Evaporimeter Pan for Evaporation Estimation

An evaporimeter is a scientific instrument specifically designed to measure the rate at which water evaporates from a surface into the atmosphere. Understanding evaporation is a critical aspect of hydrology and water resource management, as it directly impacts water availability for various uses, including agriculture, municipal supply, and environmental flows. Different designs of evaporimeters exist, but for consistent and comparable data across different regions, standardized instruments are adopted.

Indian Standard IS: 5973-1970 and Evaporation

The Indian Standard (IS) 5973-1970 provides guidelines for the estimation of evaporation in India. This standard plays a crucial role in ensuring uniformity and reliability in evaporation measurements, which are essential for effective water resource planning, irrigation scheduling, and hydrological studies. To achieve this standardization, the IS: 5973-1970 specifies a particular type of evaporimeter pan for use.

Standard Evaporimeter Pan Description

According to the Indian Standard IS: 5973-1970, the evaporimeter pan adopted for the estimation of evaporation has precise dimensions and characteristics. This specific pan is widely recognized for its design and is often compared to or is a derivative of the international "Class A Pan."

The exact specifications of the pan as adopted by IS: 5973-1970 are:

• Shape: Circular pan
• Diameter: \(\text{1.22 meters}\) (which is approximately \(\text{4 feet}\))
• Depth: \(\text{25.5 centimeters}\) (which is approximately \(\text{10 inches}\))

This standard pan is typically made of unpainted galvanized iron and is installed on a wooden platform a few centimeters above the ground. This elevation helps to ensure proper air circulation around the pan, minimizing heat transfer from the ground and ensuring the measured evaporation reflects atmospheric conditions more accurately. The water level inside the pan is usually maintained at a level several centimeters below the rim. Daily readings of the water level drop, along with precipitation data, are used to calculate the daily evaporation rate.

Understanding Evaporation Measurement

The use of a standardized evaporimeter pan, such as the one specified by IS: 5973-1970, is vital for collecting accurate and comparable evaporation data. While pan evaporation provides a good indicator of the evaporative demand of the atmosphere, it typically needs to be adjusted using a "pan coefficient" to estimate the actual evaporation from larger water bodies like lakes or reservoirs, or from natural land surfaces. This coefficient accounts for the differences in heat storage, boundary effects, and other factors between the small pan and a large natural surface.

Therefore, the evaporimeter adopted by Indian Standard IS: 5973-1970 for the estimation of evaporation is specifically a pan with a diameter of \(\text{1.22 m}\) and a depth of \(\text{25.5 cm}\).