The field capacity and permanent wilting point for a given 0·8 m root-zone soil are 35 and 10 per cent, respectively. At a given time, the soil moisture in the given soil is 20 per cent when a farmer irrigates the soil with 250 mm depth of water. Assuming bulk specific gravity of the soil as 1·6, what is the amount of water wasted from the consideration of irrigation ?
- A23·2%Correct
- B58%
- C5·8%
- D19·2%
Solution
Concept
Depth of water needed to bring the root zone to field capacity
Formula
d = (FC − MC) × (γb/γw) × Droot
Step-by-step solution
1. Depth needed = (0.35 − 0.20) × 1.6 × 800 mm = 192 mm. 2. Water applied = 250 mm, so water wasted = 250 − 192 = 58 mm. 3. As a percentage of the applied water = 58 / 250 = 23.2%.
Explanation
Only the water needed to raise the moisture from 20% to field capacity is useful; the excess percolates below the root zone and is wasted.
Why the other options are wrong
58% and 5.8% are slips of the unit (58 mm taken as a percent); 19.2% is the wasted depth expressed as 192/1000.
Shortcut
Needed = (FC − w) × γb × root depth.
Common mistake
Reporting 58 (mm) as a percentage.
Correct answer: A) 23·2%