🌱 What is Evapotranspiration? A Complete Learning Guide (2021)

🌱 What is Evapotranspiration? A Complete Learning Guide (2021)

EVAPOTRANSPIRATION

The Complete Learning Guide

written by Stanley Udegbunam || Dec 27, 2020

evapotranspiration - The complete learning guide

AFRILCATE

WHAT IS EVAPOTRANSPIRATION?

Evapotranspiration (ET) is the sum of evaporation from land surfaces and plant transpiration to the atmosphere.

Evapotranspiration = Land Surface Evaporation + Plant Transpiration

Evaporation and transpiration are water cycle processes and they are the two integral part that makes up evapotranspiration.

Let’s look at these two parts in details.

Evaporation is the phase transition of surface water from water bodies, leaf surfaces, land surfaces to the air as gasses.

It is the primary pathway that aids the movement of water back into the water cycle as atmospheric water vapor and it accounts for 90% of the moisture in the atmosphere. 

Transpiration is a process that involves the loss of water vapor from plant tissues like stomata, cuticles or lenticels.

It contributes the remaining 10 percent of the atmospheric moisture.

Transpiration in plants is required for plant growth and development, cooling the leaf surface, cell turgidity and aids the movement of vital minerals.

Evapotranspiration is also an important process of the water cycle.

POTENTIAL EVAPOTRANSPIRATION (PET)

Potential evapotranspiration (PET) is the rate of evapotranspiration from extended surfaces of short green crops that fully shades the ground.

These crops exert little or negligible resistance to the flow of water, and is always well supplied with water.

Potential evapotranspiration is the amount of evapotranspiration that is expected over a surface with no limitation of water.

It is quite different from actual evapotranspiration which is the amount of evapotranspiration that actually occurs when water is limited.

FACTORS AFFECTING EVAPOTRANSPIRATION

They are so many factors affecting evapotranspiration and they are grouped into three categories:

  1. Climatic factors
  2. Crop factors
  3. Crop management and environmental conditions.
  1. CLIMATIC FACTORS

Climatic factors that affect evapotranspiration includes:

  • Radiation – This is the main source of energy for the evapotranspiration process.

It depends on the global solar radiation flux density and vegetation

  • Temperature – An increase in temperature will lead to a corresponding increase in evapotranspiration rate.

Temperature increases the evaporation rate because the added heat increases the kinetic energy of the water molecules.

The transpiration rate increases also because at warmer temperatures, plants open up their stomata and release more water vapor.

  • Relative Humidity– The higher the relative humidity, the lesser the evapotranspiration rate and vice-versa.
  • Wind speed – If air is moving, not only will the evaporation rate increase, but because saturated air around the leaves is blown away, the transpiration rate will increase as well.

2. CROP FACTORS

    • Crop species – different crops have varying stomata distribution, sizes, internal resistance to water transport and spatial distribution of leaves.

Some plants, like cacti and other succulents, naturally hold onto their water and don’t transpire as much.

These independent properties have a direct effect on evapotranspiration.

    • Plant height – Taller plants extract more energy from the air and interact more efficiently with the atmosphere therefore increasing evapotranspiration.
    • Leaf area index (LAI) – LAI in different growth stages of a plant is directly related to the size of the transpiring foliar surface.
    • The larger the leaf area, the larger the transpiring surface and the higher the potential for water use.

3. CROP MANAGEMENT AND ENVIRONMENTAL CONDITIONS

    • The following crop management and environmental conditions influences crop ET:

    • Soil properties (structure and texture): The soil properties determines the soil holding capability of the soil.

    It also determines the easy at which water is drawn out of crops through transpiration pull and evaporation.

    For areas where the ground is covered by vegetation, the rate of transpiration is considerably higher than the rate of evaporation from the soil.

    • Crop orientation: crop oriented perpendicularly to predominant winds tend to extract more energy from the air than those oriented in parallel.
    • Plant spacing: to avoid competition for water, light, nutrient, etc. plants should be spaced properly. Optimum plant spacing boosts evapotranspiration.

    EVAPOTRANSPIRATION AND AGRICULTURE

    Evapotranspiration accounts for most of the water lost from the soil during the growth of a crop.

