**2. Soil water and its relation to the soil environment**

#### **2.1 General characteristics of soil water**

Water exists as a soil solution in the soil [1]. Gases (O2, CO2, NH3, N-oxides, S-oxides, etc.) and minerals are dissolved in this solution. Dissolved mineral substances originate from weathering processes, where they are released from rocks into the soil solution, and also from the above-ground part of forest stands, either by means of emission or percolation through tree crowns. Up to 50–250 kg of minerals per hectare a year penetrate the soil by so-called 'cloud/fog water' [2, 3]. This results in a significant enrichment of the soil surface not only in the form of plant litter but also through rainwater, including such elements as Ca, Mg, K, P and N. These elements react in the solid phase in the soil, further dissolving or precipitating. The water composition depends on the dissolution of minerals and organic compounds, on the ion exchange between the soil sorption complex and the soil solution and on the interaction of the soil solution, fine roots and soil microorganisms. Mineral (acids, bases, salts) and organic substances (colloids of dissolved compounds, saccharides, fulvic acids and amino acids, expressed as dissolved organic carbon (DOC)) are dissolved in water and then pass through the biosphere, while being regulated by climatic factors. Due to climate change and associated substantial changes in forest stand structure and functioning, the cycles and flows also change, not only at the level of soil water percolation and content but also within bulk deposition and through fall, both representing substantial sources of DOC [4].

#### *2.1.1 Water sources and losses in forest soils*

The soil water content and its availability are the results of a water balance arising from the inputs and outputs of the water cycle within the particular ecosystem [5, 6]. The actual soil moisture enters and leaves the water balance at the beginning and the end of the investigation period respectively. Individual components of the water balance [7] are subject to external influences (generally climate and topography) and internal influences (including properties, composition of the soil body and vegetation characteristics).

The most important water source is vertical precipitation in most areas of the temperate climatic zone. Horizontal precipitation is also regarded as a significant source, for example cloud/fog water in misty forests of tropical or mountain areas, dew, interception, condensation of water vapor in soil pores (especially in soils with a high proportion of macropores), capillary lift and lateral water. The water loss from the soil is primarily due to infiltration, surface run-off and evapotranspiration. Run-off is significantly regulated by forest stands, both in a precipitationrich period (run-off is lower compared to the non-forested soil) and in a drought (run-off is higher in comparison with the non-forested soil).

The character of surface run-off and water flow through the soil depends on many factors, notably the slope gradient, the amount and intensity of precipitation, soil permeability, the depth of freezing and vegetation coverage. An excessively

**47**

*Forest Soil Water in Landscape Context DOI: http://dx.doi.org/10.5772/intechopen.93003*

pedocompaction.

exceeding 1000%).

umn = 760 mm Hg = 1 atm.

soil water potential (**Figure 1**).

*2.1.2 Soil water content, forms and water regime*

dried soil surface may be characterized by poor wettability, while humus acts like a permeable filter with high hydraulic conductivity after being soaked in water. This leads to less vulnerability in forest ecosystems compared with different vegetation types [8, 9]. Humus may also be characterized by lower water loss (higher retention) compared to mineral soil. The forest floor, which is typical for forest soils [10], plays a crucial and indispensable role in terms of nutrient supply [11] but also for the water regime [12]: it absorbs several times more water than mineral horizons

The forestry-pedology nexus represents perhaps the greatest existential threat to humanity at present, requiring urgent action yet currently being ignored by the international community. Historical precedent is all too clear, yet we ignore this growing crisis at our peril. Deficiency of physiological water [13–15] and the potential risk of stress associated with water unavailability to plants [16, 17], both of which differ between vegetation types [18], cannot be overemphasized [19, 20]. The internal factors impacting water availability in the soil environment include the grain-size composition of the soil (the distinction of stoniness and fine earth in a differentiated way in sand, silt and clay fractions), the organic matter form and content and the thickness of soil horizons, affecting both the multidirectional water flow and the physiological depth of root distribution. Other factors include soil chemistry (increased hygroscopicity of salinated soils) [21], the degree of rooting (water drainage alongside the roots) [22] and the distribution and representation of soil pores of specific sizes, but also anthropogenic impacts, such as

