2.5 Scintillometry

Scintillometers consist of a transmitter and receiver, separated by a specified path length. Scintillometry uses a beam of electromagnetic radiation of known wavelength transmitted across a relatively large distance (100 m–4.5 km). The beam intensity fluctuates as it encounters gases in the air due to absorption and diffraction. These fluctuations, or scintillations, can be used to determine the structural parameters for temperature and the refractive index of air, which can be used to calculate the H. The calculations to obtain H from scintillometers are based on Monin-Obukhov Similarity Theory (MOST). This theory describes the relationship between the parameters of friction velocity, the temperature scale, and the specific humidity scale, in reference to the process of turbulence, mainly from buoyancy and horizontal shear.

MOST describes the vertical flow and turbulence properties in the lower atmospheric boundary layer or surface layer [30]. This theory provides a set of equations that relate turbulence properties, using dimensionless parameters, to atmospheric processes including H. One of the parameters derived from similarity theory is the Obukhov length, which is the height above the surface that turbulence is caused by wind shear. Above the Obukhov length, turbulence is driven more by buoyancy, or the action of radiant heat moving the air mass upwards. MOST was developed on the idea that turbulence properties, when made dimensionless using friction velocity, temperature scale, and other variables, are a universal function of the Obukhov length [31, 32]. The key parameters of MOST are the friction velocity, u∗; temperature scale, θ∗; and the specific humidity scale, q<sup>∗</sup> [33]. These parameters are calculated as

$$
\mu\_\* = \left(\frac{\tau\_0}{\rho}\right)^{1/2} \tag{12}
$$

<sup>ζ</sup> <sup>¼</sup> <sup>z</sup>

<sup>L</sup> ¼ � <sup>u</sup><sup>3</sup>

∗ k g T<sup>0</sup> q cpρ<sup>0</sup>

where k is the von Karman constant, g is the acceleration due to gravity, ρ<sup>0</sup> is the

There are different scintillometer models available, which differ based on the wavelength of the radiation beam and aperture diameter. The aperture diameter determines the path length where a larger aperture will need a longer path length. All models use a transmitter and receiver to send the beam and measure the scintillations. The most common wavelengths are visible (670 nm), infrared (880 nm), and microwave (1 mm to 1 cm). The aperture size for most infrared (large aperture) scintillometers (LAS) is 10–15 cm (see Figure 6), while the aperture for visible (surface layer) scintillometers (SLS) is 2.7 mm (see Figure 7). Microwave scintillometer aperture sizes can be much larger, up to 30 cm. The SLS is sometimes termed as a displaced-beam small aperture scintillometer (DBSAS) since the SLS beam is split into two parallel beams, displaced by 2.7 mm. Based on the correlation of the intensity fluctuations between the two beams, the inner scale parameter, l0,

The benefits of each scintillometer come from the differences between them. For instance, the visible wavelength scintillometers have a much smaller aperture, which allows for better representation of small eddies and greater sensitivity to smaller changes in temperature and wind fluctuations. The larger apertures can

Large aperture scintillometer (LAS MKII, Kipp & Zonen, Delft, the Netherlands) with aperture restrictor plate

where z is the height above the surface and L is the Obukhov length

air density at temperature T0, and q is the kinematic heat flux [34].

Field-Scale Estimation of Evapotranspiration DOI: http://dx.doi.org/10.5772/intechopen.80945

can be determined [31].

Figure 6.

15

reducing aperture from 15 cm to 10 cm.

<sup>L</sup> (18)

(19)

$$\theta\_\* = -\frac{H\_0}{\rho c\_p \mu\_\*} \tag{13}$$

$$q\_\* = -\frac{E\_0}{\rho u\_\*}\tag{14}$$

where ρ is the air density, cp is the specific heat of air, τ<sup>0</sup> is the turbulent stress at the surface, H<sup>0</sup> is the vertical flux of heat, and E<sup>0</sup> is the vertical flux of water vapor. τ0, H0, and E<sup>0</sup> can be calculated by

$$
\pi\_0 = \rho \mathbf{C}\_D \mathbf{U}\_r^2 \tag{15}
$$

$$H\_0 = \rho \mathbf{C}\_D \mathbf{C}\_H U\_r (\Theta\_s - \Theta\_r) \tag{16}$$

$$E\_0 = \rho C\_W U\_r (Q\_s - Q\_r) \tag{17}$$

where Ur is the wind speed at reference height, Θ<sup>r</sup> is air temperature at reference height, Qr is specific humidity at reference height, Θ<sup>s</sup> is air temperature at the surface, Qs is specific humidity at the surface, CD is the drag coefficient, CH is the heat transfer coefficient, and CW is the water vapor transfer coefficient [33]. Monin, Lumley [34] determined that turbulence properties at height z depend on only five quantities: z, ϱ, <sup>g</sup> <sup>T</sup>, <sup>u</sup>∗, and <sup>q</sup> cp<sup>ρ</sup>. From these parameters, one dimensionless parameter, the stability parameter ζ, can be derived. Using ζ, surface flow properties can be described as a function of ζ using dimensional analysis. ζ is calculated as Field-Scale Estimation of Evapotranspiration DOI: http://dx.doi.org/10.5772/intechopen.80945

2.5 Scintillometry

Advanced Evapotranspiration Methods and Applications

and horizontal shear.

