Evaporation Monitor: Pan Evaporimeter with Sensor

Evaporation rate is a critical parameter for agriculture, reservoir management, and water resource planning. India loses an estimated 70% of its rainfall to evaporation before it can be used for irrigation or drinking water. Building an automated evaporation monitor helps farmers and water managers track water loss and plan irrigation schedules. This guide shows you how to build a digital pan evaporimeter with Arduino sensors.

What Is Evaporation and Why Measure It

Evaporation is the process by which liquid water becomes water vapour and enters the atmosphere. The rate depends on temperature, humidity, wind speed, and solar radiation. In India, evaporation rates vary dramatically — from 2-3 mm/day in coastal Kerala during monsoon to 12-15 mm/day in Rajasthan during May.

Measuring evaporation helps determine:

• Irrigation water requirements for crops (evapotranspiration)
• Water loss from reservoirs and lakes
• Drought severity assessment
• Optimal timing for irrigation

Understanding Pan Evaporimeters

The standard Class A evaporation pan (used by IMD and USDA) is a cylindrical tank 120.7 cm diameter and 25.4 cm deep, made of galvanised iron or stainless steel. Water level is measured daily — the difference plus any rainfall is the evaporation.

A DIY version can use any cylindrical container, though for IMD-compatible data you should use the standard dimensions. The key is accurate water level measurement and rain exclusion or measurement.

Sensors for Automated Evaporation Monitoring

For automated monitoring, you need:

• Ultrasonic sensor (JSN-SR04T waterproof) — Mounted above the pan, measures water surface distance. Resolution: 1mm. Calculates water level changes without contact.
• BME280 — Measures temperature, humidity, and pressure for evapotranspiration estimation.
• Rain gauge (tipping bucket) — Measures precipitation to subtract from the water level change.
• DS18B20 waterproof probe — Measures water temperature in the pan.

Building a Digital Pan Evaporimeter

Mount the JSN-SR04T waterproof ultrasonic sensor on a bracket above the centre of the pan, pointing down at the water surface. The sensor should be 30-50 cm above the water. At this range, the JSN-SR04T provides ±1mm accuracy — sufficient to detect daily evaporation of 2-15 mm.

The BME280 goes in a radiation shield mounted on a pole beside the pan. The DS18B20 waterproof probe hangs in the water at 5 cm depth. Connect all sensors to an Arduino or ESP32 with an SD card for logging.

Arduino Code for Evaporation Rate Calculation

The code reads the ultrasonic sensor every hour, calculates the water level change, and subtracts rainfall:

// Evaporation calculation
float currentLevel = readUltrasonic(); // distance from sensor to water
float evaporation = previousLevel - currentLevel - rainfall;
// Positive = evaporation, Negative = more rain than evaporation

// Penman-Monteith estimate (simplified)
// ET0 = (0.408 * delta * Rn + gamma * 900/(T+273) * u2 * (es-ea))
//       / (delta + gamma * (1 + 0.34 * u2))

Store hourly readings with timestamps. Calculate daily totals at midnight. The Penman-Monteith equation (shown in simplified form above) estimates reference evapotranspiration from your BME280 data and is the FAO standard method used worldwide.

Evapotranspiration Estimation for Farmers

For Indian farmers, evapotranspiration (ET) determines how much water crops need. The relationship is simple: Crop Water Need = Crop Coefficient × Reference ET. Crop coefficients for major Indian crops:

• Rice (paddy): 1.05-1.20
• Wheat: 0.80-1.15
• Sugarcane: 0.85-1.25
• Cotton: 0.65-1.25
• Tomato: 0.60-1.15

Your evaporation monitor, combined with these coefficients, tells farmers exactly when to irrigate and how much water to apply. This can reduce water usage by 20-30% compared to flood irrigation schedules.

Solar-Powered Field Deployment

For field deployment, power the system with a 6W solar panel and 18650 lithium battery. The ESP32 in deep sleep draws only 10 microamps, waking every hour to take readings and once daily to transmit data via WiFi or LoRa to a gateway.

Protect the electronics in an IP67 enclosure. The ultrasonic sensor is already waterproof (JSN-SR04T), but ensure the cable connections are sealed with heat-shrink tubing and silicone.

Data Analysis for Irrigation Planning

After collecting data for a full season, analyse the trends:

• Daily evaporation patterns — peak hours and cumulative daily totals
• Correlation between BME280 humidity/temperature and measured evaporation
• Validation against IMD published pan evaporation data for your district
• Crop water stress periods — when evaporation exceeds irrigation capacity

This data is valuable for Krishi Vigyan Kendras (agricultural science centres) and local water management bodies. Share your findings to contribute to India’s water conservation efforts.