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# Evapotranspiration

**Evapotranspiration **is the the combination of evaporation from the ground surface and transpiration by vegetation. It includes both evaporation of free water from the surface of vegetation and the land surface. It also includes transpiration which is the process of vegetation extracting it from the soil through the plant root system. Whether by evaporation or transpiration, water is returned from the land surface or subsurface to the atmosphere. Even though evaporation and transpiration are taken together, transpiration is responsible for the movement of much more water than evaporation. Combined evapotranspiration is often responsible for returning 50 or even 60% of precipitation back to the atmosphere. The theoretical evapotranspiration, also called the potential evapotranspiration, serves as the upper limit for what can happen on the land surface based on atmospheric conditions. In all cases, the Meteorologic Model is computing the potential evapotranspiration and subbasins will calculate actual evapotranspiration based on soil water limitations.

The **Evapotranspiration Method **included in the Meteorologic Model is only necessary when using continuous simulation loss rate methods in subbasins:** Deficit Constant**, **Gridded Deficit Constant**, **Soil Moisture Accounting**, and **Gridded Soil Moisture Accounting**. If a continuous simulation loss rate method is used and no evapotranspiration is specified in the Meteorologic Model, then zero potential evapotranspiration is used in the subbasins. The options for evapotranspiration include a **Physically-Based Energy Balance Model (Penman Monteith)**, a **Simplified Physically-Based Model (Priestley Taylor)**, the **Hargreaves and Hamon Temperature Only Methods**, and a **Simple Monthly Average **approach. A specified method is also included so that evapotranspiration can be calculated external to the program and imported. Each option produces the potential evapotranspiration rate over the land surface where it can be used in the subbasin element to compute evaporation from the canopy and surface, and transpiration from the soil. More detail about each method is provided in the following sections.

## Annual Evapotranspiration

The **Annual Evapotranspiration Method **is designed to work with a maximum daily rate combined with an optional pattern of variation throughout the year. Specifying only a daily rate can produce good results for simulations lasting days to weeks if evapotranspiration is fairly consistent each day. The optional pattern can be used to adjust the applied evapotranspiration rate during simulations lasting weeks to years.

The **Annual Evapotranspiration Method **includes a **Component Editor **with parameter data for all subbasins in the Meteorologic Model. The **Watershed Explorer **provides access to the **Evapotranspiration Component Editor **using a picture of a water pan.

The** Component Editor **for all subbasins in the Meteorologic Model includes the **Daily Rate **and optional **Pattern **for each subbasin. When the optional percentage pattern is not used, the value entered for the daily rate should generally be the average daily potential evapotranspiration rate over the duration of the simulation. When a pattern will be added, the value entered for the daily rate should generally be the largest potential evapotranspiration for any day occurring during the simulation.

The optional **Pattern **is specified as a percent pattern in the **Paired Data Manager**. The evapotranspiration for each day of the simulation is computed by multiplying the entered rate by the percentage interpolated from the percent pattern. The available percent patterns are shown in the selection list. If there are many different patterns available, you may wish to choose a pattern from the selector accessed with the **Paired Data **button next to the selection list. The selector displays the description for each percent pattern, making it easier to select the correct one.

## Gridded Hamon

The **Gridded Hamon Method **is the same as the regular **Hamon Method **(described in a later section) except that the Hamon equations are applied to each grid cell using separate boundary conditions instead of area-averaged values over the whole subbasin. The Gridded Hamon Method requires a temperature from temperature method** **in the Meteorologic Model.

The **Gridded Hamon Method **includes a **Component Editor **with parameter data for all subbasins in the Meteorologic Model. The** Watershed Explorer **provides access to the **Evapotranspiration Component Editor **using a picture of a water pan.

The** Component Editor **requires a **Hamon Coefficient **be selected for all subbasins (shown in the following figure). A default coefficient of 0.0065 inches per gram per meter cubed is provided; this is equivalent to 0.1651 millimeters per gram per meter cubed. The units inches per gram per meter cubed are implicit in the Hamon (1963) formulation where the coefficient is presented as a constant: 0.0065. The coefficient can be adjusted by the user in the Component Editor.

## Gridded Hargreaves

The** Gridded Hargreaves Method **is the same as the regular **Hargreaves Method **(described in a later section) except that the Hargreaves equations are applied to each grid cell using separate boundary conditions instead of area-averaged values over the whole subbasin. Shortwave radiation is required in the Hargreaves method so a **Gridded Shortwave Radiation Method **should also be selected in the Meteorologic Model. The Gridded Hargreaves Method requires temperature from a temperature method** **in the Meteorologic Model.

The **Gridded Hargreaves Method **includes a** Component Editor **with parameter data for all subbasins in the Meteorologic Model. The **Watershed Explorer **provides access to the **Evapotranspiration Component Editor **using a picture of a water pan.