[33]. These parameters are calculated as

vapor. τ0, H0, and E<sup>0</sup> can be calculated by

only five quantities: z, ϱ, <sup>g</sup>

14

Scintillometers consist of a transmitter and receiver, separated by a specified path length. Scintillometry uses a beam of electromagnetic radiation of known wavelength transmitted across a relatively large distance (100 m–4.5 km). The beam intensity fluctuates as it encounters gases in the air due to absorption and diffraction. These fluctuations, or scintillations, can be used to determine the structural parameters for temperature and the refractive index of air, which can be used to calculate the H. The calculations to obtain H from scintillometers are based on Monin-Obukhov Similarity Theory (MOST). This theory describes the relationship between the parameters of friction velocity, the temperature scale, and the specific humidity scale, in reference to the process of turbulence, mainly from buoyancy

MOST describes the vertical flow and turbulence properties in the lower atmospheric boundary layer or surface layer [30]. This theory provides a set of equations that relate turbulence properties, using dimensionless parameters, to atmospheric processes including H. One of the parameters derived from similarity theory is the Obukhov length, which is the height above the surface that turbulence is caused by wind shear. Above the Obukhov length, turbulence is driven more by buoyancy, or the action of radiant heat moving the air mass upwards. MOST was developed on the idea that turbulence properties, when made dimensionless using friction velocity, temperature scale, and other variables, are a universal function of the Obukhov length [31, 32]. The key parameters of MOST are the friction velocity, u∗; temperature scale, θ∗; and the specific humidity scale, q<sup>∗</sup>

> <sup>u</sup><sup>∗</sup> <sup>¼</sup> <sup>τ</sup><sup>0</sup> ρ <sup>1</sup>=<sup>2</sup>

<sup>θ</sup><sup>∗</sup> ¼ � <sup>H</sup><sup>0</sup>

<sup>q</sup><sup>∗</sup> ¼ � <sup>E</sup><sup>0</sup> ρu<sup>∗</sup>

where ρ is the air density, cp is the specific heat of air, τ<sup>0</sup> is the turbulent stress at the surface, H<sup>0</sup> is the vertical flux of heat, and E<sup>0</sup> is the vertical flux of water

<sup>τ</sup><sup>0</sup> <sup>¼</sup> <sup>ρ</sup>CDU<sup>2</sup>

where Ur is the wind speed at reference height, Θ<sup>r</sup> is air temperature at reference

height, Qr is specific humidity at reference height, Θ<sup>s</sup> is air temperature at the surface, Qs is specific humidity at the surface, CD is the drag coefficient, CH is the heat transfer coefficient, and CW is the water vapor transfer coefficient [33]. Monin, Lumley [34] determined that turbulence properties at height z depend on

parameter, the stability parameter ζ, can be derived. Using ζ, surface flow properties can be described as a function of ζ using dimensional analysis. ζ is calculated as

<sup>T</sup>, <sup>u</sup>∗, and <sup>q</sup>

ρcpu<sup>∗</sup>

(12)

(13)

(14)

<sup>r</sup> (15)

H<sup>0</sup> ¼ ρCDCHUrð Þ Θ<sup>s</sup> � Θ<sup>r</sup> (16) E<sup>0</sup> ¼ ρCWUr Qs � Qr ð Þ (17)

cp<sup>ρ</sup>. From these parameters, one dimensionless

$$
\zeta = \frac{z}{L} \tag{18}
$$

where z is the height above the surface and L is the Obukhov length

$$L = -\frac{u\_\*^3}{k \frac{g}{T\_0} \frac{q}{c\_p \rho\_0}}\tag{19}$$

where k is the von Karman constant, g is the acceleration due to gravity, ρ<sup>0</sup> is the air density at temperature T0, and q is the kinematic heat flux [34].

There are different scintillometer models available, which differ based on the wavelength of the radiation beam and aperture diameter. The aperture diameter determines the path length where a larger aperture will need a longer path length. All models use a transmitter and receiver to send the beam and measure the scintillations. The most common wavelengths are visible (670 nm), infrared (880 nm), and microwave (1 mm to 1 cm). The aperture size for most infrared (large aperture) scintillometers (LAS) is 10–15 cm (see Figure 6), while the aperture for visible (surface layer) scintillometers (SLS) is 2.7 mm (see Figure 7). Microwave scintillometer aperture sizes can be much larger, up to 30 cm. The SLS is sometimes termed as a displaced-beam small aperture scintillometer (DBSAS) since the SLS beam is split into two parallel beams, displaced by 2.7 mm. Based on the correlation of the intensity fluctuations between the two beams, the inner scale parameter, l0, can be determined [31].

