How the small water cycle impacts climate change

Increasing precipitation recycling through restoring our landscapes can lessen floods, droughts, and global warming.

The small water cycle has a significant impact on floods, droughts, and global warming, and yet knowledge of this cycle has yet to enter the mainstream climate discourse. The small water cycle is the cyclical movement of water between land and air, where it evapotranspires into air, comes down again as rain, and then evapotranspires back up again. Currently about 65% of our rain comes from the small water cycle [ref 1], and 35% from the large water cycle ( ocean-land-ocean cycle). The small water cycle, aka precipitation cycling, is intimately linked with how well the rain can infiltrate into the earth. As we degrade our lands, and lower its ability to absorb rain, there is less water available to evapotranspire to form precipitation again, and so the small water cycle decreases.

A decrease in soil quality correlates with a decrease in the small water cycle. The decrease of soil quality, means that rain from large atmospheric rivers that blow in from the ocean in the wet season do not get absorbed into the land, and instead run-off downhill to create floods in urban areas at lower altitudes.

A decrease in soil quality and soil absorbency means less water is available in the land to evapotranspire to create rain, thus increasing the amount of droughts in the dry season.

When plants evapotranspire they convert liquid water to water vapor, which takes heat from the land, in the same way that sweat takes heat from our bodies. That heat is stored in the bonds of the vapor water molecule as latent heat. Convection then brings these water molecules high up into the atmosphere, where it condenses into clouds, and releases its latent heat as sensible heat (sensible heat is the jiggling around of atoms). Some of this sensible heat then turns into thermal radiation, radiating out into space and cooling the earth. Because this thermal radiation happens at higher altitudes, it does not have as much greenhouse gases in its path to block its exit into space.

The greater the magnitude of the small water cycle, the more loops of this convective cycle there are, the more heat is transported from the surface of the earth up into the atmosphere, and then into space. So larger the magnitude of the small water cycle, the less global warming there is.

The greater the magnitude the small water cycle, the more rain there is, and the more the vegetation grows. This means the carbon cycle increases, and carbon is sequestered (brought down) from the atmosphere. The more carbon is sequestered, the less global warming there is.

The greater the carbon cycle, the more the organic matter content increases in the soil. The soil is then able to absorb more rain, and the land then has more available water to evapotranspire into the atmosphere, thus increasing the small water cycle.

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References

[1] K.E. Trenberth, J. Fasullo, and J Mackaro, 2011: Atmospheric Moisture Transports from Ocean to Land and Global Energy Flows in Reanalyses. J. Climate, 24, 4907–4924. doi: http://dx.doi.org/10.1175