One measure of increase and loss of water in the soil is the instantaneous soil moisture, represented by the total sum of water sources and losses and the water retention capacity. It is expressed in percentages by volume (*Θ*) or the mass (w) water content and also mm of water supplied, depending on different applications. In particular, forest soil humus horizons, act differently depending on stand species composition [23], the indicator of volumetric water content is more appropriate than the mass water content. The reason (also associated with low humus bulk density) is a significant disproportion in volumetric and mass water content when the maximum volumetric water content is always less than 100% while the maximum mass water content can be far more than 100% (even

Water is bound to the soil by the range of forces [24, 25] (chemical, physicochemical, physical and biological). The components that, together, produce water potential (see below) act simultaneously to influence water behavior and water content in the soil. There is no sharp boundary between these different forces. As a rule, the water-binding forces in the soil overlap and they are frequently related to specific soil horizons. The resultant sums of forces that hold water in the soil (matrix, osmotic, sorption, capillary, pneumatic, gravitational forces) together make up the soil water potential (*Ψ*) representing the strength by which soil water is bound. It can be said that it represents energy (work) that we would have to expend to 'drain out' water from the soil. The negative pressure is then referred to as suction or tension; hence it is expressed as the negative value of the atmospheric pressure [−Pa, −kPa, −MPa], where 0.1 MPa = 1 bar = 1020 cm of the water col-

The soil water potential can also be formulated in pF curves, where pF = −log *Ψ*. The pF curves thus express the relationship between the soil water content and the

located below and, at the same time, it reduces soil water losses.

#### *Forest Soil Water in Landscape Context DOI: http://dx.doi.org/10.5772/intechopen.93003*

*Soil Moisture Importance*

water conservation.

translocation effects. Soil water is irreplaceable in a wide range of Physico-chemical, biochemical and biological processes and de facto it conditions soil formation and the development of the pedosphere. Oxygen, upon which all anaerobic life depends, is generated from the water-splitting reaction. Entire photosynthetic physiological pathways, such as Crassulacean Acid Metabolism (CAM), are engineered around

Water exists as a soil solution in the soil [1]. Gases (O2, CO2, NH3, N-oxides, S-oxides, etc.) and minerals are dissolved in this solution. Dissolved mineral substances originate from weathering processes, where they are released from rocks into the soil solution, and also from the above-ground part of forest stands, either by means of emission or percolation through tree crowns. Up to 50–250 kg of minerals per hectare a year penetrate the soil by so-called 'cloud/fog water' [2, 3]. This results in a significant enrichment of the soil surface not only in the form of plant litter but also through rainwater, including such elements as Ca, Mg, K, P and N. These elements react in the solid phase in the soil, further dissolving or precipitating. The water composition depends on the dissolution of minerals and organic compounds, on the ion exchange between the soil sorption complex and the soil solution and on the interaction of the soil solution, fine roots and soil microorganisms. Mineral (acids, bases, salts) and organic substances (colloids of dissolved compounds, saccharides, fulvic acids and amino acids, expressed as dissolved organic carbon (DOC)) are dissolved in water and then pass through the biosphere, while being regulated by climatic factors. Due to climate change and associated substantial changes in forest stand structure and functioning, the cycles and flows also change, not only at the level of soil water percolation and content but also within bulk depo-

sition and through fall, both representing substantial sources of DOC [4].

The soil water content and its availability are the results of a water balance arising from the inputs and outputs of the water cycle within the particular ecosystem [5, 6]. The actual soil moisture enters and leaves the water balance at the beginning and the end of the investigation period respectively. Individual components of the water balance [7] are subject to external influences (generally climate and topography) and internal influences (including properties, composition of the soil body

The most important water source is vertical precipitation in most areas of the temperate climatic zone. Horizontal precipitation is also regarded as a significant source, for example cloud/fog water in misty forests of tropical or mountain areas, dew, interception, condensation of water vapor in soil pores (especially in soils with a high proportion of macropores), capillary lift and lateral water. The water loss from the soil is primarily due to infiltration, surface run-off and evapotranspiration. Run-off is significantly regulated by forest stands, both in a precipitationrich period (run-off is lower compared to the non-forested soil) and in a drought

The character of surface run-off and water flow through the soil depends on many factors, notably the slope gradient, the amount and intensity of precipitation, soil permeability, the depth of freezing and vegetation coverage. An excessively

(run-off is higher in comparison with the non-forested soil).