    You can estimate the water demands of various crops if only you can predict evapotranspiration rates.

    This prediction will help you determine whether to irrigate or not.

    Evapotranspiration and water cycle

    Knowledge of predicted temperature and wind conditions from weather forecasts can give you a clue to how strong the evapotranspiration rates will be.

    Numerous approaches are available for evapotranspiration estimation.

    They include:

    • water balance methods
    • energy balance methods, and
    • aerodynamic formulations (Brutsaert, 2005).

    Estimation of evapotranspiration rates is thus important in planning irrigation schemes.

    DO YOU KNOW?

    An element such as a tree that contributes to evapotranspiration is called an evapotranspirator.

    DO YOU KNOW?

    An element such as a tree that contributes to evapotranspiration is called an evapotranspirator.

    What is Desublimation? Process, Causes and Examples (updated)

    What is Desublimation? Process, Causes and Examples (updated)

    WHAT IS DESUBLIMATION?

    Causes, Process and Examples

    written by Stanley Udegbunam || Dec 25, 2020

    desublimation - deposition

    AFRILCATE

    WHAT IS DESUBLIMATION?

    Desublimation is a phase transition in which a gas changes directly to a solid without passing through an intermediate liquid phase. 

    It is an exothermic phase change that occurs at temperatures and pressures below a substance’s triple point in its phase diagram.

    Desublimation is also called deposition.

    WHAT CAUSES DESUBLIMATION?

    Desublimation is caused by a drastic loss in thermal energy from the surrounding gas due to the presence of a much cooler surface.

    For example, frost formation occurs on window surfaces during the winter seasons.

    This window is at a much cooler temperature, this makes the water vapor in the surrounding air to lose enough thermal energy and change directly into solid which we see as frost during winter.

    glass frost

    DESUBLIMATION PROCESS

    Desublimation is an exothermic phase change, it releases energy in the form of heat to the surrounding.

    Desublimation is the reverse process of sublimation.

    Recall that:

    Recall that: ∆Hsub = ∆Hfus + ∆Hvap

    In an ideal condition, the energy required to cause desublimation is the same energy required to cause sublimation for a given substance.

    Therefore:

    Heat of Desublimation = Heat of sublimation

    DESUBLIMATION EXAMPLES

    • water vapor changing directly to ice
    • Formation of snow
    • Soot formation in chimneys
    • Formation of frost on windows
    • A carbon dioxide fire extinguisher is filled with gaseous carbon dioxide but the higher pressure inside the canister causes it to turn into solid carbon dioxide which is later released as a white powder when extinguishing.
    • Desublimation is utilized in an industrial coating. A process known as evaporative deposition.

    ♨️ What is Evaporation? Causes, Process and Examples (updated)

    ♨️ What is Evaporation? Causes, Process and Examples (updated)

    EVAPORATION ♨️ 

    Meaning, Causes, Process and Examples

    written by Stanley Udegbunam || Dec 20, 2020

    evaporation in science

    AFRILCATE

    WHAT IS EVAPORATION?

    Evaporation is the spontaneous transition of a substance from the liquid phase to the gaseous phase.

    It occurs below the boiling temperature of the liquid. 

    It is the primary pathway that aids the movement of water into the water cycle as atmospheric water vapor.

    Therefore, evaporation is an essential part of the water cycle. 

    Evaporation accounts for 90 percent of the moisture in the atmosphere.

    The remaining 10 percent is contributed by plant transpiration.

    Evaporation is different from boiling. It occurs only at the surface of the liquid.

    Water boils at 100C (212F) but it begins to evaporate at 0C (32F). This means that evaporation takes place over a wide range of temperatures.

    Evaporation occurs at a very slow rate but as the temperature increases, the rate of evaporation increases also.

    hot spring evaporating

    WHAT CAUSES EVAPORATION?

    1. Surface Exposure
    2. Heat
    3. Saturation (humidity)
    4. Air Pressure

    1. Surface Exposure

    Evaporation occurs when the surface of the liquid is exposed.

    It doesn’t occur throughout the whole body or volume of the liquid but only at the liquid’s surface.

