Improvements to Potential Evaporation calculations

[hut104]

Atmospheric Potential Evaporation, called Eo in APSIM, is currently calculated using algorithms from CERES Maize. They are supposedly implementations of a Priestly-Taylor evaporation rate (ie an Equilibrium or energy driven evaporation rate multiplied by a scalar) but the simplified implementation obscures the nature of the exact implementation. There are also issues possibly related to the current Eo values. Simulations have shown that ET rates for canopies (using Penman-monteith) in winter are often lower than the Es rates for bare soil (using Eo). This results in higher drainage under winter crops than under fallow in some simulations. Futhermore, soil evaporation rates have been known to be high in Southern Australia during winter which has led to the implementation of different evaporation parameters for winter and summer. These are likely due to inaccurate Eo values. The current implementation of Priestly-Taylor should be compared to a full implementation of the approach using the energy balance within micromet to judge if this is the cause of the difficulties described above. After this, alternative approaches could also be evaluated. The first stage of any implementation will be to remove the current duplication of Eo calculations within the 3 soil water balances. The calculation should be centralised into the microclimate model. This will also allow better calculation of Eo for multi-point simulations, such as for intercropping, where plants shade soils in nearby zones.

[peter-devoil]

It would be helpful to leave a warning somewhere if a simulation is run without MicroClimate..