**2. Soil water and its relation to the soil environment**

**2.1 General characteristics of soil water**

*2.1.1 Water sources and losses in forest soils*

and vegetation characteristics).

**46**

dried soil surface may be characterized by poor wettability, while humus acts like a permeable filter with high hydraulic conductivity after being soaked in water. This leads to less vulnerability in forest ecosystems compared with different vegetation types [8, 9]. Humus may also be characterized by lower water loss (higher retention) compared to mineral soil. The forest floor, which is typical for forest soils [10], plays a crucial and indispensable role in terms of nutrient supply [11] but also for the water regime [12]: it absorbs several times more water than mineral horizons located below and, at the same time, it reduces soil water losses.

The forestry-pedology nexus represents perhaps the greatest existential threat to humanity at present, requiring urgent action yet currently being ignored by the international community. Historical precedent is all too clear, yet we ignore this growing crisis at our peril. Deficiency of physiological water [13–15] and the potential risk of stress associated with water unavailability to plants [16, 17], both of which differ between vegetation types [18], cannot be overemphasized [19, 20]. The internal factors impacting water availability in the soil environment include the grain-size composition of the soil (the distinction of stoniness and fine earth in a differentiated way in sand, silt and clay fractions), the organic matter form and content and the thickness of soil horizons, affecting both the multidirectional water flow and the physiological depth of root distribution. Other factors include soil chemistry (increased hygroscopicity of salinated soils) [21], the degree of rooting (water drainage alongside the roots) [22] and the distribution and representation of soil pores of specific sizes, but also anthropogenic impacts, such as pedocompaction.

#### *2.1.2 Soil water content, forms and water regime*

One measure of increase and loss of water in the soil is the instantaneous soil moisture, represented by the total sum of water sources and losses and the water retention capacity. It is expressed in percentages by volume (*Θ*) or the mass (w) water content and also mm of water supplied, depending on different applications. In particular, forest soil humus horizons, act differently depending on stand species composition [23], the indicator of volumetric water content is more appropriate than the mass water content. The reason (also associated with low humus bulk density) is a significant disproportion in volumetric and mass water content when the maximum volumetric water content is always less than 100% while the maximum mass water content can be far more than 100% (even exceeding 1000%).

Water is bound to the soil by the range of forces [24, 25] (chemical, physicochemical, physical and biological). The components that, together, produce water potential (see below) act simultaneously to influence water behavior and water content in the soil. There is no sharp boundary between these different forces. As a rule, the water-binding forces in the soil overlap and they are frequently related to specific soil horizons. The resultant sums of forces that hold water in the soil (matrix, osmotic, sorption, capillary, pneumatic, gravitational forces) together make up the soil water potential (*Ψ*) representing the strength by which soil water is bound. It can be said that it represents energy (work) that we would have to expend to 'drain out' water from the soil. The negative pressure is then referred to as suction or tension; hence it is expressed as the negative value of the atmospheric pressure [−Pa, −kPa, −MPa], where 0.1 MPa = 1 bar = 1020 cm of the water column = 760 mm Hg = 1 atm.

The soil water potential can also be formulated in pF curves, where pF = −log *Ψ*. The pF curves thus express the relationship between the soil water content and the soil water potential (**Figure 1**).

Water flow in the soil is conditioned by means of two processes [27, 28]: infiltration (determined by field or laboratory infiltration tests), where empty pores are filled with soaked water, and unsaturated flow. This sort of flow gradually slows down until all the pores are filled with water and water flows freely through noncapillary pores. Thus, the soil is fully saturated with water, and saturated flow is realized. This is not uniform, but, rather, tongue-like in terms of the water column, which gradually increases from the soil surface to greater depths.

In sloping landscapes of humid areas, lateral water is also added to rainwater [29]. This means that as we descend a sloping landscape, more water flows on the slope lower down the incline than higher up because soil water from the higher slopes is added to infiltrating rainwater. This phenomenon may also contribute to the differentiation of the soil types over a short distance.

As can be seen from the characteristics of the water potential, water flow in the soil is influenced by moisture gradients, but also by temperature and the mineralogical composition of the soil. The downward direction of water percolation typifies humid areas, where this type of movement contributes to the eluviations of soil particles. Under arid or semi-arid climate conditions, prevailing water flow is upward, as a consequence of suction pressure, and thus water rises by capillary action through the soil profile.