    For instance, if a bottle of whiskey is open, the liquid will begin to evaporate with time.

    A tightly corked bottle of methylated spirit will contain the same volume of liquid content but when the cork is removed, the content of the bottle reduces gradually.

    The wider the opening or exposure, the faster the evaporation.

    2. Heat

    Heat (thermal energy) is necessary for evaporation to occur.

    Liquids evaporate faster when it is heated because the added heat results in an increase in the kinetic energy of the water molecules.

    Continuous heating makes it possible for a greater fraction of the liquid molecules to break the intermolecular bond and escape from the surface to the atmosphere.

    evaporation in hot spring

    evaporation in hot spring

    3. Saturation (Humidity)

    If the air is saturated with air particles, It will be difficult for evaporation to occur.

    If the humidity is 100 percent, it means that the air is saturated with water.

    At this point, evaporation can’t take place.

    4. Air Pressure

    Air pressure also affects evaporation.

    If air pressure is high on the surface of a body of water, then the evaporation process will be drastically reduced.

    The pressure pushing down on the water makes it difficult for water to escape into the atmosphere as vapor.

    Storms are often high-pressure systems that prevent evaporation.

    EVAPORATION PROCESS

    The collision of molecules leads to the transfer of energy from one to the other.

    When a molecule near the surface absorbs enough energy to overcome the vapor pressure, it escapes and enters the surrounding air as a gas.

    During evaporation, the energy removed from the vaporized liquid will reduce the temperature of the liquid, resulting in a cooling effect which is termed evaporative cooling.

    Evaporation will continue until an equilibrium is reached.

    At equilibrium, the rate of evaporation of the liquid equals its condensation rate.

    On a global scale, the amount of water evaporating from the earth’s surface is roughly the same amount delivered back to the earth as precipitation although, a variation may exist due to geographical location.

    Evaporation is one of the three main processes in the earth’s water cycle. The other two being condensation and precipitation.

    If the liquid is in an enclosed environment, it will evaporate until the surrounding air is saturated.

    Evaporative cooling alongside convection help balance the earth’s surface from the overheating effect of solar radiation and greenhouse emission.

    EXAMPLES OF EVAPORATION

    Evaporation is a common phenomenon. Let’s discuss some common examples.

    1. Evaporation of perspiration from the body: sweat evaporates from the body taking away the body heat thereby, causing a cooling effect.
    2. Drying clothes under the sun: wet clothes get dried when placed under the sun because the heat generated evaporates the water.
    3. Common salt preparation: Salt is recovered naturally by evaporation of seawater.
    4. Ironing of clothes
    5. Evaporation of nail paint remover
    6. Drying up of water and potholes from streets
    7. Cooling of tea and other hot drink.

    DO YOU KNOW?

    Transpiration is the evaporation of water from plant tissue mainly through the stomates of leaves.

    In hydrology, evaporation and transpiration are collectively termed evapotranspiration.

    DO YOU KNOW?

    Transpiration is the evaporation of water from plant tissue mainly through the stomates of leaves.

    In hydrology, evaporation and transpiration are collectively termed evapotranspiration.

    What is Condensation? – The Best Learning Guide

    What is Condensation? – The Best Learning Guide

    CONDENSATION

    The Best Learning Guide

    written by Stanley Udegbunam || Dec 20, 2020

    what is condensation

    AFRILCATE

    WHAT IS CONDENSATION?

    Condensation is the change of the physical state of matter from the gaseous phase to the liquid phase.

    It is a process in which water vapor in the air changes into liquid water when it comes in contact with a cooler surface.

    Let’s relate this with a real-life scenario.

    Imagine that you are having a hot shower. If you take a look at the mirror after the bath, you will notice that the mirror is covered with fog and you can’t clearly see your reflection.

    Try wiping the steamy fog from the mirror and you will notice that they are nothing but tiny water beads.

    The water vapor which is at an elevated temperature, cooled down when it came in contact with the mirror which happens to be at a much more reduced temperature.

    This cool surface changes the water vapor in the surrounding to tiny water droplets visible on the mirror.

    This phenomenon is known as condensation.

    condensation meaning

    WHAT CAUSES CONDENSATION?

