Evapotranspiration Calculator: Smart Irrigation Scheduling

Evapotranspiration (ET) — combined water loss from soil evaporation and plant transpiration — determines exact crop water needs. ET-based scheduling reduces water usage by 25-40% while maintaining yields.

What Is Evapotranspiration

Crops lose water daily through transpiration and soil evaporation. Wheat in Punjab loses 4-6 mm/day at peak growth; rice in West Bengal 6-8 mm/day. Traditional fixed-interval irrigation leads to over- or under-watering. ET-based scheduling replaces guesswork: if ET was 5 mm yesterday, irrigate 5 mm today.

The Penman-Monteith Equation Simplified

FAO-56 Penman-Monteith uses temperature, humidity, wind speed, and solar radiation. The simplified Hargreaves equation needs only temperature: ET0 = 0.0023 x (Tmean+17.8) x (Tmax-Tmin)^0.5 x Ra. Sufficient for basic scheduling.

Sensors for ET Calculation

BME280: temperature, humidity, and pressure in one I2C module — three of five parameters. DHT22 as secondary sensor at canopy level captures temperature gradient. DS18B20 waterproof probe at 10cm depth provides soil temperature for evaporation estimation.

Building an ET Weather Station

Hardware: Arduino Mega/ESP32, BME280, DHT22, DS18B20, anemometer (Rs 800-1,500), BH1750 light sensor, RTC, SD card, 12V solar panel. Mount temperature/humidity at 2m height in radiation shield. Anemometer at 2m clear of obstructions.

Arduino Code for ET Calculation

Hargreaves ET calculator on Arduino:

#include <DHT.h>
#include <Adafruit_BME280.h>
#define LAT 19.07
float calcET(float Tmax,float Tmin,int doy){
  float Tm=(Tmax+Tmin)/2, lr=LAT*3.14159/180;
  float d=0.409*sin(2*3.14159/365*doy-1.39);
  float ws=acos(-tan(lr)*tan(d));
  float dr=1+0.033*cos(2*3.14159/365*doy);
  float Ra=(24*60/3.14159)*0.082*dr*(ws*sin(lr)*sin(d)+cos(lr)*cos(d)*sin(ws));
  return 0.0023*(Tm+17.8)*sqrt(Tmax-Tmin)*(Ra/2.45);
}

Smart Irrigation from ET Data

Convert ET0 to crop water: multiply by Kc (crop coefficient). Initial stage Kc=0.3-0.5, mid-season Kc=1.0-1.2. Account for irrigation efficiency: drip 90%, sprinkler 75%, flood 50%.

Indian Conditions

Monsoon: subtract effective rainfall (80% of actual for <75mm). Summer ET exceeds 8mm/day. ICAR publishes India-specific Kc values for local varieties.

Water Savings: Real Numbers

Documented savings: sugarcane Maharashtra 28%, wheat Punjab 22%, cotton Gujarat 35%, vegetables Karnataka 30-40%. For 5 acres of sugarcane with Rs 200/hour pump, 28% reduction saves Rs 15,000-25,000/season.

Frequently Asked Questions

Simplest ET estimate?

Hargreaves needs only daily max/min temperature from a single DHT22.

ET vs tensiometer?

ET predicts proactively; tensiometers detect reactively. Best: combine both.

Government weather data?

IMD data available but 5-15km away. On-field sensors improve accuracy by 15-25%.

Greenhouse ET?

60-80% of outdoor ET. Measure inside with DHT22/BME280.

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