Peatlands do not retain water as well as regular forest soil, hydrologists find

17. 07. 2023

When it comes to retaining water, mountain peatlands do not behave like a sponge. Most of the rainwater drains out of them in the form of so-called fast water, which can raise the level in watercourses quite abruptly. Ordinary forest soils, on the other hand, allows water to filter through to greater depths. This discovery was made by researchers from the Institute of Hydrodynamics of the CAS, who compared the runoff response of peatland vs regular forest soil. The experts added that the impact of peatlands on cooling the landscape during the summer is also questionable.

Today, wetlands are often mentioned in the context of mitigating floods and the impact of drought, cooling the landscape, or increasing groundwater reserves. However, wetlands as a biome encompass several types of landscape features. These include riparian areas of flowing and standing water (i.e., dams and ponds), vernal pools, and peat bogs. Each of these elements has its own specific water regime, which is influenced by its subsoil, position within the landscape, soil characteristics, and vegetation.

It is thus clear that not every type of wetland can fulfil the above functions. Examples where different hydrological processes are most evident include wetlands in river floodplains (floodplain forests) and wetlands on mountain ridges (peat bogs).

Floodplain forests are mainly impacted by the water regime of the watercourse and the intensive ‘extraction’ of water by vegetation. Mountain peat bogs, on the other hand, are very sensitive to rainfall patterns and the water regime is influenced by the soil – peat – rather than the vegetation. “Peat bogs as unique ecosystems increase biodiversity, but their impact on the hydrological regime of the area is not as clearcut,” explains Kristýna Falátková from the Institute of Hydrodynamics of the CAS.

Monitoring two hillsides facing each other
It is mountain peat bogs in particular that experts from the Institute of Hydrodynamics of the CAS have been studying, looking at the runoff response of two hillsides facing each other. The first hillside was a peat bog, the second comprised regular forest soil. The results, based on environmental tracers (stable isotopes of water, water temperature) and a hydrological model, show very different ‘behaviours’ of the two hillsides during rainfall.

In the forest soil, infiltration into the soil and seepage of water to greater depths clearly predominates. On the other hand, the peatland, due to its typical structure (its lower layer is very poorly permeable), causes a majority of the water to run off along its surface or right beneath it. Thus, most of the rainfall (75%) runs off as so-called ‘fast water’, which raises watercourse levels quite rapidly. Conversely, where forest soil is concerned, most of the water (70%) runs off as slow water (i.e., it soaks into deeper waters belowground where it is temporarily retained), effectively subduing the rise in watercourse levels.

The light-coloured stripe along the edge of the peat pond shows how much the water’s surface has gone down during the dry season.

It will retain water but not release it
There is also minimal runoff from peatlands in the summer, which is increasingly characterised by periods of minimal rainfall, unless it is helped out by groundwater (several types of peatland exist with different water sources). It typically retains water well but does not release it further into the watercourse. Its bottom layer (as little as 10 to 30 centimetres below the surface), permanently saturated with water, is very poorly permeable – various studies give values of water flow rates in this layer in the order of centimetres per day.

“In the long term, water in peatland is more likely to be accumulated. Even in peat bogs that are several metres deep, the retention space, i.e., the place where a greater degree of rainfall water is retained, is a depth of up to 50 centimetres. However, this space is always full of water for over three quarters of the year. It is only ‘empty’ during dry periods. If the peat bog is ever able to hold back a flood wave, it is only during storms usually in high summer,” says Lukáš Vlček from the Institute of Hydrodynamics of the CAS. “Springs from peat bogs dry up during periods of drought. Thus, the amount of water that flows through peat bogs decreases significantly in the summer,” the researcher adds.

Under normal conditions, the peatland is full of water and the depressions are flooded.

In summer, it does not cool down the surrounding area much
The results from the hydrological model also suggest that overall, the peatland evaporates significantly less water in a year than a hillside with forest soil. Over the period 2014–2022, the average was between 65 and 70% of the evaporation value of the forest hillside (i.e., about 1.5 times more water evaporated from the forest biome than from the peatland).

Unless the peatland is covered with trees such as spruce or bog pine, evapotranspiration (i.e., evaporation from the soil surface and from plants) comprises primarily evaporation from the earth’s surface itself, which is low due to the specific properties of peat and peatland. In the case of peat spruce forests, the total annual evapotranspiration is assumed to be equivalent to that of the surrounding forest with regular soil. This is confirmed by the results of air temperature and soil surface temperature measurements.

“During the cold season, peat bogs are the place with the lowest temperatures in the landscape. However, recent data shows that on dry summer days, these places become warmer than their surroundings. Their cooling effect is therefore highly questionable. Peatland without taller trees in its vicinity thus cools the landscape less than the surrounding forest,” Vlček adds.

Author: Markéta Wernerová, Division of External Relations, CAO of the CAS, drawing on the press release of the CAS
Translated by: Tereza Novická, Division of External Relations, CAO of the CAS
Photo: Institute of Hydrodynamics of the CAS

Licence Creative Commons The text is released for use under the Creative Commons license.

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