The moisture regime represents the distribution and movement of water in spatial and temporal terms [30]. It incorporates water inputs into the soil, water retention in the soil and water leakage from the soil. The water regime is conditioned by climate, vegetation, the soil-forming substrate, the groundwater location, the terrain relief and the landscape history. The water regime is generally expressed in terms of the relationships among temperature, potential evapotranspiration, precipitation and actual evapotranspiration. The soil water regime can be classified into several categories: aquic, udic, perudic, ustic, aridic and xeric [30]. Based on the resulting balance, there is a water deficit (percolates into underground layers) or a water surplus (retained in the soil).

#### *2.1.3 Soil hydrolimits and plant-available water*

Soil hydrolimits (**Figure 1**) represent the strength of water binding in the soil [7, 26, 31, 32]. They denote qualitative and quantitative alterations in soil-water relations, or how strongly water is retained in the soil (in what volume) at the given soil moisture level. Soil hydrolimits are soil moisture values achieved under welldefined conditions and they describe the relation of water and soil according to the flow of water in the soil and its accessibility to plants.

#### **Figure 1.**

*Relationships between various forms of water and binding forces in the soil (modified according to Vavříček, Kučera [26]).*

**49**

*Forest Soil Water in Landscape Context DOI: http://dx.doi.org/10.5772/intechopen.93003*

The significant hydrolimits are:

saturation degree without water

identifying the water-holding capacity

by interrupting capillary water

sometimes it is perceived as hygroscopic water

where it is also detected by field methods

−3.1 MPa, and so it is immobile and unavailable to plants

with water

limit, which corresponds to the soil porosity

• Maximum retentive capacity: soil fully saturated with water achieves a hydro-

• Gravitational water: *Ψ* = −33 to −10 kPa or more; under natural conditions, its presence in the soil is qualified mainly by precipitation. The direction of the gravitational water flow is in the direction of gravity to the lower soil strata

• Maximum capillary capacity: volume of capillary and partly semi-capillary pores. Suction forces at this level of the soil water content are in the range of pF 1.6–2.0 (*Ψ* = −0.01 to −0.007 MPa). Only coarse pores are present at this

• Water-holding capacity (WHC): corresponds to the pF curves in the interval of pF = 2.0–2.7 (*Ψ* = −0.08 to −0.01 MPa), expressing the ability of soil to retain a certain amount of water for a longer period (24 hours). We can ascertain the division of soil pores into capillary and semi-capillary pores by

• Point of limited availability: the initial phase of the deteriorated availability of water and its soil mobility. Water still flows continuously through the soil, but merely in the thinnest pores. The water flow is interrupted in semi-capillary and non-capillary pores and water only encapsulates the soil particles

• Lentocapillary point: occurs at pF = 3.0–3.3 (*Ψ* = −0.3 to −0.1 MPa). It is the soil moisture, which is in the range between slightly and scarcely mobile capillary water. It corresponds with a state where a sudden drop in mobility begins

• Wilting point: starts at pF = 4.18 (*Ψ* = −2 to −1 MPa; conventionally −1.5 MPa). It indicates the soil moisture level at which plants are insufficiently supplied

• Pellicular water: at pF = 2.1–4.0 (−5.0 to −0.1 MPa). Water encapsulates soil particles in a thicker layer, not moving with gravity, but merely from particles with a larger pellicle to particles with a smaller one. It is unavailable to plants;

• Hygroscopic water: is bound to the soil by means of adsorption and osmotic forces. As a rule, it only encapsulates soil particles, and *Ψ* is generally less than

• Field capacity: represents the ability of the soil to retain the maximum amount of water in the natural profile (in site conditions) against the effects of the Earth's gravity, that is without further active water removal, for a longer period of time (24 hours). This hydrolimit, which is de facto compatible with the water-holding capacity, is widespread, especially in agronomic soil science,

Plant-available water capacity (PWC) reflects the increase and loss of water in the soil. It is expressed in %, but also in mm of supply [25, 33]. The determination is based on the assumption that a column of water of 1 mm in height represents

The significant hydrolimits are:

*Soil Moisture Importance*

action through the soil profile.