The** Component Editor **for all subbasins includes a **Hargreaves Evapotranspiration Coefficient **(shown in the following figure). A default coefficient of 0.0075 per degree Fahrenheit is provided; this is equivalent to 0.0135 per degree Celsuis. If the **Gridded Hargreaves Evapotranspiration Method **is combined with the **Gridded Hargreaves Shortwave Radiation Method**, the resulting default coefficient is 0.0023 per degree Celsius raised to the 3/2 power. This is equivalent to the form presented by Hargreaves and Allen (2003), Eq. 8.

## Gridded Penman Monteith

The **Gridded Penman Monteith Method **is the same as the regular **Penman Monteith Method **(described in a later section) except that the Penman Monteith equations are applied to each grid cell using separate boundary conditions instead of area-averaged values over the whole subbasin. Shortwave and longwave radiation are inputs to the Penman Monteith Method so both a **Shortwave Radiation Method **and a **Longwave Radiation Method **should also be selected in the Meteorologic Model.

The **Gridded Penman Monteith Method **includes a** Component Editor **with parameter data for all subbasins in the Meteorologic Model. The** Watershed Explorer **provides access to the **Evapotranspiration Component Editor **using a picture of a water pan.

The **Component Editor **for all subbasins in the Meteorologic Model is shown in the following figure. **Temperature, ****Windspeed, Pressure, **and **Dew Point **methods are also required.

A **Reference Albedo **is required for computing the energy balance at the ground surface. The same value is applied to all grid cells in all subbasins. A default value of 0.23 is provided.

## Gridded Priestley Taylor

The **Gridded Priestley Taylor Method **is the same as the regular **Priestley Taylor Method **(described in a later section) except the Priestley Taylor equations are applied to each grid cell using separate boundary conditions instead of area-averaged values over the whole subbasin. Net shortwave radiation is an input to the Priestley Taylor Method so a **Shortwave Radiation Method **should also be selected in the Meteorologic Model.

The **Gridded Priestley Taylor Method **includes a **Component Editor **with parameter data. The **Watershed Explorer **provides access to the **Evapotranspiration Component Editor **using a picture of a water pan.

The** Component Editor **for all subbasins in the Meteorologic Model includes a **Dryness Coefficient **which must be entered for all subbasins. The same coefficient is applied to all grid cells in all subbasins. The coefficient is used to make small corrections based on soil moisture state. A coefficient should be specified that represents typical soil water conditions during the simulation. A value of 1.2 can be used in humid conditions while a value of 1.3 represents an arid environment.

A **Temperature method **must be selected in the meteorologic model.

## Hamon

The **Hamon Method **(Hamon, 1963) is based on an empirical relationship where saturated water vapor concentration, at the mean daily air temperature, adjusted by a day length factor, is proportional to potential evapotranspiration. The day length factor accounts for plant response, duration of turbulence, and net radiation. **Daily Average Temperature **is the only data requirement. The method has proven effective for estimating potential evapotranspiration in data-limited situations. The method calculates daily potential evapotranspiration given daily average temperature. For simulation time steps less than one day, potential evapotranspiration is redistributed for each time step based on a sinusoidal distribution between sunrise and sunset.

The **Hamon Method **includes a** Component Editor **with parameter data for each individual subbasin in the Meteorologic Model. The** Watershed Explorer **provides access to the **Evapotranspiration Component Editor **using a picture of a water pan. A **Temperature method **must be selected in the meteorologic model.

The **Component Editor **for each subbasin includes a **Hamon Coefficient**. A default coefficient of 0.0065 inches per gram per meter cubed is provided; this is equivalent to 0.1651 millimeters per gram per meter cubed. The units inches per gram per meter cubed are implicit in the Hamon (1963) formulation where the coefficient is presented as a constant: 0.0065. The coefficient can be adjusted by the user in the **Component Editor**.

## Hargreaves

The **Hargreaves Evapotranspiration Method **(Hargreaves and Samani, 1985) is based on an empirical relationship where reference evapotranspiration was regressed with solar radiation and temperature data. The regression was based on eight years of precision lysimeter observations for a grass reference crop in Davis, CA. The method has been validated for sites around the world (Hargreaves and Allen, 2003). The method is capable of capturing diurnal variation in potential evapotranspiration for simulation time steps less than 24 hours. Combining the Hargreaves Evapotranspiration Method with the **Hargreaves Shortwave Radiation Method **will yield the Hargreaves evapotranspiration form equivalent to Hargreaves and Allen (2003) Eq. 8.

The **Hargreaves Evapotranspiration Method **includes a** Component Editor **with parameter data for each individual subbasin in the Meteorologic Model. The **Watershed Explorer **provides access to the **Evapotranspiration Component Editor **using a picture of a water pan (Figure 13). A **Temperature method **must be selected in the meteorologic model.

