20. Measurement of Evapotranspiration

Evapotranspiration is the combined process of evaporation from soil and water surfaces and transpiration from plants. It is a critical component of the hydrologic cycle, influencing the availability of water for irrigation, urban supply, reservoir design, and overall water resource management. Accurate estimation of evapotranspiration is essential for understanding water balance, drought assessment, and planning of irrigation systems. Various direct and indirect methods are used to measure evapotranspiration, ranging from empirical equations to sophisticated field instruments.

Evapotranspiration (ET) is the total loss of water from land surfaces, combining two processes: evaporation and transpiration. Evaporation occurs when water changes from a liquid to a vapor state, mainly from surfaces like lakes, rivers, or moist soil. Transpiration is the process where plants absorb water through their roots and release it as vapor from tiny openings in their leaves called stomata. Understanding ET is vital because it affects water availability for agriculture, cities, and managing water resources. To estimate ET accurately, we use various methods, including simple equations and complex instruments.

Think of ET like a sponge that absorbs and releases water. When you soak a sponge in water (evaporation), it becomes heavy and eventually, as it dries out in the air (transpiration), it releases moisture back into the environment. Understanding how much water is lost through this 'sponge' helps farmers know how much water to provide for crops.

• Evaporation: The process of water converting into vapor from open surfaces like rivers, lakes, and soil.
• Transpiration: The process through which water is absorbed by plant roots, moves through plants, and is lost as vapor through stomata in leaves.
• Evapotranspiration (ET): The total water loss from both evaporation and transpiration.
• Potential Evapotranspiration (PET): The amount of evapotranspiration that would occur with unlimited water supply.
• Actual Evapotranspiration (AET): The actual water loss, which may be less than PET due to limited water availability.

This section outlines the basic terms and concepts related to evapotranspiration. Firstly, evaporation is when water turns into vapor from surfaces like lakes. Transpiration refers to how plants take in water and release it as vapor. Together, these processes make up evapotranspiration (ET), which signifies how much water is lost to the atmosphere. There are two main types of ET: Potential Evapotranspiration (PET), which is the maximum rate if water is not limited, and Actual Evapotranspiration (AET), which shows the real water loss in conditions where water may be restricted.

Consider a sponge soaked in water. If the sponge has unlimited access to water (like PET), it can release maximum moisture into the air easily. However, if the sponge has restricted water access (like AET), it will release less moisture. This analogy helps illustrate how ET operates under different conditions.

Several climatic, plant-related, and soil factors influence evapotranspiration:
• Solar radiation
• Air temperature
• Humidity
• Wind speed
• Soil moisture content
• Crop type and stage of growth
• Plant density
• Management practices (mulching, irrigation, etc.)

Evapotranspiration is influenced by various factors. Climatic conditions like solar radiation, temperature, and humidity play a crucial role; for instance, higher temperatures can increase ET. Wind speed can also enhance evaporation by removing saturated air near the water surface. Soil moisture content affects how much water plants can access, whereas plant type, how dense they are, and their growing stage can determine how much water they lose through transpiration. Additionally, management practices, such as irrigation and mulching, can significantly impact ET rates by either supplying water or retaining soil moisture.

Imagine a garden on a sunny day. If it's hot (like high air temperature) and windy, water evaporates quickly from the soil and plants (increasing ET). If you add mulch to the soil (management practice), it helps retain moisture, lowering ET. Alternatively, if it rains (increasing soil moisture), plants can access more water, potentially increasing AET.

Evapotranspiration can be measured or estimated using direct methods involving actual field measurements of ET.

• Lysimeter Method:
  • A lysimeter is a tank embedded in the ground filled with soil and crops identical to the surrounding field.
  • Measures change in weight due to water loss.
  • Water inputs (rainfall, irrigation) and outputs (drainage, ET) are carefully monitored.
  • Advantages: High accuracy.
  • Disadvantages: Expensive, labor-intensive, not suitable for large areas.
• Field Experimental Plots:
  • Known quantity of water is applied.
  • Change in soil moisture and plant growth is observed.
  • ET is calculated as a water balance.
  • Used mostly for calibration of models.

