HYDROLOGY

EVAPORATION 

  •  the process by which a liquid is converted into gaseous state. Evaporation is often described in hydrology as the net of water vapor transfer because water molecules continuously move between a surface and the atmosphere. 


EVAPOTRANSPIRATION

  • A related process involving water loss from plants. It occurs mainly by day when small pores called stomata, on leaves of plants open up under the influence of sunlight.


FACTORS AFFECTING EVAPORATION (METEOROLOGICAL FACTORS)


  1. SOLAR RADIATION

  • Evaporation is a process of energy exchange.

  • Solar radiation supplies the energy necessary for the liquid water molecules to evaporate.

  1. HUMIDITY

  • As the humidity of air increases, its ability to absorb more water vapor decreases, and the rate of evaporation becomes slower.

  1. WIND SPEED

  • As the liquid water vaporizes from a water body, the air adjacent to this body will be saturated. For the continuation of evaporation, this saturated air should be removed by wind.

  • 10% change in the wind speed = 1-3% change in evaporation. 

  1. TEMPERATURE

  • High liquid water temperature = high molecular motion in the water = the number of molecules leaving the water will be high.


MEASUREMENT OF EVAPORATION

  1. OPEN TANK

  • They are classic and straight forward methods for estimating evaporation. They mimic a natural body of water and allow for direct measurement of water loss due to evaporation. 

  • They are large, circular or square metal tanks filled with water, installed in open areas to avoid obstructions affecting wind flow or solar radiation.

  1. EVAPORATION PANS

  • They are the most widely used method of finding reservoir evaporation.

  • They are relatively small instruments with either circular or square plan sections, and are easily managed and can be transported to any required location for simple installation. 

TYPES OF EVAPORATION PANS

  • CLASS A EVAPORATION PAN

  • It is the standard pan of 1210 mm diameter and 255 mm depth used by the US Weather Bureau and is known as a Class A Land Pan.

  • The depth of water is maintained between 18 cm and 20 cm.

  • ISI STANDARD PAN

  • A pan of diameter 1220 mm and depth 255 mm. It is also known as Modified Class A pan. 

  • The top of the pan is covered in the pan from birds. It makes the temperature of water more uniform during the day and night. 

  • COLORADO SUNKEN PAN

  • This square pan is 920 mm sides, 460 mm deep, and buried into the ground within 100 mm of the top.

  • It is aerodynamic but difficult to detect leaks, expensive to install, extra care is needed to keep the surrounding area free from tall grass, dust, etc.

  • US GEOLOGICAL SURVEY FLOATING PAN

  • A square pan of 900mm sides, 450 mm deep. 

  • Supported by drum floats in the middle of a raft size 4.25m x 4.87m, it is set afloat in a lake with a view to simulate the characteristics of a large body of water.

  • Water level in the pan is maintained at the same level as that in the lake, leaving a rim of 75 mm.

  1. ATMOMETERS

  • It is a scientific measurement used for measuring the rate of water evaporation on a wet surface to the atmosphere.

  • They are mainly used by the farmers and growers to measure evaporation rates of crops at any field location.

TYPES OF ATMOMETERS

  • PICHE ATMOMETER

  • It consists of graduated glass tubes of 15mm diameter and 300 mm in length. 

  • It is filled with water and covered with filter paper.

  • The tube is kept in an inverted position so that there is a continuous supply of water to the filter paper.

  • LIVINGSTONE ATMOMETER

  • It consists of a 50 mm diameter spherical surface of 2.5 mm thick porous material. 

  • The bottle is filled with distilled water that is supplied continuously to the porous bulb.

  1. LYSIMETER

  • A measuring device which can be used to measure the amount of evapotranspiration which is released by plants. 

  • Lysimeter is a large container holding soil and plants.

  • By recording the amount of precipitation that an area receives and the amount of water lost through the soil, the amount of water lost to evapotranspiration can be calculated.

  • Lysimeters are a tool for water balance studies and solute transport determination.

