Water balance is about balancing water input (rain, irrigation) against water output (evaporation and plant use). ET – evapotranspiration – measures how much water is actually consumed. In Agdir, ET is calculated based on degree days, temperature, and humidity, and compared with rainfall to show the real water demand per field. This way, irrigation decisions are based on actual needs, not assumptions.

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ET and water balance explained simply‍

ET is the sum of water evaporating from the soil and water released from the plants. High temperature and low humidity give high ET; lower temperature and high humidity give lower ET. In Agdir, ET is calculated continuously based on weather data and compared with rainfall to show accumulated water demand.

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How Agdir calculates water demand per field

ET calculation based on weather
· Temperature: Higher temperatures increase both evaporation and plant water use
· Humidity: Low humidity increases evaporation; high humidity reduces it
· Wind: Increases evaporation by moving humid air away from plant surfaces
· Sunlight: Raises temperature and drives evaporation

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Water balance accounting per field
Input: Rain + irrigation + stored soil moisture
Output: ET (evaporation + plant use)
Balance: Positive = surplus, Negative = deficit/water stress

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Crop-specific adjustments
Different crops have different water consumption. Agdir adjusts ET calculations based on crop and growth stage.

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Practical use of ET data in irrigation planning

Daily accumulation
Agdir sums ET minus rainfall day by day. A negative balance indicates increasing water demand.

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Threshold values per soil type‍

· Sand: Start irrigation at 15–20 mm accumulated deficit
· Silt: Start irrigation at 25–30 mm accumulated deficit
· Clay: Start irrigation at 35–45 mm accumulated deficit

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Weather forecast adjustments‍

Upcoming rain can delay irrigation; upcoming heat peaks accelerate the need.

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ET under different weather conditions

Hot, dry days (high ET)
High temperature + low humidity + sun = 6–10 mm ET per day. Frequent irrigation needed on sandy soil.

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Cool, humid days (low ET)
Lower temperature + high humidity + cloudy = 1–3 mm ET per day. Longer intervals between irrigations.

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Windy days
Wind increases ET significantly even at moderate temperatures. Adjust irrigation needs upward.

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Integration with soil moisture sensors‍

ET calculations are combined with actual soil moisture measurements for higher precision:
· Sensor confirms stress: Low soil moisture + high accumulated ET = irrigation needed now
· Sensor corrects ET: If soil moisture is high despite high ET, roots may have access to deeper water layers

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Seasonal variation in ET calculation

Early season
Low ET values due to small plants and lower temperatures. Focus on establishment rather than volume calculation.

Mid-season
Highest ET values when plant biomass is large and temperatures are high. Most critical period for irrigation planning.

Late season
Declining ET as plants mature. Gradually reduce irrigation frequency.

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Typical water balance calculations

Example 1: Warm, dry period
ET = 7 mm/day for 5 days = 35 mm accumulated. No rain. Sandy soil needs irrigation.

Example 2: Mixed weather
ET = 4 mm/day for 3 days = 12 mm. Rain 8 mm on day 2. Accumulated deficit = 4 mm. Waiting is OK.

Example 3: Cool, humid period
ET = 2 mm/day for 4 days = 8 mm. Rain 15 mm. Surplus = 7 mm. No irrigation needed.

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Optimizing water use with ET data‍

· Irrigate in the evening when ET is lowest to reduce immediate evaporation loss
· Adjust irrigation volume to accumulated ET deficit, not fixed doses
· Postpone irrigation when forecasts show rain that will cover the ET demand

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Summary ‍

Water balance and ET calculation make irrigation decisions more precise and less guesswork-based. When Agdir calculates actual water use based on weather and crop, irrigation can be adapted to real needs rather than routines.

Activate ET calculation per field in Agdir and follow the water balance for a week – see how weather conditions affect irrigation demand and adjust supply accordingly.