    Condensation is caused by a change in pressure and temperature of water vapor.

    It occurs when:

    • a vapor is cooled or pressured to its saturation limit.
    • when the molecular density in the gas phase reaches its maximal threshold.
    • water vapor comes in contact with hygroscopic condensation nuclei in the atmosphere.

    In free air, condensation occurs due to the presence of tiny particles (about 0.2 – 10.0 microns) light enough to remain suspended in the air.

    These micro-particles are called condensation nuclei.

    Condensation nuclei are formed from a variety of water nucleates including dust, smoke, pollen, salt from ocean spray and sulfates.

    They provide surfaces upon which water vapor can condense to create cloud droplets.

    Condensation is largely dependent upon the amount of cooling and the relative humidity of the air. 

    causes of condensation

    CONDENSATION PROCESS

    Just like any other matter, water also consists of molecules.

    In a vapor form, the molecules are energetic, fast-moving and far apart.

    As the vapor encounters cooler temperatures, the molecules become slower, less energetic and closer together.

    Subsequently, the vapor turns into liquid upon reaching a threshold energy level.

    CONDENSATION HOME EXPERIMENT

    Anyone can demonstrate the process of condensation through an easy experiment.

    All you need is just two cups and hot water.

    Let’s get started…

    • Fill one cup half-level with the hot water.
    • Observe the steam gushing out of the hot water.
    • Now, take the other cup, flip it upside down, and carefully put it on top of the half-filled cup.
    • Wait for 5 mins, and take 2nd cup down.

    You will see that the steam condenses back to liquid at the top surface of the 2nd cup when placed upside down.

    As condensation occurs, the water molecules become closely packed and this results in the release of heat to the atmosphere.

    EXAMPLES OF CONDENSATION

    You don’t have to look at something as far away as the cloud to notice condensation.

    It is a daily occurrence and everywhere around us.

    Some examples of condensation phenomenon include:

    1. Droplets on can or bottle: Having a cold soda on a hot day, the can “sweats.” Water molecules in the air as a vapor hit the colder surface of the can and turn into liquid water.
    2. Morning Dew: Dew forms in the morning on leaves and grass because of the warmer air deposits water molecules on the cool leaves.
    3. Foggy windshield: When you get in your car in the morning and are breathing warmer air, the warm air comes into contact with the cooler windshield and forms a foggy window.
    4. Foggy mirror: The mirror in the bathroom during a warm shower becomes foggy because warmer water vapor in the air hits the cooler surface of the mirror.
    5. Clouds: Clouds form by condensation, warmer air comes into contact with cooler dust particles and the vapor deposits into a liquid on those particles forming clouds.
    morning dew

    morning dew on plants

    DO YOU KNOW?

    Vapor cooling and compressing equipment that collects condensed liquids is called a “condenser”.

    The reverse of condensation is vaporization.

    DO YOU KNOW?

    Vapor cooling and compressing equipment that collects condensed liquids is called a “condenser”.

    The reverse of condensation is vaporization.

    What is Vaporization? Factors, Types and Examples (explained)

    What is Vaporization? Factors, Types and Examples (explained)

    What is Vaporization? Factors, types and Examples

    Compiled by Stanley Udegbunam || Dec 08, 2020

    AFRILCATE

    WHAT IS VAPORIZATION?

    Vaporization is the phase transition from the liquid phase to the vapor phase.

    The molecules of a liquid are in constant motion and possess a wide range of kinetic energy.

    With the addition of heat, some molecules gain energy sufficient enough to break the molecular bond.

     The molecules then escape from the liquid to the atmosphere as gas or vapor.

    TYPES OF VAPORIZATION

    There are two types of vaporization:

    1. Evaporation
    2. Boiling

    Evaporation is the spontaneous transition of a substance from the liquid phase to the gaseous phase and it occurs at temperatures below the boiling temperature at a given pressure.

    Evaporation occurs on liquid surfaces and is only initiated when the partial pressure of the vapor of a substance is less than the equilibrium vapor pressure.

    As water molecules evaporate, the liquid surface gets cooler, a phenomenon called evaporative cooling.

