Determining the crop coefficient (Kc) for perennial fruits like mango involves a multi-step process that combines field observations, climate data, and crop water use measurements. The **Kc** is crucial for calculating the actual crop water requirement (ETc) based on the reference evapotranspiration (ET₀). Here’s a methodology you can follow to determine the **Kc** values for a new mango variety in Sri Lanka:

### Steps for Determining Kc for Mango:

#### 1. **Selection of Reference Evapotranspiration (ET₀) Method:**

- Choose a method for calculating reference evapotranspiration (ET₀). The **Penman-Monteith equation** is widely recommended by the FAO (Food and Agriculture Organization). It requires climate data like temperature, solar radiation, wind speed, and humidity.

- Install a weather station near the field to collect reliable climate data.

#### 2. **Field Water Use Measurement:**

- Set up a **lysimeter** to measure the actual crop evapotranspiration (ETc). Lysimeters provide accurate measurements of water use by weighing the soil mass and recording water loss due to plant transpiration and soil evaporation.

- Alternatively, use **soil moisture sensors** or **sap flow measurements** to monitor water consumption by mango trees over time.

#### 3. **Growth Stage Monitoring:**

- Mango trees have distinct growth stages, and the **Kc** values change depending on the growth period:

- **Initial Stage (Kc ini):** After planting or at the start of the growing season (before the canopy develops).

- **Crop Development Stage (Kc mid):** During the phase of active vegetative growth.

- **Mid-Season (Kc mid):** Flowering and fruit development stages.

- **Late Season (Kc end):** Harvesting and post-harvest stages.

- Monitor the tree development and record dates for key stages.

#### 4. **Water Balance Calculations:**

- Determine the crop water requirement (ETc) based on the measured **ET₀** and the observed water use (ETc).

- The formula for determining Kc is:

\[

Kc = \frac{ETc}{ET₀}

\]

- Calculate **Kc** for each growth stage.

#### 5. **Field Replication and Adjustment:**

- Repeat measurements over several growing seasons and in multiple fields to capture variability in climate and soil conditions.

- Adjust the **Kc** values to local conditions in Sri Lanka, such as soil types, rainfall patterns, and local climate influences (humidity, wind, etc.).

### Additional Tools for Accuracy:

- **Remote Sensing**: Utilize satellite imagery and NDVI (Normalized Difference Vegetation Index) to track canopy development, which influences water use.

- **FAO Guidelines**: Consult FAO Irrigation and Drainage Paper No. 56 for comprehensive guidelines on Kc determination and ET₀ calculation.

### Important Considerations:

- Ensure regular irrigation scheduling for consistency.

- Take into account local climatic and soil characteristics.

- Validate results with existing Kc values for other mango varieties grown in similar climates.

By following this methodology, you can develop reliable **Kc** values specific to the new mango variety in Sri Lanka, which will be crucial for determining its water requirements and optimizing irrigation.

Noted & thank you for your information