The **Component Editor **for each subbasin includes a **Hargreaves Evapotranspiration Coefficient**. A default coefficient of 0.0135 per degree Celsius is provided; this is equivalent to 0.0075 per degree Fahrenheit. If the **Hargreaves Evapotranspiration Method **is combined with the **Hargreaves Shortwave Radiation Method**, the resulting default coefficient is 0.0023 per degree Celsius raised to the 3/2 power. This is equivalent to the form presented by Hargreaves and Allen (2003) Eq. 8.

## Monthly Average

The **Monthly Average Method **is designed to work with measured pan evaporation data. However, it can also be used with data collected with the **Eddy Correlation Technique **or other modern methods. Regardless of how they are collected, the data are typically presented as the average depth of evaporated water each month. Maps or tabular reports can be found for each month and used with this method.

The **Monthly Average Method **includes a **Component Editor **with parameter data for each individual subbasin in the Meteorologic Model. The** Watershed Explorer **provides access to the **Evapotranspiration Component Editor **using a picture of a water pan.

The** Component Editor **for each subbasin includes the **Evapotranspiration Rate **for each month of the year (shown in the figure below). It is entered as the total amount of evapotranspiration for the month. Every time step within the month will have the same evapotranspiration rate.

The **Coefficient **must also be entered for each month. The specified rate is multiplied by the coefficient to determine the final potential rate for each month. The coefficient is usually used to correct actual evaporation pan data to more closely reflect plant water use.

## Penman Monteith

The **Penman Monteith Method **implements the Penman Monteith equations for computing evapotranspiration at less than a daily time interval as detailed by Allen, Pereira, Raes, and Smith (1998). The equations are based on a combination of an energy balance with a mass transfer. The maximum possible evapotranspiration is moderated by an aerodynamic resistance due to friction as air flows over the vegetation. A bulk surface resistance is added in series with the aerodynamic resistance to account for limitations to water vapor flow at the leaf surfaces and at the soil. The parameterization is entirely dependent on the atmospheric conditions.

The **Penman Monteith Method **requires **Shortwave Radiation, ****Longwave Radiation, Temperature, Windspeed, and Dew Point **be included in the Meteorologic Model. The algorithm of Allen, Pereira, Raes, and Smith (1998) is followed most closely when the **Specified Pyranograph Shortwave Method **and the **FAO56 Longwave Method **are selected. When shortwave data is not available, Allen, Pereira, Raes, and Smith (1998) recommend using the **Hargreaves Shortwave Method**.

The **Watershed Explorer **provides access to the **Evapotranspiration Component Editor **using a picture of a water pan (shown in the following figure). An **Air Temperature Gage **and a **Windspeed Gage **must be selected in the atmospheric variables for each subbasin.

The **Component Editor **for each subbasin includes a selection for the **Reference Albedo**.

## Priestley Taylor

The **Priestley Taylor Method **(Priestley and Taylor, 1972) uses a simplified energy balance approach where the soil water supply is assumed to be unlimited. Simplified forms of latent and sensible energy are used. The method is capable of capturing diurnal variation in potential evapotranspiration through the use of a net solar radiation gage, so long as the simulation time step is less than 24 hours. **Shortwave Radiation **and **Temperature Methods **must be selected in the Meteorologic Model. The Shortwave Radiation Method should be configured to provide the net radiation; this is usually accomplished by computing the required value externally and inputting it using a **Radiation Time-Series Gage**.

The **Watershed Explorer **provides access to the **Evapotranspiration Component Editor **using a picture of a water pan.

The **Component Editor **for each subbasin includes a **Dryness Coefficient**. The coefficient is used to make small corrections based on soil moisture state. A coefficient should be specified that represents typical soil water conditions during the simulation. A default coefficient of 1.26 is provided. This coefficient has been found to vary for regions around the world (Aschonitis, VG et al., 2017).

## Specified Evapotranspiration

The** Specified Evapotranspiration Method **allows the user to specify the exact time-series to use for the potential evapotranspiration at subbasins. This method is useful when atmospheric and vegetation data will be processed externally to the program and essentially imported without alteration. This method is also useful when a single evapotranspiration observation measurement can be used to represent what happens over a subbasin.

The **Specified Evapotranspiration Method **uses a **Component Editor **with parameter data for all subbasins in the Meteorologic Model. The **Watershed Explorer **provides access to the **Gage Component Editor **using a picture of a water pan.

The** Component Editor **for all subbasins in the Meteorologic Model includes the gage selection for each subbasin. An **Evapotranspiration Time-Series **must be stored as an **Evapotranspiration Gage **before it can be used in the Meteorologic Model. The data may actually be from daily pan measurements, hourly eddy covariance measurements, or could be the result of complex calculations exterior to the program. Regardless, the time-series must be stored as a gage. You may use the same gage for more than one subbasin. For each subbasin in the table, select the gage to use for that subbasin. Only Evapotranspiration Gages already defined in the **Time-Series Data Manager **will be shown in the selection list.