    Similar to evaporation, boiling is also the phase transition from the liquid phase to the gaseous phase.

    The major difference between boiling and evaporation is that boiling is a bulk process and only occurs when a liquid is heated to its boiling point, unlike evaporation that’s a surface phenomenon, occurs slowly and across any temperature and it’s driven only by pressure difference.

    During boiling, the entire bulk of the liquid, the liquid molecule (interior and surface inclusive) all gain sufficient energy to change to vapor state.

    The boiling process begins when the equilibrium vapor pressure of the substance is greater than or equal to the environmental pressure.

    The temperature at which boiling occurs is the boiling temperature or boiling point.

    The boiling point varies with the pressure of the environment.

    definition of vaporization - water vaporizing at boiling point

    At boiling point, water vaporizes into steam

    FACTORS AFFECTING THE RATE OF VAPORIZATION

    There are different factors that affect the rate of vaporization. 

    They Include:

    1. Temperature
    2. Pressure
    3. Molecular Bond
    4. Surface Area
    5. Wind speed

    1. Temperature: With an increase in temperature, the molecules gain more kinetic energy and the rate of vaporization increases.

    2. Pressure: The higher the atmospheric pressure, the more difficult it is to reach boiling point and more energy will be required to cause boiling.

    3. Molecular bond: A substance with a weak molecular bond will boil faster than one with a stronger bond because less energy is required to overcome the force of attraction.

    4. Surface area: With the increase in surface area, the rate of vaporization also increases as a greater number of particles is exposed to the change in temperature.

    5. Wind speed: Impurities like particles in the air are the major cause of high external pressure. With an increase in wind speed, the vaporization rate increases as particles are driven away by the wind.

    WHAT IS HEAT OF VAPORIZATION?

    Heat of vaporization is the amount of energy required to transform a given quantity of liquid to gas.

    The heat of vaporization is temperature-dependent, though a constant heat of vaporization can be assumed for small temperature ranges.

    Don’t get confused, the heat of vaporization is the same as enthalpy of vaporization since heat is synonymous with enthalpy.

    Heat of vaporization is a function of the pressure at which that transformation takes place. 

    At 100 °C (boiling point), the heat of vaporization for water is 5 40 cal/g (2,260 kJ/kg).

    EXAMPLES OF VAPORIZATION

    • wet clothes get dried when placed under the sun due to the process of vaporization.
    • Salt is recovered naturally by vaporization of seawater.
    • Vaporization is utilized in many industrial processes for separating the components of a mixture.

      4 Complete Geological Formations of Groundwater (The 4-Aqui’s)

      4 Complete Geological Formations of Groundwater (The 4-Aqui’s)

      4 Complete Geological Formations of Groundwater (The 4-Aqui’s)

      Compiled by Stanley Udegbunam || Dec 06, 2020

      In today’s article, we will be looking at the 4 different geological formations of groundwater which I call the 4 -Aqui’s.

      What does the term “4-Aqui’s” means? You will find out shortly as we proceed.

      There’s a comparison table towards the end of the article to give you a clearer understanding of the various geological formations of groundwater.

      Without further ado, let’s get started.

      AFRILCATE

      GEOLOGICAL FORMATIONS OF GROUNDWATER

      Groundwater is the largest source of freshwater for mankind and approximately 30% of the freshwater on Earth is groundwater.

      It is the water present beneath earth’s surface in soil pore spaces, underground bedrocks and in the fractures of rock formations.

      Groundwater forms when rainwater infiltrates the soil through precipitation and percolates downwards until it reaches the water table.

      In hydrology, this process of groundwater formation is called recharge.

      They are four different types of geological formations of groundwater:

      1. Aquifer
      2. Aquitard
      3. Aquiclude
      4. Aquifuge

      This is what we refer to as the 4-Aqui’s

      WHAT IS AN AQUIFER?

      An aquifer is a body of porous rock or sediments saturated with groundwater.

      They are highly permeable and porous geological formations.

      Aquifers are made of unconsolidated materials like sand, gravel, and rocks that store and transmit water.

      Generally, there are two types of aquifers:

      1. Unconfined Aquifer
      2. Confined Aquifer

      The difference between an unconfined and confined aquifer is that:

      An unconfined aquifer has a layer of permeable material above it which allows for water seepage into the aquifer.

