For irrigation in one ha wheat field, the level of CPE is fixed at 80 mm, compute the amount of irrigation water if desired IW/CPE ratio is 0.8.
Show Hint
Always pay close attention to the units requested in the options (cm vs mm).
Multiplying the ratio directly by the CPE value in mm gives the depth in mm, which can be easily converted to cm by dividing by 10.
Step 1: Understanding the Concept:
The IW/CPE ratio is an irrigation scheduling approach based on the climatological water balance.
IW represents the Depth of Irrigation Water applied to the crop during each irrigation event.
CPE represents the Cumulative Pan Evaporation, which is the sum of daily water evaporation measured from a standard open pan evaporimeter since the last irrigation.
By scheduling irrigation at a specific IW/CPE ratio, we match the water applied to the evapotranspirational demand of the crop. Key Formula or Approach:
The relation between irrigation water depth (IW) and cumulative pan evaporation (CPE) is given by:
\[ \text{IW/CPE Ratio} = \frac{\text{IW}}{\text{CPE}} \]
Rearranging this formula to find the depth of irrigation water (IW):
\[ \text{IW} = \text{Ratio} \times \text{CPE} \] Step 2: Detailed Explanation:
Let us identify the parameters provided in the problem:
The level of CPE at which irrigation is scheduled is \(80\text{ mm}\).
The desired scheduling ratio (IW/CPE) is \(0.8\).
Using the rearranged formula:
\[ \text{IW} = 0.8 \times 80\text{ mm} \]
\[ \text{IW} = 64\text{ mm} \]
To express the depth of irrigation water in centimeters, we divide the value by \(10\):
\[ \text{IW (in cm)} = \frac{64\text{ mm}}{10} = 6.4\text{ cm} \]
This indicates that a \(6.4\text{ cm}\) depth of irrigation water must be applied over the one-hectare field when the cumulative pan evaporation reaches \(80\text{ mm}\). Step 2: Final Answer:
The amount of irrigation water required is 6.4 cm.