Two main direct methods are outlined for measuring ET. The Lysimeter Method involves using a sealed tank with soil and plants to track how much water is lost over time by measuring changes in weight. Although it offers high accuracy, this method is costly and not practical for large areas. The Field Experimental Plots method involves applying a known amount of water to a section of land and then tracking changes in soil moisture and plant growth to calculate ET based on the water balance. This method is more useful for designing models and gaining insights into water usage.

Imagine a scientist using a special water-filled tank (lysimeter) to monitor a small section of a garden. By weighing the tank repeatedly, they can see how much water the plants are losing. On another part of the garden, they might sprinkle a precise amount of water and note how much remains in the soil, using it as a measurement of how much the plants consumed. This way, they're figuring out how thirsty the garden is based on different methods!

Evapotranspiration can be estimated using indirect methods involving meteorological and empirical data.

• Water Balance Method:
  • Based on the continuity equation:
  ET = P − R − D − ΔS
  Where:
  o P = Precipitation
  o R = Runoff
  o D = Deep percolation
  o ΔS = Change in soil water storage
  • Effective for basin-scale studies.
• Energy Balance Method:
  • Based on conservation of energy:
  R = G + H + LE
  Where:
  o R = Net radiation,
  o G = Soil heat flux
  o H = Sensible heat flux
  o LE = Latent heat flux (related to ET)
• Aerodynamic Method:
  • Uses principles of mass transfer.
  • Based on wind speed and vapor pressure difference:
  ET = C⋅u⋅(e_s − e_a)
  Where:
  o u = wind speed
  o e_s − e_a = vapor pressure deficit
  o C = empirical constant
• Combination Method (Penman Equation):
  • Combines energy balance and aerodynamic methods.
  • Penman equation:
  ET = rac{Δ(R − G) + γ⋅f(u)(e_s − e_a)}{Δ + γ}

Indirect methods estimate ET without needing direct measurements on-site. One popular approach is the Water Balance Method, which uses a simple equation to consider precipitation, runoff, deep soil water movement, and changes in soil moisture, making it great for larger areas. Another method is the Energy Balance Method, which focuses on how energy absorbed from the sun is distributed between soil heat and water evaporation. The Aerodynamic Method relies on wind speed and vapor pressure to estimate how quickly water evaporates. Lastly, the Combination Method, often using the Penman equation, integrates several factors to determine a more precise ET estimate.

Think of a carpenter measuring wood without physically cutting it. Instead, he calculates the length needed based on plans (water balance). He considers how much sunlight affects wood drying time (energy balance) and uses local weather data to estimate moisture loss (aerodynamic). Using a combination of these ideas, he figures out how to create precise cuts without trimming too much or too little. Similarly, researchers can estimate ET using indirect methods!

When direct data is unavailable, empirical equations are used for estimation:

• Blaney-Criddle Method:
  ET = k⋅p⋅(0.46T + 8)
  • k = crop coefficient
  • p = monthly % of annual daylight hours
  • T = mean monthly temperature
  • Simple and suitable for planning purposes.
• Thornthwaite Method:
  • Based on air temperature and day length.
  • Monthly PET:
  (10T)^a
  PET = 1.6I
  Where:
  o T = mean monthly temperature in °C
  o I = annual heat index
  o a = empirical exponent
• Hargreaves Method:
  • Simpler than Penman but more accurate than Thornthwaite:
  ET = 0.0023⋅(T_avg + 17.8)⋅R_a
  Where:
  o T_avg, T_max, T_min = average, max, and min temperature
  o R_a = extraterrestrial radiation.

In situations where direct measurements of ET are not available, empirical methods provide useful estimations. The Blaney-Criddle Method uses crop coefficients, daylight hours, and temperature data to estimate ET simply. The Thornthwaite Method focuses on air temperature and day length to calculate potential ET based on historical temperature data via a complex formula. The Hargreaves Method, while simpler, integrates temperature ranges and extraterrestrial radiation for a more accurate ET estimate. These empirical methods are often easier to apply in planning and assessing water needs for agriculture.

