• The IW/CPE ratio method schedules irrigation when Cumulative Pan Evaporation ($CPE$) reaches a designated threshold based on depth of irrigation water ($IW$):
  $$\text{Ratio} = \frac{\text{IW}}{\text{CPE}}$$
• Rearranging the formula to solve for $CPE$:
  $$\text{CPE} = \frac{\text{IW}}{\text{Ratio}}$$
• Substituting the given parameters ($IW = 5\text{ cm}$, $\text{Ratio} = 0.5$):
  $$\text{CPE} = \frac{5\text{ cm}}{0.5} = 10\text{ cm}$$
• Thus, irrigation should be scheduled when cumulative pan evaporation reaches 10 cm, confirming Option 2.

Calculation of irrigation depth via IW/CPE ratio:

• Formula: $\frac{\text{IW}}{\text{CPE}} = \text{Ratio} \implies \text{IW} = \text{Ratio} \times \text{CPE}$
• Given: $\text{Ratio} = 0.8$, $\text{CPE} = 80\text{ mm}$
• Depth of irrigation water (IW): $\text{IW} = 0.8 \times 80\text{ mm} = 64\text{ mm} = 6.4\text{ cm}$.