Soil Water and Atmospheric Processes
Learning Outcomes: This exercise has been designed to reinforce some of the material on radiation and energy balance discussed in the lectures. By the end of this exercise you should have an improved understanding of concepts such as: emissivity, albedo, the Stefan-Boltzmann law, net radiation and energy balance. You should also start to get a feel for typical values of solar and net radiation and be confident making simple calculations of radiation and energy balance.
Before you begin:
- Think about the relative magnitude of the fluxes you calculate – ask yourself the question “are they realistic?” If you are in doubt about the relative magnitude of the fluxes you calculate, have a look either at your lecture notes or Oke’s book to see typical sizes of radiative and heat fluxes during the day and at night. Are your values within that range?
- Be sensible when reporting your calculated fluxes – fluxes can only be measured to an accuracy of about 10-15% so write down your calculated fluxes as integers – just as in the Tables of values.
————————————————————————————————————-
Radiation and Energy balance questions to answer
- The accompanying Field Log shows some measurements of radiation and energy balance made by a first-year postgraduate research student. They made the observations on four different land-surface types on the same day and at the same time but unfortunately their field log book was partially destroyed when they slipped into a peat bog on the way back to the lab. It is possible to re-create the missing data given the data that did survive and it is your job to do just that. You should complete the Field Log by filling in the blanks as appropriate. [40 marks]
(hint: step 1 would be to write down the equations of radiation and energy balance to help you do the arithmetic for each of the surfaces; step 2 would be to collate all the information for the surfaces separately so you know which data to use for each surface).
- Write about what can you tell from the measurements about each of the surface types? Can you identify each surface type? (hint: which one is wettest; which one shows the greatest soil heat flux or albedo. They are all natural surface types found in Scotland). [60 marks]
Field Log – A.N Student
| Measurement | Units | Surface 1 | Surface 2 | Surface 3 | Surface 4 |
| Incoming Solar Radiation (St) | W m-2 | 800 | |||
| Reflected Solar Radiation | W m-2 | 40 | 720 | ||
| Albedo (a) | 0.4 | 0.3 | |||
| Surface Temperature (Ts) | ºC | 15 | |||
| Outgoing Longwave Radiation (Lu) | W m-2 | 512 | 391 | 307 | |
| Incoming Longwave Radiation (Ld) | W m-2 | 340 | |||
| Net Radiation (Rn) | W m-2 | ||||
| Sensible Heat flux (H) | W m-2 | 100 | 50 | ||
| Latent Heat flux (LE) | W m-2 | 300 | |||
| Ground Heat flux (G) | W m-2 | 21 | 17 | ||
| Bowen Ratio (b) | 40 | 0.167 | 0.5 |
Assume: emissivity is the same for all surface types (for simplicity assume e=1 here); the measurements were made by identical sets of equipment on the same day at the same time i.e. the incoming radiation streams are the same in all instances.
Useful Formulae and definitions
Equation of Radiation Balance
Equation of Energy Balance
Bowen Ratio is the ratio of sensible to latent heat flux densities
Stefan Boltzmann Constant (σ) = 5.67 x 10-8 W m-2 K-4
Stefan-Boltzmann Law: emittance (or Lu here) = esTs4 (W m-2)
