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Definition
Soil water in the vegetation area
To get a measure of soil water content, the first step is to make a rough estimate of the depth down to the average level of the groundwater table during the growing season (see diagram below). The groundwater table is the level at which the water stabilises in a pit dug in the soil or in a perforated pipe driven down into the soil. The water pressure at the groundwater table is equal to the atmospheric pressure.
In the groundwater zone, all pores are waterfilled (the soil is saturated) and the water pressure is greater than the atmospheric pressure. In the soil water zone (zone between groundwater table and soil surface) there is also air in the pores and the water pressure is therefore negative compared to the atmospheric pressure.
Examples of how different soil water classes occur in terrain. |
The groundwater table generally follows the slope of the soil surface. This is clearest in areas where the groundwater table lies close to the soil surface, which is often the case in Sweden. The deeper the groundwater table, the weaker its relationship with local soil topography.
The groundwater table is determined with the help of the topography, the possible presence of groundwater in hollows, and in some cases the soil class. The presence of wetland plants (incl. sphagnum and common golden maidenhair mosses) can be a help. However, do not draw conclusions based on the presence of lichen!
With the help of the soil class, the thickness of the bleached (eluvial) horizon and to some extent the colour of the B horizon, an attempt to determine the degree of podsolisation can be made. Determination of podsol type in this way has its value, since to a certain extent it characterises the normal degree of wetness of the soil, independent of temporary changes in water content due to prevailing climatic conditions. However, care and discretion must be exercised when using soil class as an aid to soil water determination. In northern Sweden it is generally successful, but in southern Sweden soil profile formation is often considerably more complicated.
Note that the groundwater table often rises rapidly after clear-cutting, which in some cases can mean that the characteristics described above bear no clear resemblance to the new, actual soil water conditions.
After heavy rain or snowmelt, the test area can be completely or partly inundated for a time, particularly where the soil texture is fine-grained or where the soil is still frozen. This must not lead to the soil being classed as wet (or wetter than it actually is) - it is always the estimate made during the growing season that is decisive for this classification.
In the variable WatDom, the dominant soil water class in the veg-area/part veg-area must be specified. However, observations of the water conditions in hollows and more low-lying terrain in the vicinity of this area must be used to support the distinction into different classes. The variable must always be recorded if VegArea is > 0 m2.
Furthermore, if partial areas with different soil water classes are present within the veg-area/part veg-area, a so-called also-class must be specified in the variable WatAlso.
Note that the also-class may not be used to express doubtful borderline cases between two soil water classes. It is used only in cases where there are clearly delineated part areas within the veg-area with distinctly different soil water classes.
If there are rock pools that are > ¼ of the veg-area, the water in these must be taken into consideration (poss. as also-class in the variable WatAlso).
If one is in doubt between two water classes, one must not be afraid to set the extreme class, e.g. ’dry‘ if one is deliberating between dry and mesic; ’moist‘ if one is deliberating between mesic-moist and moist, or ’wet‘ if one is deliberating between moist and wet.
The appendix shows examples of combinations of soil water classes and classes of Lateral soil water movementes within different areas of hummocky terrain sections with different soil types.
Soil water classes are characterised in the following way:
Code and name |
Description |
(1) Very dry soil 
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General rule:
The groundwater table must not be found even in deep boreholes (>5 m).
Otherwise, see definition of dry soil
NB! The vegetation area must not be topographically aligned so that it receives water from higher sections of the terrain, i.e. Lateral soil water movement must be lacking (lateral flow class is always SA).
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(2) Dry soil
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General rule:
The groundwater table is deeper than 2 m.
The groundwater table is deeper than 2 m.
Eskers, hills, marked crowns and ridge crests.
Plateaus and flat, high terrain sections with bedrock outcrops (i.e. the variable soil depth has code 2-4,) or with coarse texture (coarse texture here means that the variable texture has a code less than or equal to 5).
If the soil depth is great and the variable texture has a code greater than or equal to 6, the class ’dry soil‘ is thus only used in exceptional cases; the class 'mesic soil' is generally used even if the groundwater level is at depths greater than 2 metres.
If the variable LatSoilW has code 2-3, the class should not be used in the variable WatDom. However even in this case the class can be used in the variable WatAlso, e.g. on bedrock outcrop.
The soil class is generally bedrock outcrop, lithosol, boulder soil, ’coarse soil texture‘ or iron podsol (though often with a thin humus blanket and a thin bleached horizon).
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(3) Mesic soil
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General rule:
The groundwater table is on average at a depth between 1 and 2 m.
Flat land and slopes.
No water collecting on the soil surface, either in the veg-area or in neighbouring areas in line with the veg-area. Overall on the veg-area one can walk dry footed (low shoes) even immediately after rain or shortly after snowmelt.
Sites with mesic soil can periodically receive water from higher terrain so that the groundwater table is temporarily higher than 1 m below the surface. This can occur during persistent heavy rain and in snowmelt periods.
If the soil depth is great and the variable texture has a code greater than or equal to 6, the class 'mesic soil' is often used even if the groundwater table is deeper than 2 metres below the surface, see above under class ’dry soil‘.
All classes in the variable LatSoilW can occur on mesic soil.
Test areas situated on the moss plane of raised bogs (nutrient-poor and domed bogs) are included in the class ’mesic soil‘. Raised bogs are mainly found south of Limes Norrlandicus.
On podsolised soil, the soil class is most often iron podsol with a rather thin (4-10 cm) humus layer of mor type. The bottom layer consists mainly of dryland mosses (e.g. wall, house and feather mosses). The bleached horizon is grey-white and relatively well delineated against the B horizon, which is rust-yellow, rust-red or brownish rust-red in colour (the darker the colour, the wetter the soil).
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(4) Mesic-moist soil
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General rule:
The groundwater table is on average at less than 1 metre depth
Flat land within relatively low-lying terrain sections.
Central and lower parts of long slopes.
Flat land adjoining large hills. In flat areas in particular, a small decrease in surface level in relation to the surrounding area can give rise to mesic-moist soil.
In summer, one should be able to walk dry footed (in low shoes) over the entire veg-area, although not after heavy rain or shortly after snowmelt, when water can temporarily collect in marked depressions.
Small areas of wetland mosses (sphagnum and common golden maidenhair moss) occur relatively often.
Trees often grow on ’pedestals‘ (small mounds on the soil surface), which either indicates that the tree stand has drained the soil to a certain extent or that the trees grew on elevated spots from the beginning.
On podsolised soil, the soil class is most iron humus podsol or humus podsol. The humus layer is thicker than on mesic soil and the humus form is often peaty mor.
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(5) Moist soil
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General rule:
The groundwater table is on average at less than 1 metre depth. It is generally visible in marked depressions in the veg-area or in its immediate vicinity.
Flat land within relatively low-lying terrain sections.
Lower parts of weak-gradient slopes.
Flat land adjoining large hills.
In summer, one should be able to walk dry footed (in low shoes) over the entire veg-area, if one steps on tussocks in the wetter parts. After long dry spells, a pool should form around the shoe if one steps in a deep hollow.
There are patches of wetland mosses here and there. It is not uncommon for moss patches to dominate the bottom layer.
Tree often grow on pedestals. Other signs of wetland include ditches or channels (often free of vegetation) in the humus layer around boulders, a tussocky soil surface and small pools.
The soil class is most often humus podsol or fen soil.
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(6) Wet soil
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General rule:
The groundwater table forms permanent water standing on the soil surface.
Sites with very poor drainage conditions.
One can not walk dry footed (in low shoes). Conifers can only occasionally occur in stands.
The soil class is most often fen soil.
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