Forests and water – two strong partners

Forests have unique impacts on the water balance

The im­pacts of forests on the local mi­cro-cli­mate, i.e. air hu­mid­ity, tem­per­a­ture, and their ef­fec­tive­ness in fil­ter­ing pol­lu­tants, are cru­cial.

The for­est acts like an um­brella, a fil­ter and a water reser­voir. It in­ter­cepts the rain, resulting in some of it being evap­o­rated back into the air. The pro­por­tion of pre­cip­i­ta­tion which is re­tained in the nee­dle and leaf mass and di­rectly re­leased into the at­mos­phere by way of evap­o­ra­tion ac­counts for up to 50% for conifers, and for up to 30% for de­cid­u­ous trees.

Comparatively, forests use a very large amount of water due to the process of tran­spi­ra­tion. On a warm sum­mer day, 20,000 to 60,000 litres of water, per hectare, will be evaporated after transpiration by an av­er­age tree pop­u­la­tion. This there­fore has a bal­anc­ing ef­fect on the mi­cro-cli­mate, as the for­est keeps the sur­round­ing land fresh and moist, while humidity is 10% higher in the forest itself than in the surrounding area. Water not evap­o­rated by the for­est will slowly fall to the ground.

For­est soil - water reser­voir

The for­est soil is porous and thus able to ab­sorb water in a spongelike manner. Due to their high wa­ter-stor­age ca­pac­ity, which can be up to 6 times higher than in open areas (e.g. fields), forests have vir­tu­ally no sur­face run-off, unlike in open areas, like alpine pas­tures or ski slopes, where surface run-off can be a genuine problem. Rain and meltwater are re­tained and dis­charged at a very slow rate, if at all. Fur­ther­more, in for­ests, snow melts com­par­a­tively slowly, and therefore nei­ther heavy rain­fall nor snow-melt­ing leads to ero­sion, unlike in unforested areas.

How­ever, despite its high wa­ter-re­ten­tion po­ten­tial, for­est soil is unable to cope with short-du­ra­tion and high-in­ten­sity pre­cip­i­ta­tion events, which can result in disastrous consequences. Under these precipitation events,  sur­face run-off will occur in forested areas and may result in soil ero­sion, which in turn can lead to the emergence of natural hazards such as land­slides or mud­flows, although usu­ally the re­sul­tant dam­age is far less sig­nif­i­cant than in areas with­out forest cover.

Springs, water bod­ies, and flood­plain forests

Forests aid in regulating water flows from springs into streams. More­over, the water stemming from forest area is of ex­cel­lent qual­ity, due to the process of natural filtration.

So forests pro­vide us with sustainable source of pure drink­ing water, in both wet and dry seasons

The forests located along­side streams and rivers, also know as flood­plain forests, are very closely linked to the water level and quality in the river itself. Further, these forests are constantly oscillating between being completely flooded or completely dry, and offer im­por­tant habi­tats for rare an­i­mal and plant species.

Func­tion­ing as nat­ural flood and water re­ten­tion areas, flood­plain forests slow down water run-off in flood events and there­fore play a major role in ‘flood con­trol’, or mitigating the consequences of a flood event.

In the event of a flood, forests also have a ‘cleans­ing ef­fect’, as they trap or fil­ter or­ganic mat­ter and min­eral sub­stances. Water seeps through the thick layers of the soil and replenishes the groundwater reserves.

Plant roots help strengthen the soil sur­face, which helps in reducing the impact of wind and water erosion on the loss of topsoil. At particular lo­ca­tions, favourable to the growth of plants, the shoots and leaves of various plant species fil­ter sed­i­ment par­ti­cles from the water. In this way, a fertile top­soil grad­u­ally de­vel­ops on the gravel banks, which therefore allows nutrient-de­mand­ing plants to grow.

How im­por­tant are do­mes­tic forests for Aus­tria’s drink­ing water?

Sus­tain­able for­est man­age­ment safe­guards our water sup­ply for both present and future generations, while at the same time continuing to ensure that it is of the highest quality. In fact, many peo­ple around the world envy us for our water! Hence, the importance of for­est areas cannot be emphasised enough, with forests being cru­cial for the local cli­mate, the fil­ter­ing of pol­lu­tants from air and water cy­cles, as well as for the sus­tain­able and con­tin­u­ous de­liv­ery of water in abundance.

In Aus­tria, almost 50% of the areas that di­rectly or in­di­rectly pro­tect­ springs and other water sources are lo­cated in for­est areas!

‘For­est and Water’ Ini­tia­tive

The planned ‘Forest and Water’ Initiative aims to raise aware­ness about the important role forests play in preserving and maintaining Austria’s water supply. The im­ple­men­ta­tion of the fu­ture agri­cul­tural pol­icy has at its heart the performance of a sustainable forest ecosystem.

published at 03.07.2018, Kommunikation und Service (Abteilung Präs. 5)