Final answer:
A total of 135 mm of the water applied by the farmer is wasted, since it exceeds the capacity of the soil to hold water that is useable to plants, which is only 90 mm above the current soil moisture level.
Step-by-step explanation:
We need to calculate the amount of water wasted in the irrigation process, considering the soil's field capacity and permanent wilting point. The amount of water that plants can use is stored in the soil between these two points. The root zone depth is 0.9 m, field capacity is 0.32, permanent wilting point is 0.12, currently the soil moisture is at 25%, and the farmer applies 225 mm of water.
The total water available to plants is the difference between the field capacity and the permanent wilting point. Multiply this difference by the root zone depth to find the capacity of water the soil can hold for plant use:
- (Field capacity - Permanent wilting point) × Root zone depth = Available water
- (0.32 - 0.12) × 0.9 m = 0.18 × 0.9 m = 0.162 m or 162 mm
The soil currently has 25% moisture, so we calculate how much water is present compared to the total available:
- 25% of field capacity = 0.25 × 0.32 × 0.9 m = 0.072 m or 72 mm
The farmer irrigates the field with 225 mm of water, so we need to find out how much of that actually goes to raising the soil moisture to field capacity:
- 162 mm (capacity) - 72 mm (current) = 90 mm (needed to reach capacity)
The amount of water wasted can then be calculated by subtracting the amount needed to reach capacity from the amount applied:
- 225 mm (applied) - 90 mm (needed) = 135 mm (wasted)
Therefore, 135 mm of the water applied by the farmer is wasted, as it exceeds the amount needed to bring the soil back to its field capacity.