The benefits of each scintillometer come from the differences between them. For instance, the visible wavelength scintillometers have a much smaller aperture, which allows for better representation of small eddies and greater sensitivity to smaller changes in temperature and wind fluctuations. The larger apertures can

#### Figure 6.

Large aperture scintillometer (LAS MKII, Kipp & Zonen, Delft, the Netherlands) with aperture restrictor plate reducing aperture from 15 cm to 10 cm.

which are used to determine the H. Without determining the l0, the LAS requires additional measurements to estimate the friction velocity. The SLS has been found to be more accurate than the LAS with errors of 15–30% [29] compared to greater

Many ET models are available for use with remote sensing data. In addition, there are a variety of satellite data sources such as Moderate Resolution Imaging Spectrometer (MODIS), Landsat, Advanced Very High-Resolution Radiometer (AVHRR), Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), and many others. Additional information on satellite sources and available models can be found in [39]. Most remote sensing models are based on the energy balance where the reflectance from remote sensing is used with weather data from nearby weather stations and the four components of the energy balance are calculated. Typically, LE is calculated as the residual of the energy balance and

The biggest issue with using satellite data for creating ET maps is poor spatial and temporal resolution. Many energy balance-based models such as METRIC [40], SEBS [41], SEBAL [42], and others require thermal data to calculate surface temperature. These models are more limited on available data. ASTER, MODIS, and Landsat are the main data sources available with thermal sensors. ASTER has the highest spatial resolution at 15 m for visible wavelengths and 90 m for thermal wavelengths but has a return interval of 16 days [43]. Landsat data has 100 m resolution for thermal wavelengths and can provide data on an 8 day interval if Landsat 7 and Landsat 8 are both used [44]. MODIS provides daily data but has poor spatial resolution of 1000 m [45]. Although the models typically only provide hourly and daily ET estimates, methods are available to interpolate between satellite

The aforementioned remote sensing models not only provide ETa maps but can also provide estimates of leaf area index, surface temperature, surface albedo, and many others. Although the spatial and temporal resolution of existing satellites limits applications to field-scale agricultural use, the rapid increase in unmanned aerial vehicle (UAV) technology shows vast potential to acquire remote sensing data with spatial resolution at a centimeter scale and as frequent as desired. Satellite-based ET maps typically have accuracy of 20–30% at best [47]; however,

The methods mentioned above can all be used to determine ET; however, there are disadvantages to each one of them. With the soil water balance approach, the drainage and runoff terms can be difficult to determine. Although they are commonly miniscule in arid and semiarid regions, they would still need to be accounted for to obtain the greatest accuracy. Lysimeters are the most accurate but are very expensive and intrusive to install and operate. In addition, they require a high level of knowledge and experience to obtain the best measurements. The Bowen ratio method has been used to determine ET from the energy balance, but it is an indirect measurement. EC is a direct measurement method of turbulent fluxes but is known to have energy balance closure and other errors associated with it. Scintillometers are another indirect measurement method that has been extensively used, but they also have known errors. EC and scintillometers are two of the more common

the accuracy of using UAV data for ET maps is not currently known.

than 30% for the LAS [38].

Field-Scale Estimation of Evapotranspiration DOI: http://dx.doi.org/10.5772/intechopen.80945

converted to ET at an hourly and daily time step.

passes and for monthly and seasonal sums [46].

2.6 Remote sensing

3. Conclusions

17

Figure 7. Surface layer scintillometer (SLS-20, Scintec AG, Rottenburg, Germany).

detect larger eddies better. Microwave scintillometers are more sensitive to humidity fluctuations than the other wavelengths, providing an inference to accurate ETa determination [35]. Infrared scintillometers have been used for some time; however, visible and microwave scintillometers are relatively new and have not been used as extensively.

Scintillometers have been considered to be very beneficial for ET remote sensing studies due to their large path length. Specifically, large aperture scintillometers (LAS), which can have path lengths up to a few km, can have a large enough spatial footprint to be similar to most remote sensing data resolution. In addition, the path averaging of the scintillometer provides an integrated benefit in that a homogenous surface is not required to meet any assumptions. This allows the scintillometer to be used across varying terrain and provide an averaged value. The averaging for variable surfaces is similar to that of remote sensing data. The previous points illustrate how the scintillometer can serve as a ground-truthing instrument or as a source of validation data for remote sensing. Since most of the ET remote sensing models are based on the surface energy balance, similarly to scintillometry, measurements other than just ET can be evaluated. The surface fluxes H and LE are determined by both scintillometers and ET remote sensing models, which provide more data for comparison. One benefit scintillometry has over EC is the lack of corrections [36].

One advantage SLS offers over other point source measurements is that the fluxes can be determined over shorter lengths and at heights closer to the surface [37]. In addition, the fluxes can be calculated on shorter temporal scales, as low as 1 minute, compared to EC, for example, which typically uses a 30 minute interval. An advantage the SLS has over the LAS is that the SLS determines the l0, which is proportional to the dissipation rate of the turbulent kinetic energy, ε, and CT 2 ,

which are used to determine the H. Without determining the l0, the LAS requires additional measurements to estimate the friction velocity. The SLS has been found to be more accurate than the LAS with errors of 15–30% [29] compared to greater than 30% for the LAS [38].
