# What is the difference between Water Holding Capacity, Field Capacity and TAWC?

I am reading the paper Characterization of yield reduction in Ethiopia using a GIS-based crop water balance model by Gabriel B. Senay and James Verdin, and it says on page 3:

Soil water content (SW) is estimated through a simple mass balance equation where the total volume is defined by the water holding capacity (WHC) of the soil. SW is the amount of soil water present at a given time step. The value of SW varies from a minimum of 0 to a maximum equal to WHC (in mm)

What is the WHC here? Is it the Field capacity or Field Capacity - Wilting Point?

• I am not 100% sure but I think WHC is just the volume of the pores of the soil, the total water it would have if it would be completely saturated, nothing related with wilting point. Then SW go from 0 (dry soil) to WHC (water satured soil).
– user12525
Jun 5 '18 at 11:46
• Please look at how your question looks now. Next question, you make it look good yourself, OK ;-) If you click edit you see the formatting. BTW Is that your personal Dropbox link? Does that mean the document can disappear? Is there no copyright issue placing it there? Jun 5 '18 at 20:17

Okay Water Holding Capacity(WHC) is how much water the soil has in it at total saturation, if you filled every soil pore space completely with water.

Field Capacity(FC), if I remember correctly from first year, is shorthand for the water that stays in the soil after surface runoff and groundflow has ceased; the sub-saturation "ground state" of the moist soil if you will.

Soil Water(SW) would appear based on the excerpt provided to be referring to the percentage of WHC present at time of measurement.

You've also asked about Total Available Water Capacity(TAWC) which is the water that can be accessed by plants, this is the water stored in macro, as opposed to micro, pores in the soil (it's more complex than that but we always worked it as a two grade problem; pores big enough for plant roots and pores too small for them). Usually to quantify this figure WHC, or FC depending on what's of interest, is multipled by the percentage of plant accessible pore spaces in the soil, this gives the volume of water plants can access before they start to wilt.

Please note I'm mostly working from my memory of an entry level class I took nearly a decade ago so this may not be perfect, but it it does check against the research material I quickly skimmed while writing.

• Maybe with clays works a bit different than from sand soils, meaning TAWC <= WHC is my guess.
– user12525
Jun 5 '18 at 18:15
• @Universal_learner TAWC < WHC, always, there is always some water in any soil that isn't plant accessible, the sandier the soil is the more closely TAWC approaches WHC, also the lower FC becomes, but even pure sand holds some water plants can't extract.
– Ash
Jun 5 '18 at 18:22
• well I can imagine at a little soil of 3cm from the rock mass, you could find little plants and maybe close to =
– user12525
Jun 5 '18 at 18:24
• I have studie too just hydrology courses some years ago. Maybe there is somo coefficient TAWC/WHC or so...?
– user12525
Jun 5 '18 at 19:30
• Makes me wonder why they don't use porosity or connected porosity instead of WHC Jun 6 '18 at 22:37