evaporation

Jun

2022

Article: Working with plants, soils and water to cool the climate and rehydrate Earth’s landscapes

My UNEP paper on “Working with plants, soils and water to cool the climate and rehydrate Earth’s landscapes“.

The continued destruction of forests, the deterioration of soils, the subsequent loss of terrestrial soil water storage and the reduction of water retention in the landscape are disrupting the movement of water in and through the atmosphere. This disruption causes major shifts in precipitation that could lead to less rainfall and more droughts in many areas of the world, increases in regional temperatures and an exacerbation of climate change. These changes affect regional climate, but can also impact regions far away. Understanding the interwoven relationships and the subsequent fluxes of energy between plants, soils and water on the ground, as well as in the atmosphere, can help mitigate climate change and create more resilient ecosystems.

Translations into French, Spanish, Chinese to be found here (#25).

Apr

2022

Drought self-propagation in drylands due to land–atmosphere feedbacks

It has consequences, if (agricultural or forest) soils can hold less and less water and dry out. Although this research is on global drylands, I dare to guess that the same pattern applies to our desiccating agricultural landscapes.

Reduced evaporation due to dry soils can affect the land surface energy balance, with implications for local and downwind precipitation. […] We show that dryland droughts are particularly prone to self-propagating because evaporation tends to respond strongly to enhanced soil water stress. In drylands, precipitation can decline by more than 15% due to upwind drought during a single event and up to 30% during individual months. In light of projected widespread reductions in water availability, this feedback may further exacerbate future droughts.

Description of the figure: »Meteorological drought is frequently triggered by weaker-than-usual dynamical saturation-enabling mechanisms (conceptualized as low precipitation efficiency; Peffʹ < 0), which in turn may respond to a remote forcing, such as anomalous sea surface temperatures. Once that happens, limited precipitation (Pʹ < 0) causes soil desiccation (SMʹ < 0) and soil stress, exacerbated by the high potential evaporation due to clear skies and elevated temperatures. Then, evaporation becomes (more) water limited (E’ < 0). The reduction in near-surface air moistening — extending across the troposphere via vertical mixing—causes a reduction in water vapour being exported downwind (Qʹ < 0). Therefore, further downwind, for the same precipitation efficiency, even less precipitation is expected (Pʹ < 0), contributing to downwind drought onset (SMʹ < 0, Eʹ < 0). Moreover, since water vapour is known to enhance uplift, additional reductions are possible for convective precipitation (Peffʹ, Pʹ, SMʹ, Eʹ « 0).«