Viable Water Content - Maximum

This constant represents the percentage water content above which most plants won't thrive.  This value is approximated as the the field capacity upper bound.

We are assuming in this equation that the field capacity represents a reasonable estimate of the static state moisture required after a gardener waters their plants, including plantings in pots with adequate drainage.  Since water content exceeding field capacity is considered "saturated water content", this is upper bound for hospitable plant growth.

DERIVATION

Field Capacity is the amount of soil moisture or water content held in the soil after excess water has drained away and the rate of downward movement has decreased. This usually takes place 2–3 days after rain, irrigation, or manual watering in pervious soils of uniform structure and texture. The physical definition of field capacity (expressed symbolically as θ_fc) is the bulk water content retained in soil at −33 J/kg (or −0.33 bar) of hydraulic head or suction pressure. The term originated from Israelson and West and Frank Veihmeyer and Arthur Hendrickson.¹ 

Veihmeyer and Hendrickson realized the limitation in this measurement and commented that it is affected by so many factors that, precisely, it is not a constant (for a particular soil), yet it does serve as a practical measure of soil water-holding capacity. Field capacity improves on the concept of moisture equivalent by Lyman Briggs. Veihmeyer & Hendrickson proposed this concept as an attempt to improve water use efficiency for farmers in California during 1949.

Field capacity is characterized by measuring water content after wetting a soil profile, covering it (to prevent evaporation) and monitoring the change soil moisture in the profile. Water content when the rate of change is relatively small is indicative of when drainage ceases and is called Field Capacity, it is also termed drained upper limit

Usage

In soil science, hydrology and agricultural sciences, water content has an important role for groundwater recharge, agriculture, and soil chemistry. Many recent scientific research efforts have aimed toward a predictive-understanding of water content over space and time.

Observations have revealed generally that spatial variance in water content tends to increase as overall wetness increases in semiarid regions, to decrease as overall wetness increases in humid regions, and to peak under intermediate wetness conditions in temperate regions .

1. ^ http://en.wikipedia.org/wiki/Water_content#Earth_and_agricultural_sciences