Calculating Water Requirements - Group Info

1. Calculating Water Requirements of Crops

• Measurement of evapotranspiration;

Accomplished by the use of a pan evaporimeter The data gathered from this instrument is known as

“Reference ET or ETo” Or Need a reference point for ET defined as amount of water required by healthy turfgrass completely covering ground and at a uniform height of 4 – 6 in.

2. Calculating Water Requirements of Crops

The objective is to replace the water lost through ET starting with the last irrigation.

Steps to Developing the Irrigation Program Obtain Reference ET Data

Use onsite evaporimeter / weather station data Use local weather station ET data

• Use historical average ET reference data

3. Calculating Water Requirements of Crops

• Measurement of evapotranspiration;

ETo must be adjusted for types of planting (Kc or crop coefficient), microclimate conditions (Kh or microclimate coefficient), and crop density factors (Kd or density coefficient).

This adjustment is referred to as ETadj This information can be read from Table 1.1

4. Calculating Water Requirements of Crops

Table 1.1. Estimated Coefficients for Crop Coefficient (Kc), Microclimate Coefficient (Kh), Density Coefficient (Kd)

• Vegetation type
• Species Coefficient (Kc) Microclimate Coefficient (Kh)

Density Coefficient (Kd) High Ave Low High Ave Low

• High
• Ave
• Low
• Trees
• 9 0.6 0.2 1.4 1.0 0.5 1.2 1.0 0.5 Shrubs 0.7 0.5
• 2 1.3 1.0 0.5 1.1 1.0 0.5 Ground Cover 0.7 0.5
• 2 1.2 1.0 0.5 1.1 1.0 0.5 Mixed 1.0 0.6 0.2 1.4
• 0 0.5 1.3 1.1 0.6 Flowers 1.0 0.7 0,3 1.3 1.0 0.6
• 3 1.0 0.6 Vegetables 0.8 0.7 0.6 1.2 1.0 0.8 1.4
• 0 0.6

5. Calculating Water Requirements of Crops

Steps in implementing a Water Management Program include;

Obtain current Reference ET (ETo) data Obtain Kc, Kh ,Kd

• Calculate ETadj by the following equation;

ETadj= ETo x (Kc x Kh x Kd)

6. Calculating Water Requirements of Crops

• Steps to Developing the Irrigation Program
• Determine Water Application Efficiency (WAE) of the irrigation system

7. Calculating Water Requirements of Crops

Steps to Developing the Irrigation Program Determine Water Application Efficiency (WAE);

This is a decimal value that reflects the portion of water that reaches the rootzone.

• High maintenance = 0.8 (80%) Medium maintenance = 0.7 (70%)
• Low maintenance = 0.6 (60%)

WAE is the portion of water taken from the water source that reaches the root zone. So a WAE of 80% means that 80% of the water reaches the rootzone while the remaining 20% is lost to system leakage, surface runoff etc

8. Calculating Water Requirements of Crops

• Steps to Developing the Irrigation Program

Plug your figures into the following equation calculate the amount of water that is required.

Water Applied (WA) = ETo x (Kc x Kh x Kd) WAE (as a decimal)

9. Calculating Water Requirements of Crops

Irrigation interval refers to the time period between irrigation cycles.

Determine Irrigation Intervals by using the following equation;

Irrigation Interval = 0.5 (Available Moisture) Water Applied