Picture planning a garden without being able to measure moisture directly. Instead, you watch how much sunlight the plants get and observe average temperatures (Blaney-Criddle). You use past weather patterns to predict growth (Thornthwaite) and general temperatures to gauge how much water predicted growth requires (Hargreaves). These methods help projects run smoothly without constant checks or measurements!

Evaporation pans are widely used to estimate reference ET:

• Class A Evaporation Pan:
  • Circular pan, 120.7 cm diameter, 25 cm depth.
  • Water is filled and level is observed daily.
  • ET is calculated using a pan coefficient:
  ET = K_p⋅E_p
  Where:
  o K_p = pan coefficient (0.6–0.8)
  o E_p = pan evaporation.

Class A Evaporation Pans are practical tools for estimating ET. These are round containers filled with water that measure how much water evaporates over time. The water level in the pan is checked daily, and this change is recorded. To get the estimated ET, researchers use a pan coefficient, which adjusts for the differences in environment between the pan and natural surfaces. This method allows for a simplified and relatively low-cost way to estimate potential ET in a given area.

Think of the evaporation pan like a petri dish with water left outside. If it’s sunny and windy (good evaporation conditions), you will see less and less water day by day. By knowing how deep the pan is (the size) and calculating how much water evaporated, you estimate how much water plants might lose naturally. It’s a simple science experiment that gives vital information for farmers and hydrologists!

• Satellite-based sensors can estimate ET using surface temperature, vegetation index (NDVI), and albedo.
• Models like SEBAL (Surface Energy Balance Algorithm for Land) and METRIC (Mapping EvapoTranspiration at high Resolution with Internalized Calibration) are used.

Remote sensing technology is changing how we estimate ET. Satellites can gather data on surface temperatures, the health of vegetation through indices like NDVI, and surface reflectivity (albedo). Models such as SEBAL and METRIC use this satellite data to calculate ET over large landscapes without needing to place instruments on every square meter of land. This method is immensely beneficial for large scale agricultural assessments and water resource management.

Imagine having a drone fly over a farm. Instead of you walking through the field with a ruler and thermometer, the drone can take pictures and measure temperature from the sky. By analyzing these pictures, you can get an overall picture of how much water the crops are likely using. This efficient technique harnesses technology to replace old labor-intensive methods!

• India has lysimeter stations in research centers like IARI (New Delhi), PAU (Ludhiana), and ICRISAT (Hyderabad).
• Help in developing crop coefficients for different agro-climatic zones.

In India, lysimeter stations are used for research to better understand ET across different agricultural zones. Institutions like IARI and PAU have deployed lysimeters to monitor water consumption and develop crop coefficients, which help farmers understand water needs for specific crops in their local climates. This targeted research ensures better crop management and water conservation strategies are in place.

Think of these lysimeter stations as specialized laboratories for plants, where scientists study how much water they use in various weather conditions. Just like a chef experiments with ingredients to find the best recipe, researchers are fine-tuning water management practices to suit different regions' agricultural needs. This tailored approach helps optimize yield while conserving water.

• FAO Penman-Monteith is the most accepted method to compute reference ET.
• Crop coefficient Kc values vary with crop type and growth stage.
ET = Kc⋅ET₀

The FAO Penman-Monteith method is widely regarded as the standard for calculating reference ET (ET₀), setting a baseline for comparisons. Crop coefficients (Kc) allow farmers to adjust ET values based on specific crop types and their growth stages. These coefficients serve as multipliers to estimate how much water a particular crop will need compared to the reference ET.

Imagine setting a thermostat in a house. You have a standard temperature (reference ET) that’s comfortable for most people. However, depending on who’s visiting or what activities are occurring, you might adjust the thermostat (crop coefficient) to find the right comfort level for each situation. This analogy illustrates how farmers make adjustments based on specific crop requirements for optimal water management.

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