precipitation to absorption into the land, to evapotranspiration. An understanding of this small water cycle, this moisture feedback loop is key if we want to understand what we can do about extreme weather and climate change.

Here is an excerpt from Hubert Savenije’s paper ‘Does moisture feedback affect rainfall significantly?’ :

“Desertification, land degradation and the occurrence of droughts are problems of a global dimension which are primarily of anthropogenic nature. Most of the research into these phenomena takes place through the link with global climate change, which can be summarised as the enhanced emission of greenhouse gasses triggering global and regional climate change, which in turn may provoke droughts, desertification and land degradation.

Recent research, however, demonstrates that impacts resulting from land-use change are more important than the greenhouse effect to explain regional climate change, occurrence of droughts and desertification. The most important mechanism, in this respect, is the feedback of moisture to the atmosphere through evaporation from vegetation which is required to sustain continental rainfall. Land use change, in general, reduces moisture feedback and enhances runoff, resulting in reduction of rainfall, regional climate change, occurrence of droughts, land degradation, and desertification. Particularly in semi-arid continental regions this problem is predominant. In the Sahel the importance of moisture feedback has been clearly demonstrated, but these feedback mechanisms are evident on other continents as well (Eastern Europe, Continental Asia, Southern Africa, South America, USA).

In this light, it is remarkable that in the international research environment so little attention is paid to the analysis and quantification of moisture feedback processes, whereas enormous research efforts are dedicated to global warming and GCM predictions, which will not yield any tangible actions to be taken locally. The problem of desertification, however, having its cause in local anthropogenic influences on the land-atmosphere interaction, can be addressed by local and regional management actions. Therefore it is of paramount importance that the physical processes leading to regional climatic change and desertification can be established scientifically and quantified.

In addition to the research in the physics of moisture recycling, there is an important need for research into the policy implications of the link between land and water use, climate and water resources availability. If the impact of land and water use on rainfall and water resources availability can be quantified, than the full environmental costs and benefits of certain land use practices can be determined. Land use practises that minimise runoff, and hence maximise moisture feedback, do not only prevent erosion and nutrient losses, but they also sustain continental rainfall. The policy implications for socio-economic development are both of a national and international nature, which may influence the water resources negotiations between upstream and downstream countries.

It can be demonstrated that in West Africa land- use change has affected climate in the Sahel region, often within countries, but certainly at a river basin scale. This means that national policy adjustments, awareness raising, physical planning, implementation incentives, research, education, training, extension and strategic planning at national and international (river basin) scales, can have a direct effect on restoration of the climatic and hydrological systems."

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References

1. Savenije, Hubert HG. "Does moisture feedback affect rainfall significantly?." Physics and Chemistry of the Earth 20, no. 5-6 (1995): 507-513 https://www.sciencedirect.com/science/article/abs/pii/S0079194696000146 , and for pdf of paper https://sci-hub.yncjkj.com/10.1016/s0079-1946(96)00014-6

2. Water for the recovery of the planet : A new water paradigm http://www.waterparadigm.org/download/Water_for_the_Recovery_of_the_Climate_A_New_Water_Paradigm.pdf

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