A crop with a root zone depth of $80\text{ cm}$ is grown on a sandy loam soil. The field capacity and permanent wilting point of the soil on a dry weight basis are $22\%$ and $10\%$, respectively. The apparent specific gravity of the dry soil is $1.50$. If irrigation is to be applied when $50\%$ of available soil water is depleted, the net irrigation requirement in $\text{mm}$ is ________ (answer in integer).
Answer Key:
72
Step-by-Step Solution & Explanation
1. Available water capacity ($AWC$):
$AWC = \frac{\rho_b}{\rho_w} \cdot d \cdot (FC - PWP)$
Where:
• Apparent specific gravity $\frac{\rho_b}{\rho_w} = 1.50$
• Root zone depth $d = 80\text{ cm} = 800\text{ mm}$
• Field capacity $FC = 0.22$
• Permanent wilting point $PWP = 0.10$
$AWC = 1.50 \times 800 \times (0.22 - 0.10) = 1200 \times 0.12 = 144\text{ mm}$
2. Net irrigation requirement ($NIR$) at $50\%$ allowable depletion:
$NIR = 0.50 \times AWC = 0.50 \times 144 = 72\text{ mm}$