*2.1.3 Soil hydrolimits and plant-available water*

flow of water in the soil and its accessibility to plants.

Water flow in the soil is conditioned by means of two processes [27, 28]: infiltration (determined by field or laboratory infiltration tests), where empty pores are filled with soaked water, and unsaturated flow. This sort of flow gradually slows down until all the pores are filled with water and water flows freely through noncapillary pores. Thus, the soil is fully saturated with water, and saturated flow is realized. This is not uniform, but, rather, tongue-like in terms of the water column,

In sloping landscapes of humid areas, lateral water is also added to rainwater [29]. This means that as we descend a sloping landscape, more water flows on the slope lower down the incline than higher up because soil water from the higher slopes is added to infiltrating rainwater. This phenomenon may also contribute to

As can be seen from the characteristics of the water potential, water flow in the soil is influenced by moisture gradients, but also by temperature and the mineralogical composition of the soil. The downward direction of water percolation typifies humid areas, where this type of movement contributes to the eluviations of soil particles. Under arid or semi-arid climate conditions, prevailing water flow is upward, as a consequence of suction pressure, and thus water rises by capillary

The moisture regime represents the distribution and movement of water in spatial and temporal terms [30]. It incorporates water inputs into the soil, water retention in the soil and water leakage from the soil. The water regime is conditioned by climate, vegetation, the soil-forming substrate, the groundwater location, the terrain relief and the landscape history. The water regime is generally expressed in terms of the relationships among temperature, potential evapotranspiration, precipitation and actual evapotranspiration. The soil water regime can be classified into several categories: aquic, udic, perudic, ustic, aridic and xeric [30]. Based on the resulting balance, there is a water deficit (percolates into underground layers) or a water surplus (retained in the soil).

Soil hydrolimits (**Figure 1**) represent the strength of water binding in the soil [7, 26, 31, 32]. They denote qualitative and quantitative alterations in soil-water relations, or how strongly water is retained in the soil (in what volume) at the given soil moisture level. Soil hydrolimits are soil moisture values achieved under welldefined conditions and they describe the relation of water and soil according to the

*Relationships between various forms of water and binding forces in the soil (modified according to Vavříček,* 

which gradually increases from the soil surface to greater depths.

the differentiation of the soil types over a short distance.

**48**

**Figure 1.**

*Kučera [26]).*


Plant-available water capacity (PWC) reflects the increase and loss of water in the soil. It is expressed in %, but also in mm of supply [25, 33]. The determination is based on the assumption that a column of water of 1 mm in height represents

a water volume of 1 l per 1 m<sup>2</sup> . For the practical application of this relation, it is essential that volumetric percentages of the ascertained soil moisture content, or the given hydrolimit, express the soil water supply in mm for a soil layer of 100 mm. In forest soils, this value is depicted in terms of the root distribution for the upper 20 cm of soil, and the observed volumetric % of the soil moisture is therefore multiplied by two to express the value of the plant-available water capacity. The plantavailable water capacity formulates the height of the water column of the soil within the range of the wilting point and the water-holding capacity. Thus, PWC represents the condition of the soil moisture where soil water is bound for a relatively long time, but it is still available to plants. The highest values of the plant-available water capacity are in loamy soil. Lower values exist in clay soils, and the lowest values are found in sandy soils [24, 34]. In addition to the texture characteristics, it is necessary to take into account the degree of soil stoniness, which practically does not participate in water retention and represents an inactive soil component in terms of water retention capacity, when determining the plant-available water capacity. PWC also expresses how much torrential rainfall the soil is capable of collecting. From this standpoint, it is an important indication of the water-retaining capacity of the landscape of which the soil is a part as a geological formation, which, with great efficiency, counteracts the flood distribution caused by torrential as well as prolonged rainfall. In this respect, soil, especially forest soils, with several times higher PWC in comparison with agricultural land and much higher than urbanized areas, plays an irreplaceable role in water management in the landscape.

Another soil property, soil moisture storage, relates directly to the actual soil water status, and shares the same units and the same principles as PWC. It can be expressed as the variance between the current soil moisture and the wilting point in mm, representing the current content of physiologically available water.

