Vegetation cover cools Earth when it intercepts the sun’s energy. This is not just by providing shade, but also through evapotranspiration, which is how plants regulate their own internal temperatures.
For a plant … transpiration[5] is a necessity by which a plant maintains its inner environment within the limit of optimal temperatures. And at the level of landscape, evapotranspiration is the most efficient air conditioning system developed by nature [Eiseltova 2012:10].
The water in plant tissues contains the sun’s energy in the form of latent heat, which is released from plants through evapotranspiration. In the absence of water, solar energy reaching Earth becomes sensible heat – the heat we can feel and measure in rising temperatures.
Without water, the energy of the incoming radiation is transformed into sensible heat and the local area becomes overheated during the day and likewise far cooler at night (as is well known from desert areas, with differences between day and night temperatures typically exceeding 50°C). Water-saturated landscapes provide much more stable environments than do dry terrestrial systems. In landscapes with water – abundant aquatic ecosystems, wetlands and soils with high water retention capacity – about 80% of incoming solar energy is stored as latent heat of water vapour via evapotranspiration, whilst in de-watered landscapes (with a low-water retention capacity) the vast majority of solar energy is transformed into sensible heat (Pokorný et al. 2010b) [Eiseltova 2012: 307].
With respect to landscape management for sustainability, the authors introduce the idea of a “dissipative-ecological-unit,” meaning “the smallest functional unit that is capable of forming internalized cycles of matter and water while dissipating energy” [Eiseltova 2012: 312]. This term emphasizes the importance of small, local water cycles, which occur naturally in undisturbed ecosystems, resulting in “an efficient local resource economy and … relatively even temperatures and moisture conditions” [Eiseltova 2012: 312].
In catchments with a well-developed vegetation cover, water and matter are bound to short-circuited cycles and losses are minimal. In contrast, the increased clearance of forest, exposure of bare land, and drainage of agricultural land have accelerated matter losses from catchments [Eiseltova 2012: 11].
There is an urgent need that agricultural research focuses on how to close water cycles[6] in the landscape and the development of farming systems with a more vertically-layered vegetation structure keeping water and lower temperatures during a sunny day[7] [Eiseltova 2012: 324].
The water cycle is akin to the ‘bloodstream’ of the biosphere. Returning water to the landscape and restoring more natural vegetation cover is the only way to restore landscape sustainability. More attention in present-day science needs to be devoted to the study of the role of vegetation in the water cycle and climate amelioration. Restoration of a more natural vegetation cover over the landscape seems to be the only way forward.
Based on our current scientific knowledge, we can propose two criteria for assessing sustainable land management. These criteria are: the efficiency of an ecosystem to recycle water and matter, and its efficiency to dissipate solar energy. Land managers can substantially contribute to the restoration of the water cycle, climate amelioration and reduction of irreversible matter [soil and nutrient] losses with river water flows to the sea.
It is in the interest of society as a whole that land managers (farmers, foresters) be rewarded for their actions towards sustainable management of their land. Suitable tools to assess the achievements of individual land managers with respect to sustainable management of their land are: (1) continuous monitoring of conductivity – a measure of dissolved load – and flow rates in streams in order to estimate matter losses; and (2) the regular evaluation of satellite thermal channel images to assess temperature damping, i.e., the effectiveness of land use to dissipate solar energy. Restoration of natural ‘cooling structures’ – vegetation with its evapotranspiration and condensation-induced water circulation – is essential to renew landscape sustainability [Eiseltova 2012: 325].
Eiseltova, Martina, et al., 2012, Evapotranspiration - a driving force in landscape sustainability, Evapotranspiration - Remote Sensing and Modeling, Dr. Ayse Irmak (Ed.), InTech. , Chap. 14: 305-324, https://www.intechopen.com/books/evapotranspiration-remote-sensing-and-modeling/evapotranspiration-a-driving-force-in-landscape-sustainability
[5] Transpiration is the movement of water from plant roots up through the stem into the leaves, where it is vaporized and released through leaves’ stomatal openings.
[6] The water cycle is the constant movement of water through land and atmosphere via evapotranspiration and condensation. To close a water cycle within a landscape is to enhance water recycling and limit water loss through vegetative cover.
[7] There are also productivity reasons for layering vegetation structure. See, for example, Mark Shepard 2013, Restoration Agriculture, Acres USA.