In most pedotransfer functions, field capacity and permanent welting point predicted using some easily measured data such as soil texture, bulk density and or organic matter percentage. None of the well known pedotransfer functions used the hygroscopic water as an input variable. Do you know any work used the soil hygroscopic water to predict field capacity and permanent welting point?
Soil Water Potential and Moisture Condition
0 bar saturation
-1/3 bar (4.1 psi) field capacity
-15 bar (220 psi) wilting point
-31 bar air dry
0 to -1/3 bar all gravitational water drains
-1/3 to -31 bars capillary water
-31 bar on we have hygroscopic water
Some PDFs are enclosed fro further reading on the subject :
Soil pores act as capillaries, and water tension can be related to "equivilent pore diameters". -60 hPa correspond to 50 µm, -300 hPa to 10 µm, -1.5 MPa to 0.2 µm pore sizes etc. Hygroscopic water (< -3 MPa) represents the tiniest pores, it binds to iron oxides and humic matter. It is not possiple to extrapolate from the tiniest pores to the whole pore size distribution.
The hygroscopic water of soils is the amount of water held by the soil when it is in equilibrium with an atmosphere of 98% water vapour saturation or 98% relative humidity. The hygroscopic coefficient is the boundry between moist and dry soil and arbitrarily set at -31 bar. The amount of water that a soil holds at the hygroscopic coefficient depends largely upon the amount and type of clay minerals, the nature of the exchangeable ions and salt concentration. Soils high in expanding type of clay and organic matter have a high hygroscopic coefficient.
The matric potential corresponding to the permanent wilting point moisture is at-15 bar or pF of 4.18. Soil structure has very little influence on moisture content at -15 bar and only texture plays the role.Concentration of salts in soil solution also has great influence on wilting point.
Field capacity is defined as the amount of water retained in the soil after the downward movement from a pre-saturated soil has materially ceased or practically zero. The matric potential corresponding to this moisture content is affected by soil texture, type of clay, organic matter, depth of wetting, the presence of impending layers and evapo-transpiration. Generally, moisture percentage at 1/10th bar to 1/3rd bar represents a measurable field capacity. Moisture percentage at 1/10th bar is more reasonable field capacity for course textured soils while 1/3rd bar for medium and fine textured soils.
Dear Younes- yes, such relation exits. Please find attached a relevant paper. With best wishes-
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