#### *2.1.4 Soil porosity and capillarity*

Apart from the soil structure, porosity is a major factor in the spatial arrangement of the soil and is fundamentally involved in the characterization of water and soil-air regimes, and in the soil–plant (forest stand) relationship. Pores exist in the soil both between soil particles and structural elements (aggregates). If the porosity value between aggregates is marked with the symbol A and the porosity value within the aggregates with the symbol B, the optimum soil porosity may be expressed as A:B = 1:2.

Water is bound most weakly in non-capillary and semi-capillary pores. This kind of water is called gravitational water. Non-capillary porosity occupies pores with very low water retention capacity, in which water moves under the influence of gravity. This is also why the term gravitational water is used for water contained in non-capillary pores. When non-capillary pores dominate, the soil has a low available water content due to its rapid flow to depths unavailable to plant roots.

Capillary water is present only in capillary pores. It is not tied to the Earth's gravity and can move in all directions in the soil. Capillary water is bound thanks to capillary adhesion and the surface tension of menisci. The optimal proportion of capillary pores is approximately two-thirds of total porosity [35–37]. An excess of capillary pores complicates infiltration of water, and it also inflicts an elevation in surface run-off, increasing the risk of erosion. A lack of capillary pores prefigures low plant water supply, low water retention capacity and low water absorption.

Water can rise above a continuous groundwater table by means of capillaries. This is called capillary rise [24, 25, 38]. The capillary rise is approximately the same as the soil particle size (pore diameter = 0.3–0.7 times the soil particle diameter). The capillary rise varies from 10 of centimeters to metres within a given year.

**51**

*Forest Soil Water in Landscape Context DOI: http://dx.doi.org/10.5772/intechopen.93003*

acceptable limit, with a risk value of 25%.

*2.2.1 Effect of climate change on forest water cycle*

neously also by its scope in the specific cycle.

functions among vegetation zones [45].

**2.2 Hydric functions of forest soils**

of climate change [41].

The volume relation of capillary and non-capillary pores is expressed by the minimum aeration capacity [26, 39]. This represents the volume of air-filled pores when the soil has reached maximum capillary capacity. The lower limit value of the minimum aeration capacity of forest soils can be considered to be 8% vol, while the average value (e.g. for topsoil in forest nurseries) is 10% vol. If the soil is excessively aerated, the soil is easier to heat, vapourization increases and soils are contrarily dehydrated. Therefore, a value above 20% can be considered an upper but still

A global (large) water cycle can be defined as a water cycle in which water is transferred between the land and oceans and a local (small) water cycle is defined as a displacement merely over oceans or drainless areas of the land. The water cycle governs all of the natural forest functions. However, forest ecology represents an important aspect of the hydrological cycle at the planetary level, and so these effects impact at a global level. Whereas the global water cycle is related to the adaptation of forests to climate change, the local water cycle interlinks mutual interactions between related forest complexes within the catchment. In general, the impact of forests on global climate change is at its most significant due to cloud formation in the tropics. The formed clouds reflect solar radiation more effectively and, therefore, cool the atmosphere more than does the absorption of greenhouse gases by vegetation [40]. Environmental pollution, deforestation and transformation of the tree species composition reduce the natural ability of forests to adapt to climate change. Monitoring of soil properties focused on water and nutrient cycles in different forest ecosystems offers a tool for assessing the impacts

Forest functions are the outcome of the interactions between the environmental, soil and vegetation subsystems. Natural functions are based on processes that support self-organization, recovery and development of the ecosystem. The interrelated processes of biodiversity, organic matter formation and nutrient cycles promote production, air circulation, (in-)filtration, evapotranspiration and site differentiation [42]. The water cycle controls the carbon cycle through which forests modify local cycling of all nutrients. The parts of the water and carbon cycles within soils have linked individual forest functions to the self-organized ecosystem [43]. The degree of interconnection is subject to the flow of soil water, but simulta-

The global effect of forest functionality consists in the transfer of evaporated water through cloudiness within the catchments from the areas with more significant vapor in the lower parts than the areas in the upper parts, where cloudiness condenses into more frequent precipitation. Precipitation in the upper parts of the catchments flows to the lower parts, where additional water complements the higher evaporation and lower precipitation [44]. As temperatures rise, this phenomenon is intensified: evaporation elevates, and drought deepens in drier areas, while precipitation in wetter areas increases. The consequent accentuation in disparities between drier and wetter areas disrupts the interconnection of forest

Even though the local water cycle defines the hydric functions of forests to catchments, their response to climate change depends on the interconnected monitoring of variability not only within the catchment but also among remote catchments. The link between the effects of global and local water cycles also

#### *Forest Soil Water in Landscape Context DOI: http://dx.doi.org/10.5772/intechopen.93003*

*Soil Moisture Importance*

a water volume of 1 l per 1 m<sup>2</sup>

*2.1.4 Soil porosity and capillarity*

expressed as A:B = 1:2.

