Forest water balance model
- Silva Joint Research Unit
Soil water reserve is defined as the volume of water contained in the soil at a given time. This volume of water is generally expressed in the height of the water content (mm), easily comparable to that of rainfall and evapotranspiration. It is a dynamic variable that changes over time, influenced jointly by rainfall and evapotranspiration. However, all soil water is not available to be utilized by plants, either because the roots have not colonized the entire volume of the soil or because the water is too firmly retained by the soil to be extracted by the roots.
The soil water status can be characterized in situ by considering the bond of water to soil particles (suction, also called water potential) or by the amount of water contained in the soil (humidity). Monitoring soil moisture levels over time can be achieved by taking direct measurements (mass humidity of soil samples), or through indirect measurements considering relationships between physical or chemical soil properties and its water content. Volumetric water content can be determined through neutron scattering measurements, electrical conductivity or dielectric constant measurements of the soil. The state of water binding is quantified by the soil water potential; negative when the water is bound to the soil and null when water is free. The amount of water (in mm) contained in a soil layer is equal to the product of the volumetric water content expressed as a percentage of its thickness (in decimetre). The soil water content is defined as being the sum of water contained in each layer of soil. However, as all soil water is not usable by the roots, it is never measured at zero as even under severe drought conditions, the remaining water is too firmly bound to soil particles.
Soil auger drill: a soil sample is extracted and weighed, then dried in an oven at 105 ° C to determine its mass water content
Two examples of equipment used to measure volumetric water content by the time domain reflectometry (TDR) measurement technique either by using waveguides inserted into the soil (left: Trase model) or by moving the probe to a tube (right: Trime model)
Neutron probe (Troxler) for measuring volumetric water content repeatedly over time at different depths by displacing the probe to an access tube
1. By measuring the total soil thickness containing fine roots. This is the first potential source of imprecision facing the forester. As such, direct observation of a trench wall often proves useful. In the absence of obstacles (slabs, stones) the rooting depth of adult trees usually measures at least 2 m.
Map of the distribution of fine roots, performed by counting the roots on a trench wall using a square grid measuring 10 cm x 10 cm. Colours indicate the different root densities.
|Texture class (according to the Jamagne’s triangle)||% humidity of field capacity (pF = 2.5)||% humidity of permanent wilting point (pF = 4.2)||Available water (g water per 100 g of soil)||Bulk density ( dimensionless)||Available soil water reserve (water per cm of soil, mm)|
Table of available soil water, from soil texture, Service soil mapping of the Aisne, Jamagne et al, 1977. Baize and Jabiol in 1995).
The water potential gradient, primarily driven by transpiration, allows trees to extract water from the soil through suction. The soil water is absorbed by the non-suberized fine roots and will often function in association with mycorrhizal fungi to "drain" water to the roots.
Fine roots and mycorrhizae of laccaria fungi observed in oak in a forest soil using an endoscope (photo N Breda, 2008)
Extra-matricial hyphaes of a mycorrhizal fungus spread widely in the soil and increase the water and nutrients absorption area. Observations made in oak forest soil using an endoscope (photo N Breda, 2008)
The efficiency of a root system depends on: 1) its spatial extension particularly its depth, 2) its water absorption capacity at low water potential (water firmly bound), 3) its potential for length growth, especially when the soil is re-watered following a drought. The path of water (and of solutes) between the soil and the tree is controlled by well-established biophysical laws, which are based on principles of liquid phase diffusion in porous media and osmotic mechanisms. This path consists of three steps:
Anatomical cross section of a root..
When the soil is wet, water is easily absorbed through light suction; the rate of absorption is proportional root density. As such, the upper soil horizons provide trees with more water than deeper horizons as this is where the majority of roots are located. During a drying period, absorption by the soil is initially evenly distributed to cover the whole rooted zone; water uptake is then limited to deeper levels where water binding to soil is less strong. During a period of severe drought, only the deepest layers of soil have the capacity to provide the tree with water. Transpiration flow, at this stage, is driven by a relatively low percentage of deep fine roots. The degree to which each soil layer contributes to the trees water supply varies according to season and depends on the facility of water extraction.
Variation in water uptake in different soil layers during drought development in an ash stand. Measurements were taken using a neutron probe. Right, distribution of fine roots in different soil layers of the same stand. (From Breda et al. 2002)
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