      While a confined aquifer has layers of impermeable rock or clay above it which hinders the passage of water into the aquifer.

      The restricting layers of confined aquifer offer some protection from surface contamination, unlike the unconfined aquifer.

      Geological formations of groundwater

      Geological Formations of Groundwater: confined and unconfined aquifer

      STILL CURIOUS?

      We have a stand-alone article that takes you through everything you need to know about aquifers.

      see it here:

      WHAT IS AN AQUITARD?

      Aquitard is a saturated, poorly permeable geological unit that retard the flow of groundwater.

      They partially disrupt the flow of groundwater by acting as separation layers between aquifers.

      Although water cannot flow very fast through an aquitard, significant quantities of water can still seep through aquitards in some conditions.

      This signifies that aquitards are porous and partially permeable.

      A typical example of aquitard is sandy clay and a poorly fractured igneous or metamorphic rock.

      One common misconception is that Aquitard stops underground water flow, this is wrong.

      Aquitard only slows down the groundwater flow, it doesn’t stop it completely.

      Therefore, an aquifer with layers of aquitard above it is unconfined.

      WHAT IS AN AQUICLUDE?

      Aquiclude is a saturated, geological formation, which is porous but not permeable.

      By been porous, It means that aquicludes have pore spaces that can contain and store large amount of water. 

      Not permeable – signifies that aquicludes don’t allow for the percolation of water downstream to the aquifers.

      Unlike aquitard, aquicludes completely block the passage of groundwater unless been acted upon by external pressure.

      An aquifer with a layer of aquiclude above it is said to be confined.

      Typical examples of aquicludes are argillaceous rocks like shale and clay.

      WHAT IS AN AQUIFUGE?

      Aquifuge is a geological formation that is neither porous nor permeable.

      Aquifuges don’t store water and since there are no interconnected openings, water can’t also flow through.

      Naturally, aquifuges are not suitable for groundwater occurrence.

      Typical examples of aquifuges include:

      • Compact rocks like massive granites
      • basalt and
      • quartzite 

      Aquifers with layers of aquifuge above it are said to be confined.

      COMPARISON OF THE DIFFERENT GEOLOGICAL FORMATIONS OF GROUNDWATER

      PropertiesAQUIFERAQUITARDAQUICLUDEAQUIFUGE
      POROSITY
      PERMEABILITYPartially
      YIELD OF WATERSlow Yielding
      EXAMPLESUnderground gravel and sand layerssandy clay and a poorly fractured igneous or metamorphic rock.Argillaceous rocks like shale and clay.Compact rocks like massive granites, basalt and quartzite

      Slide sideways to view the complete table.

      PropertiesAQUIFERAQUITARDAQUICLUDEAQUIFUGE
      POROSITY
      PERMEABILITYPartially
      YIELD OF WATERSlow Yielding
      EXAMPLESUnderground gravel and sand layerssandy clay and a poorly fractured igneous or metamorphic rock.Argillaceous rocks like shale and clay.Compact rocks like massive granites, basalt and quartzite

      SUMMARY – Geological Formations Distinction (at a glance)

      While porosity is a measure of empty space (pores) in a material, permeability measures the ease of water flow as a result of the interconnection of these pore spaces.

      You can see more distinction between porosity and permeability here: porosity vs permeability

      These two soil properties can be used to give a summarized distinction between the 4-aqui’s of groundwater geologic formations (aquifers, aquitard, aquiclude, and aquifuge).

      • Aquifers – geologic formations having both porosity and permeability.
      • Aquitard- geologic formations having porosity but low permeability.
      • Aquicludes – geologic formations having porosity but no permeability.
      • Aquifuge – geologic formations having neither porosity nor permeability. 

       

      DO YOU KNOW?

      Unlike surface water, groundwater can move as slow as a meter per year.

      This means it can take several thousands of years for underground aquifers to become replenished.

      DO YOU KNOW?

      Unlike surface water, groundwater can move as slow as a meter per year.

      This means it can take several thousands of years for underground aquifers to become replenished.

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