. For the practical application of this relation, it is

essential that volumetric percentages of the ascertained soil moisture content, or the given hydrolimit, express the soil water supply in mm for a soil layer of 100 mm. In forest soils, this value is depicted in terms of the root distribution for the upper 20 cm of soil, and the observed volumetric % of the soil moisture is therefore multiplied by two to express the value of the plant-available water capacity. The plantavailable water capacity formulates the height of the water column of the soil within the range of the wilting point and the water-holding capacity. Thus, PWC represents the condition of the soil moisture where soil water is bound for a relatively long time, but it is still available to plants. The highest values of the plant-available water capacity are in loamy soil. Lower values exist in clay soils, and the lowest values are found in sandy soils [24, 34]. In addition to the texture characteristics, it is necessary to take into account the degree of soil stoniness, which practically does not participate in water retention and represents an inactive soil component in terms of water retention capacity, when determining the plant-available water capacity. PWC also expresses how much torrential rainfall the soil is capable of collecting. From this standpoint, it is an important indication of the water-retaining capacity of the landscape of which the soil is a part as a geological formation, which, with great efficiency, counteracts the flood distribution caused by torrential as well as prolonged rainfall. In this respect, soil, especially forest soils, with several times higher PWC in comparison with agricultural land and much higher than urbanized

areas, plays an irreplaceable role in water management in the landscape.

mm, representing the current content of physiologically available water.

Another soil property, soil moisture storage, relates directly to the actual soil water status, and shares the same units and the same principles as PWC. It can be expressed as the variance between the current soil moisture and the wilting point in

Apart from the soil structure, porosity is a major factor in the spatial arrangement of the soil and is fundamentally involved in the characterization of water and soil-air regimes, and in the soil–plant (forest stand) relationship. Pores exist in the soil both between soil particles and structural elements (aggregates). If the porosity value between aggregates is marked with the symbol A and the porosity value within the aggregates with the symbol B, the optimum soil porosity may be

Water is bound most weakly in non-capillary and semi-capillary pores. This kind of water is called gravitational water. Non-capillary porosity occupies pores with very low water retention capacity, in which water moves under the influence of gravity. This is also why the term gravitational water is used for water contained in non-capillary pores. When non-capillary pores dominate, the soil has a low available water content due to its rapid flow to depths unavailable to plant roots. Capillary water is present only in capillary pores. It is not tied to the Earth's gravity and can move in all directions in the soil. Capillary water is bound thanks to capillary adhesion and the surface tension of menisci. The optimal proportion of capillary pores is approximately two-thirds of total porosity [35–37]. An excess of capillary pores complicates infiltration of water, and it also inflicts an elevation in surface run-off, increasing the risk of erosion. A lack of capillary pores prefigures low plant water supply, low water retention capacity and low water absorption. Water can rise above a continuous groundwater table by means of capillaries. This is called capillary rise [24, 25, 38]. The capillary rise is approximately the same as the soil particle size (pore diameter = 0.3–0.7 times the soil particle diameter). The capillary rise varies from 10 of centimeters to metres within a given year.

**50**

The volume relation of capillary and non-capillary pores is expressed by the minimum aeration capacity [26, 39]. This represents the volume of air-filled pores when the soil has reached maximum capillary capacity. The lower limit value of the minimum aeration capacity of forest soils can be considered to be 8% vol, while the average value (e.g. for topsoil in forest nurseries) is 10% vol. If the soil is excessively aerated, the soil is easier to heat, vapourization increases and soils are contrarily dehydrated. Therefore, a value above 20% can be considered an upper but still acceptable limit, with a risk value of 25%.

## **2.2 Hydric functions of forest soils**