  1. EDDY COVARIANCE SYSTEM

  • One of the most accurate ways to measure evapotranspiration at the field scale. 

  • It calculates the covariance between vertical wind speed and the concentration of water vapor in the air. It provides a measurement of the evaporation rate at which water vapor is moving upward from the surface. 

  1. ROLE OF METEOROLOGICAL DATA IN WEATHER STATIONS

  • By providing important atmospheric conditions like temperature, humidity, wind speed, and atmospheric pressure, which directly affect the energy available for water molecules to transition from a liquid to a gaseous state and thus impact the rate of evaporation at a given location, meteorological data from weather stations is vital for understanding and calculation evaporation rates. This data is important for a number of applications, including hydrology, agriculture, and climate studies.


EVAPORATION FROM DIFFERENT SURFACES


EVAPORATION is the process where liquid water transforms into vapor and enters the atmosphere. Its rate depends on the surface type and environmental conditions. 

  • Evaporation is particularly significant over large bodies of water such as lakes, reservoirs, and the ocean.

  • Evaporation from a particular surface is directly related to the opportunity for evaporation (availability of water) provided by that surface. 


UNDERSTANDING EVAPORATION ACROSS SURFACES

  1. OPEN WATER

  • Evaporation is at 100% opportunity since water is fully available. Common in lakes, reservoirs, and oceans.

  1. SOIL

  • Evaporation varies from 100% (high saturation, e.g., after storms) to nearly 0% (low moisture content).

  1. PAVED SURFACE

  • Evaporation occurs from rainwater or moisture content on these surfaces, but rates can vary depending on the temperature and surface type.

  1. VEGETATION

  • Evaporation levels fluctuate over time and depend on factors like plant type and environmental conditions. 


EVAPORATION IN DIFFERENT CLIMATES AND ENVIRONMENT

  1. TROPICAL REGIONS

  • High evaporation: due to high temperatures, abundant sunlight, and moisture availability,

  1. DESERT/ARID REGIONS:

  • High evaporation: despite low water availability, high temperatures and dry conditions promote rapid evaporation.

  1. COLD REGIONS

  • Low evaporation: due to low temperatures and frozen surfaces. examples : Arctic Regions.

  1. URBAN AREAS

  • Moderate to High Evaporation: evaporation from moisture on concrete, rooftops, and vegetations in cities. Example: Metro Manila, Cebu City, and Davao City.



TWO MAIN FACTORS INFLUENCING EVAPORATION FROM AN OPEN WATER SURFACE

  1. Energy Supply

  2. Water Transport


THREE FACTORS AFFECTING EVAPOTRANSPIRATION

  1. Energy Supply

  2. Vapor Transport

  3. Supply of Moisture


The rate of evaporation depends on weather, soil, and vegetation, with water availability acting as a limiting factor. Climate change impacts these elements in different ways, causing variation in evaporation rates across surfaces like bare soil, vegetation, and water bodies.






APPLICATION IN WATER RESOURCE MANAGEMENT

  1. WATER SUPPLY PLANNING

  • Evaporation affects how much water remains available for irrigation, drinking water, and industrial use. It also helps in predicting storm runoff and river flow forecasts, which are essential for managing water supply projects. 

  1. EVAPORATION CONTROL FOR WATER CONSERVATION RESERVOIR

  • Selecting the site and design yielding a minimum of reservoir area per unit of storage is advantageous.

  1. CLIMATE ASSESSMENT FOR RESERVOIRS

  • To maintain a stable reservoir level, engineers and water managers must account for the heat transfer process (radiation, conduction, and energy stored in water bodies).

  1. IRRIGATION PLANNING AND SCHEDULING

  • Evaporation data helps calculate the exact water needed for crops. Using irrigation techniques and planning minimizes soil moisture loss.

  1. EXCESSIVE WATER LOGGING & SALINITY BUILDUP 

  • Excessive irrigation + high evaporation = salt accul=mulation in soil, damaging crops.