# Feasibility of Obtaining Surface Layer Moisture Flux Using an IR Thermometer
## 4 Potential Flux Measurement Method  
The word 'flux' can be defined in very simple terms: how much of something passes through a unit area, or a unit volume, per unit time. For instance, if 100 birds fly through a 1 \(\times\) 1 m\({}^{2}\) window each minute, then the flux of birds is 100 birds m\({}^{-2}\) min\({}^{-1}\). If the window was 5 \(\times\) 5 m\({}^{2}\), the flux would be 4 birds m\({}^{-2}\) min\({}^{-1}\). Therefore, the flux is dependent on: (i) the number of things crossing an area/volume, (ii) the size of an area/volume being crossed and (iii) the time it takes to cross this area or pass through this volume. In more scientific terms, the flux can be defined as the amount of flow of a fluid, the amount of radiation or the number of particles incident through a flat surface in a given time [14; 15; 16]. Flux is also synonymously stated as flow rate per unit area. In transport phenomena (heat transfer, mass transfer and fluid dynamics), flux is defined as the rate of flow of a property per unit area, which has the dimensions [quantity]\(\cdot\)[time]\({}^{-1}\) [-area]\({}^{-1}\)[17]. There is a constant exchange of thermal energy, moisture (i.e., water vapor) and gases between the soil and the atmosphere. Atmospheric moisture is the resultant product of evapotranspiration and precipitation [14; 15; 16]. There are several procedures employed to measure heat and moisture flux. Their applications and usages are situational and, moreover, dependent on the nature of objects, equipment and resources. Each technique has its own advantages and disadvantages. Nonetheless, the proposed technique of using a handheld IR Thermometer for deriving the heat and moisture flux is inexpensive and easy to use for various ground and water surfaces, such as: a green and dry vegetation canopy, tree lines of a uniform surface/canopy, mixed vegetation with varying canopy heights, as well as seas, lakes, rivers and ponds. Although using an IR Thermometer to determine heat flux appears to be a relatively straightforward argument, using a standard handheld IR Thermometer to accurately evaluate moisture flux requires more discussion. The following suggests that the proposed use of an IR Thermometer Psychrometer indeed has merit. Both sensible heat flux, \(Q_{H}\) and moisture flux, \(Q_{E}\), (also called latent heat flux) can be quantified per unit area from vertical profile measurements of temperature and specific humidity [8; 18]. These vertical profile gradients can be obtained from the dry- and wet-bulb IR temperature measurements, as demonstrated with the paper towels oriented in a vertical fashion as described in the previous section. Specific humidity can be calculated from the obtained vapor pressures and ambient pressure via  
\[q=\frac{0.622e}{P-0.378e}\, \tag{6}\]  
where \(q\) is the specific humidity of air vapor mixture, \(e\) is the vapor pressure and \(P\) is the atmospheric air pressure. According to Oke [8], \(Q_{H}\) and \(Q_{E}\) (W m\({}^{-2}\)) are given by  
\[Q_{H}=-\rho c_{pd}K_{H}\left(\frac{\partial\overline{T}}{\partial z}-\Gamma \right)\, \tag{7}\]  
\[Q_{E}=-\rho L_{\varphi}K_{\varphi}\frac{\partial\overline{q}}{\partial z} \tag{8}\]  
where \(\rho\) is the air density (kg m\({}^{-3}\)), \(c_{pd}\) is the specific heat of dry air (J kg\({}^{-1}\) K\({}^{-1}\)) at constant pressure, \(K_{H}\) is the eddy conductivity or diffusivity for heat (m\({}^{2}\) s\({}^{-1}\)), \(\partial\overline{T}/\partial z\) is the vertical temperature gradient derived from the dry-bulb temperatures, \(\Gamma\) is the dry adiabatic lapse rate (\(-9.8\) K km\({}^{-1}\)), \(L_{\varphi}\) is the latent heat of vaporization (J kg\({}^{-1}\)), \(K_{\varphi}\) is the eddy diffusivity for water vapor (m\({}^{2}\) s\({}^{-1}\)) and \(\partial\overline{q}/\partial z\) is the vertical moisture gradient derived from the wet-bulb temperatures. This method to obtain \(Q_{H}\) and \(Q_{E}\), however, requires knowledge of \(K_{H}\) and \(K_{\varphi}\), which can vary significantly with location and environmental conditions.