Determination of Soil Moisture Release Curve Parameters
using particle composition
The relationship between matric potential and water content in soil is called
the moisture release curve. The soil moisture release curve is described by:
where:
The values for the key parameters for the moisture release curve can be
determined for representative soils based on the soil texture classification.
The moisture characteristics of soil are dependent on pore spaces and size
in the soil, which are in turn related to the proportions of partical
sizes in the soil and the bulk density.
The parameters for the moisture release curve are related to the geometric
mean particle diameter and the geometric standard deviation. These are
computed from the texture based on the standard partical sizes for
clay (0.001mm), silt (0.026mm), and sand (1.025mm). The geometric mean
partilcle diameter is:
where:
is the mass fraction of particle i, and
is the diameter arithmetic mean diameter of particle i.
The geometric standard deviation is:
where:
For the purposes of this discussion, bulk density will be assumed to be a
given parameter, based on the soil type. The moisture characteristics are
determined using bulk density and texture information.
Air Entry Potential
This is the matric potential at the point which the largest water
filled pores just drain. It is usually determined as the intercept
of a linear fit on a plot of ln(soil moisture) against
ln(matric potential). It can be estimated according to the soil
texture and bulk density as:
where:
Saturated Moisture Content
This is the total pore space of the soil. It can be expressed as:
Power of the Soil Moisture Release Curve
The power of the moisture release curve is usually determined as the slope
of a linear fit on a plot of ln(soil moisture) against
ln(matric potential). It can be estimated according to the soil
texture as:
References:
- Campbell, G. S., Soil Physics with Basic
Transport Models for Soil - Plant Systems,
Elsevier Science Publishing Company INC., 1985
Updated: 05/22/96
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