How it works
After heavy rain drains away, a soil holds water at field capacity, around 20% in sandy soils and 35% or more in black cotton soils. Below the wilting point, roughly half of that, plants can no longer draw water.
The water between the two levels is what the crop can use. FAO-56 calls it total available water, and recommends irrigating before the crop has used about half of it.
Why it matters for Indian farmers
Keeping the root zone between those two levels is the whole job of irrigation. Black soils of Vidarbha and the Deccan hold a lot of water but drain slowly; sandy soils hold little and dry quickly.
How VarshaKrishi uses it
VarshaKrishi estimates root-zone water with an FAO-56 water balance from weather and the crop's water use. Where a research contract installs a soil-moisture kit on a reference plot, its readings anchor and calibrate that estimate.
Questions about Volumetric water content
What is field capacity?
The water a soil holds after free drainage stops, usually one to two days after heavy rain or irrigation.
Can a satellite measure soil moisture in my root zone?
Not directly at field scale. Satellites see the crop's response to water (greenness and canopy water) and radar sees surface wetness; the root zone is estimated with a water balance.