Principle

This apparatus measures the increase in temperature and vapour pressure over a fixed distance. If the gradient can be ascertained, then an indication of the ability of evaporation occurring from the soil surface can be calculated. This experiment is agriculturally and environmentally important as it shows site managers where the water is going and also show them the importance of ground cover.

Method

To calculate the bowen ratio,a number of parameters need to be measured. In the field this is best done with a mobile weather station, this is because it can at regular intervals (5 mins) measure all parameters simultaneously and then record them to a datalogger (see picture).

The bowen ratio sensor used in the experimentation recorded the ambient air temperature at two heights a fixed distance apart (1m). The sensor also records the vapour pressure or humidity of the air at the same height as the temperatures were measured. From these measurements both a pressure gradient and temperature gradient can be established.

Each temperature and humidity sensor at the two heights is housed within a protective cover to prevent direct exposure to the elements. Before each reading is taken a fan situated in the housing runs to draw in fresh air for sampling. A fixed weather station, which was approximately 10m from the survey site was used to measure the net radiation. The data was then transferred into the statistical program JMP 5.1. where the evaporation was calculated.

Analysis of Data

When analysing the data to calculate evaporation, the temperature and relative humidity are measured at two heights and net radiation is obtained from the fixed weather station. With the data that is now available analysis can begin. Initially the partial pressure of water in the air (kPa) for both heights is calculated using the equation:

Where T1 = Temperature at height 1

Then the difference between the two temperatures and two saturated vapour pressures is calculated (T1-T2 and es1-es2). After this the Bowen Ratio is calculated using the equation:

Where Beta is the Bowen ratio and gamma is the psychrometric constant:

Where Vh is the latent heat of vapourisation of water (2.45*10⁶ J/kg) and Pa is the atmospheric pressure which is determined using the equation:

where Ta = average temperature and Alt = height above sea level (m).

Finally to be able to calculate the flux of water vapour or the evaporation rate in kg/m2/s the following equation needs to be solved:

Where E = evaporation rate (kg/m²/s)

Rn = net radiation (W/m²)

S = soil heat flux (W/m²)

Beta = Bowen ratio

Vh = latent heat of vapourisation of water (2.45*10⁶ J/kg)

However the soil heat flux needs to be calculated using Fourier's law:

Where lambda h is the soil thermal conductivity (W m^-1 K^-1), T is temperature and z is verical distance.

Results

Initially all the results were plotted against time (see graphs below). It can be seen that there are definite differences between day and night temperatures and relative humidities, temperature and relative humidity increase during daylight hours and decrease from sunset onwards (which is logical but it is now proven).

Graph of relative humidity at height 1 (closest to the ground).

Graph of relative humidity at height 2.

Graph of temperature at height 1 (closest to the ground).

Graph of temperature at height 2.

Following this the partial pressure of water vapour in the air was calculated and plotted against time, however the data aquired was noisy so a spline was fitted (with lamda = 1) to both; and predicted e (partial pressure of water vapour) was saved (see graphs below). The predicted e was then used for the rest of the calculations.

Graph of the calculated partial pressure of water vapour at height 1.

Graph of the calculated partial pressure of water vapour at height 2.

After the calculations of the Bowen Ratio and the soil heat flux the evaporation rate was calculated in millimetres, again the results were noisy so a spline was fitted with lamda = 2.

Graph of the final evaporation rate against time.

Overall, approximately 18.66 mm of evporation occured at the survey site over about 4.5 days. This is slightly lower than usual and that may be due to the fact that an area above the soil sensors was covered with a mulch for another experiment. Otherwise the results obtained are a good indication of the amount and the rates at which evaporation occurs at the survey site.

This is very important agriculturally and environmentally because it numerically displays that the soil lost 18.66mm of water while we were conducting the experiment. This amount highlights the importance of ground cover such as mulches in controlling the soil moisture loss. This shows site managers that the soil is always losing water - with or without plants, but the rate of water loss can be dramatically reduced through the retaining of crop residues to act as mulch on the soil surface.

Problems with Method

In this experiment a mobile weather station was not placed in the uncultivated area of the survey site and therefore a direct difference between the two sites cannot be drawn. This method also requires a large number of calculations on the computer to be able to produce a worthwhile piece of information and maybe in the future the data logger could have a computer that could calculate the evaporation rate.