19.5. Methods of Estimating Evapotranspiration

Various methods are used depending on data availability, accuracy required, and scale. They are broadly categorized as:

This chunk introduces the different methods used to estimate evapotranspiration (ET). It highlights that the choice of method depends on the availability of data, the level of accuracy needed, and the scale at which the estimation is required. Understanding this framework helps identify which methods might be most applicable in different situations.

Think of estimating ET like measuring the distance you need to travel. If you have a GPS (lots of data), you can get an accurate reading. If you only have a rough map (limited data), you might guess or use a simpler method to figure out the distance.

(a) Empirical Methods • Based on observed climatological data and regression equations.

Empirical methods rely on existing observations and statistical relationships to estimate ET. These approaches use climatological data, meaning data related to weather patterns and measurements, along with regression equations to derive ET estimates. They are typically simpler and can be used where direct measurements are not feasible.

Empirical methods are like making a recipe based on one's experience from previous cooking. Instead of measuring every ingredient precisely, you rely on what worked before and adjust based on observation.

1. Blaney-Criddle Method • Uses mean daily temperature and daylight hours. • Suitable for monthly PET estimates. ET = p(0.46T + 8) where p = monthly % of annual daylight hours, T = mean monthly temperature (°C)

The Blaney-Criddle Method is a specific empirical method used to estimate potential evapotranspiration (PET). It incorporates mean daily temperature and the ratio of daylight hours in a month. This makes it particularly useful for monthly calculations of ET based on climatic data.

Imagine you are planning a picnic and decide the best time to go based on past weather patterns. If it’s usually sunny for most of the day in the peak summer months, you use that knowledge to decide on the best day for the picnic.

2. Hargreaves Method • Simple temperature-based method. ET = 0.0023(T + 17.8)(T − T )^0.5 * Ra • where Ra is extraterrestrial radiation.

The Hargreaves Method is another empirical approach that uses temperature data to estimate ET. This method also factors in extraterrestrial radiation, which represents the total solar energy received outside the Earth’s atmosphere. It's a straightforward calculation that requires minimal data.

Using the Hargreaves method is similar to checking the weather forecast to decide whether to carry an umbrella. If the temperature is higher, you might think it's a drier day, hence less need for an umbrella (similar to less ET).

3. Pan Evaporation Method • Based on evaporation from a Class A evaporation pan. ET = K · Ep where K = pan coefficient, Ep = pan evaporation.

The Pan Evaporation Method estimates ET by measuring the evaporation from a Class A pan, which is a standardized device. The actual ET is calculated by applying a pan coefficient to the measured evaporation. This method is easy to implement and provides real-time data.

Think of a pan of water left outside in the sun. You can observe how much water evaporates each day. If you use that measurement to predict how much water might be lost from a farm, it’s similar to how the Pan Evaporation Method works.

(b) Energy Balance Methods • Based on the conservation of energy at the land surface.

Energy balance methods estimate ET by considering the laws of energy conservation. They calculate the balance between energy inputs (like solar radiation) and outputs (like heat loss and evaporation). This holistic approach accounts for all energy fluxes at the land surface.

Consider a bank account where money is deposited and withdrawn. To know your balance for the day, you compute how much was added and how much was spent. Similarly, energy balance methods compute energy gains and losses to estimate ET.

1. Penman Method • Combines energy balance and aerodynamic approach.

The Penman Method is a sophisticated approach that merges energy balance principles with considerations of aerodynamic data, such as wind speed. It accounts for how the atmosphere affects evaporation and provides a more accurate ET estimate by incorporating multiple environmental variables.

This method is like cooking where you not only consider the ingredients (energy balance) but also the cooking temperature and method (aerodynamics), resulting in a well-prepared dish.

2. FAO Penman-Monteith Method • Recommended standard method by FAO for reference ET estimation: 0.408∆(R − G) + γ 900 u (e − e )/(∆ + γ(1 + 0.34u ))

The FAO Penman-Monteith Method is considered a gold standard for estimating reference ET. It is widely used because it is both accurate and applicable in many global contexts. The formula integrates multiple factors, including net radiation, temperature, wind speed, and humidity to provide a reliable ET estimate.

Using this method is like having a detailed recipe that has been perfected over time. It includes all necessary ingredients and precise steps, ensuring the best possible outcome for estimating ET.

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