Soil water reserve and root water uptake

What is the soil water reserve? How is this measured?

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.

What is extractable soil water?

The extractable soil water (RU) is the fraction of the soil water reserve available to the plant, or that which can be extracted by the plant through root suction. It is expressed in millimetres of water. It is a state variable, which depends on the physico-chemical properties of the soil, its particle size, on the arrangement of soil particles and porosity distribution. The soil water available to the plant is equal to the volume of water stored in the slice of soil containing the root system between two water content characteristics: the field capacity (the amount of water held in the soil following gravitational water flow) and the permanent wilting point (beyond which plants can no longer extract water). This amount of water available to the plant is determined strictly by establishing water retention curves (also called pF-humidity curves) on undisturbed soil samples taken from each soil horizon (cylinder or clods). As an optional default, coefficient tables (see table below) or functions can be uesd to estimate the soil moisture characteristics at these two states by considering slightly more complex properties (texture, carbon content, density, cationic exchange capacity ...). It should be noted that few functions exist that are calibrated specifically for forest soils. Use of these functions for agricultural land can lead to biased results.

How can water status and soil water reserve be measured?

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.

How is extractable soil water measured?

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.

2. By determining the porosity of the soil layers (horizons) by considering three distinguishable components: texture, coarse elements, structure. For BILJOU ©, it is also important to define the layer boundaries which will depend on the distribution of fine roots, which in turn determine physical constraints influencing their vertical distribution. In BILJOU ©, a layer is a hydrodynamically homogeneous compartment containing fine roots. Once the boundaries of the different soil layers have been identified, it may be useful to take a sample of fine soil to measure texture. This is usually detectable through touch.

3. By assessing the extractable water content in each soil layer in measured in millimetres, the point between the humidity percentage of field capacity and the permanent wilting point. In the absence of retention curves for the studied soil, extractable water can be estimated, as described above, by using tables (example below).

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)
S	8	3	5	1.35	0.7
SL	12	5	7	1.40	1.0
SA	19	10	9	1.50	1.35
LlS	15	7	8	1.50	1.20
LS	19	9	10	1.45	1.45
LmS	20	9	11	1.45	1.60
LSA	22	11	11	1.50	1.65
LAS	24	12	12	1.45	1.75
Ll	17	8	9	1.45	1.30
Lm	23	10	13	1.35	1.75
LA	27	13	14	1.40	1.95
AS	33	22	11	1.55	1.70
A	37	25	12	1.45	1.75
AL	32	19	13	1.40	1.80
A lourde	29	18	11	1.50	1.65

Table of available soil water, from soil texture, Service soil mapping of the Aisne, Jamagne et al, 1977. Baize and Jabiol in 1995).

4. By gauging the bulk density of each soil layer. This can be measured in trenches using cylinders of established volume but can also be extracted from tables (example above). Care should be taken, however, when considering bulk densities of forest soils which are often quite different to those of agricultural soils: bulk density of forest soils often measures lower than that of agricultural soils due to their higher content of organic matter and to high biological activity. In contrast, forest soils may be more dense in depth than agricultural soils which are mechanically reworked. Bulk density tends to increase with depth.

5. By estimating the proportion of bulk elements or coarse-textured soil (% of soil volume).

6. By calculating the extractable soil water in each layer: extractable water x bulk density x (100 -% coarse elements) / 100

7. By adding these values taken throughout the entire depth of rooted depth

How is soil water absorbed by the root system?

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.

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:

• water transfer into the soil in proximity to the absorbing root (a few mm to a few cm),
• radial water penetration into the root, across the epidermis-parenchyma - endoderm – stele cells,
• water transfer in the vertical elements of the root xylem and higher (trunk, branches, leaves).

How is the soil water reserve used by trees ?

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.

A few key figures

The figure below shows the distribution of extractable soil water in the permanent plots within the RENECOFOR network. These estimates represent soil descriptions and rooting depth; texture and coarse elements were then evaluated and textural coefficients were applied.

Useful References

Badeau V, Ulrich E (2008) : RENECOFOR - Etude critique de faisabilité sur : la comparabilité des données météorologiques « RENECOFOR » avec celles de Météo France, l’estimation de la réserve utile en eau du sol et le calcul des volumes d’eau drainée en vue du calcul de bilans minéraux sur les placettes du sous-réseau CATAENAT. Editeur : Office National des Forêts, Direction Technique et Commercial Bois, ISBN 978 – 2 – 84207 – 323 – 7, 108 p. et 166 pages annexes.

Badeau V, Bréda N (2008) Modélisation du bilan hydrique : étape clé de la détermination des paramètres et des variables d’entrée. RDV techniques hors-série n°4 – ONF.

Bréda N, Lefèvre Y, Badeau V (2002) Réservoir en eau des sols forestiers tempérés : spécificité et difficultés d’évaluation. La Houille Blanche, 3-2002, Forêts et Eau, 25-40

Brethes A, Ulrich E (coordinateurs) (1997) RENECOFOR - Caractéristiques pédologiques des 102 peuplements du réseau. Editeur : Office National des Forêts, Département des Recherches Techniques, ISBN 2 – 84207 – 112 3, 573 p.

Quentin C, Bigorre F, Bréda N, Granier A, Teissier D (2001) Etude des sols de la foret de Hesse (Lorraine). Contribution a l'étude du bilan hydrique. Etude et gestion des sols 8: 279-292.

Garbaye J, Guehl J M (1997) Le rôle des ectomycorhizes dans l'utilisation de l'eau par les arbres forestier. Revue Forestière Française, XLIX n°sp 1997, 110-120.

Bréda N, Zapater M, Barlet C, Lefèvre Y, Granier A (2008) Une pompe biologique performante : contribution des racines fines des arbres forestiers dans le dessèchement des couches argileuses du sol lors d'épisodes de sécheresse. In: SEC2008-Volume 1, International Symposium Drought and construction, Magnan J. P., Cojean R., Cui Y.J eds, Laboratoire Central des Ponts et Chaussées, Marne-La-Vallée, 103-111.

Bréda N, Granier A, Barataud F, Moyne C (1995) Soil water dynamics in an oak stand. Plant and Soil 172(1): 17-27.

Cermak J, Prax A (2001) Water balance of a Southern Moravian floodplain forest under natural and modified soil water regimes and its ecological consequences. Annals of Forest Science 58(1): 15-29.

Rambal S (1984) Water Balance and Pattern of Root Water Uptake by a Quercus Coccifera L. Evergreen Scrub. Oecologia 62: 18-25.

Vincke C, Bréda N, Granier A, Devillez F (2005b) Evapotranspiration of a declining Quercus robur (L.) stand from 1991 to 2001. II. Daily actual evapotranspiration and soil water reserve. Annals of Forest Science, 62(7): 615-623