Understanding and calculating crop water requirements is crucial for efficient irrigation planning. Below are the actionable steps to determine daily and seasonal water needs based on evapotranspiration (ET) data, crop growth stages, local climate conditions, multi-cropping systems, and soil properties.
1. Calculate Daily and Seasonal Water Needs Using Evapotranspiration (ET)
Evapotranspiration (ET) is the combined loss of water from soil and plant surfaces due to evaporation and transpiration.
Steps:
- Find Reference ET (ETo): Obtain ETo data from meteorological stations or online sources. It is expressed in mm/day.
- Determine Crop Coefficient (Kc): This varies by crop type and growth stage.
- Calculate Crop Water Requirement (ETc): ETc=ETo×KcETc = ETo \times KcETc=ETo×Kc Example: If ETo is 6 mm/day and Kc for a crop is 1.2, then: ETc=6×1.2=7.2 mm/dayETc = 6 \times 1.2 = 7.2 \text{ mm/day}ETc=6×1.2=7.2 mm/day
- Convert to Volume: Multiply by the cultivated area (ha) to get total water requirement in cubic meters (m³).
2. Factor in Crop Growth Stages and Peak Water Demand Periods
Different growth stages require different water amounts.
Steps:
- Identify Growth Stages:
- Initial Stage: Germination and early growth (low water demand).
- Vegetative Stage: Rapid leaf and stem growth (moderate water demand).
- Flowering and Fruiting Stage: Peak water demand.
- Maturity Stage: Water demand reduces.
- Adjust Kc Values Accordingly:
- Example for wheat:
- Initial Stage: Kc = 0.3
- Vegetative Stage: Kc = 0.75
- Flowering Stage: Kc = 1.15
- Maturity Stage: Kc = 0.6
- Example for wheat:
3. Account for Local Climate Conditions
Climate factors directly influence water needs.
Steps:
- Monitor Temperature: Higher temperatures increase ET.
- Consider Humidity: Low humidity leads to higher transpiration.
- Factor in Wind Speed: Wind accelerates water loss.
- Include Rainfall in Calculations:
- Subtract effective rainfall from total water requirement.
- Example:
- Total Water Need: 600 mm
- Effective Rainfall: 200 mm
- Irrigation Requirement: 400 mm
4. Adjust for Different Crop Varieties in Multi-Cropping Systems
When growing multiple crops in the same field, their water needs should be balanced.
Steps:
- Determine Individual ETc for Each Crop:
- Example:
- Tomato: ETc = 6 mm/day
- Maize: ETc = 4.5 mm/day
- Example:
- Use Weighted Averages if Necessary:
- If 50% of the land is tomato and 50% is maize: Avg ETc=(6×0.5)+(4.5×0.5)=5.25 mm/day\text{Avg ETc} = (6 \times 0.5) + (4.5 \times 0.5) = 5.25 \text{ mm/day}Avg ETc=(6×0.5)+(4.5×0.5)=5.25 mm/day
- Schedule Irrigation for High-Water-Demand Crops First:
- Example: If tomatoes need daily irrigation and maize requires irrigation every 3 days, prioritize water allocation accordingly.
5. Consider Root Depth and Soil Water-Holding Capacity for Irrigation Frequency
The deeper the root system, the less frequently irrigation is needed.
Steps:
- Determine Root Depth:
- Shallow-rooted crops (e.g., lettuce): 30 cm
- Medium-rooted crops (e.g., wheat, maize): 60 cm
- Deep-rooted crops (e.g., fruit trees): 1–2 meters
- Estimate Available Water-Holding Capacity (AWHC) Based on Soil Type:
- Sandy soil: 25 mm/m
- Loamy soil: 100 mm/m
- Clay soil: 180 mm/m
- Calculate Irrigation Frequency:
- If maize has 60 cm root depth in loamy soil (AWHC = 100 mm/m),
- Available water in root zone = 60 cm × 100 mm/m = 60 mm
- If ETc is 5 mm/day, irrigation is needed every 60 ÷ 5 = 12 days.
Final Thoughts
By following these actionable steps, farmers can accurately determine water requirements, schedule irrigation efficiently, and optimize crop growth while conserving water.