Hi,

I would like to investigate the impact that different biochars have on the actual evaporation using Hydrus. I know that Hydrus can estimate the potential evaporation using atmospheric data, and I know that the actual evaporation can be obtained through Richard's equation using equations 2.70 at Hydrus Manual 4.08:

| -K(dhdx + 1) | leq. E , where E is the maximum potential rate of evaporation (Ep)

Based on the above, my understanding is that the actual evaporation at every time step can be found by calculating the RHS of the above equation ( with an upper threshold E ), where RHS is the surface flux. Is this accurate?

If yes, why the surface flux at the upper surface should equal the actual evaporation?

## estimate actual evaporation

### Re: estimate actual evaporation

The evaporation process is usually assumed to have two stages (sometimes also the third, when flow towards the soil surface is only in the vapor phase). During the first stage, actual and potential evaporations are the same (enough water in the soil profile to satisfy the atmospheric demand), during the second stage, actual evaporation is smaller than potential evaporation (not enough water in the soil profile to satisfy the atmospheric demand). During the first stage, when the surface pressure head is larger than hCritA, the Neumann flux BC is applied at the soil surface. Once the pressure head drops below hCritA, the second stage starts. During this stage, the Dirichlet BC is applied at the soil surface (with hCritA), and the actual evaporation flux is calculated. J.