Definitions of Evapotranspiration quantities in Hydrus

A discussion forum for Hydrus-1D users.
kpan84
Posts: 2
Joined: Tue May 28, 2019 11:28 am
Location: Cyprus

Definitions of Evapotranspiration quantities in Hydrus

Hi,
I am new in Hydrus, thus I would like to ask 4 questions related to the Evapotranspiration quantities.
1) My understanding is that the reference crop evapotranspiration ETo can be calculated by the Penman-Monteith combination equation based on meteorological data. One way to convert ETo to potential evapotransiration rate ETp is to multiply it by crop coefficient Kc. Since Kc is not used in Hydrus-1D, is it accurate to say that Hydrus-1D sets ETo equivalent to ETp (thus assuming Kc=1)?
2) Is it accurate to state that the actual evaporation rate is calculated in Hydrus-1D as the product between the water stress function alpha and ETp, where alpha is the function used to obtain the actual water root uptake (Feddes model)?
3) If 1) is correct, what would be the right way to calculate ETp without assuming that it is equal to ETo?
4) In Hydrus Manual 4.08, in the section that discusses the hysteresis in soil hydraulic properties, it states that "where the subscripts d and w indicate wetting and drying, respectively." Shouldn't be the inverse way: w-wetting and d-drying?

Regards,
Kostas

Jirka
Posts: 4827
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

Re: Definitions of Evapotranspiration quantities in Hydrus

1. Yes, it is. However, HYDRUS does not consider ET to be a single process. It divides it into two independent processes, i.e., transpiration and evaporation. HYDRUS divides ETp into Ep and Tp using LAI and then calculates actual values (e.g., lower evaporation from the soil surface, lower transpiration from the soil profile) by solving the Richards equation with appropriate boundary conditions and stress response functions. Using lower Kc values than 1, which usually accounts for reduced evaporation from the soil surface, would thus consider this factor twice (via the crop coefficient and the BC). If you want to use crop coefficients, you can calculate reference ETo outside of the HYDRUS environment, multiply it by Kc to get ETp, divide ETp into Ep and Tp, and provide as input directly Ep and Tp, similarly as many other HYDRUS users have done in the past.
2. As discussed above. HYDRUS divides ET into E and T processes. Potential transpiration is then redistributed over the root zone depending on root spatial distribution. Resulting potential root water uptake is then reduced using the stress response function to get the actual root water uptake (which is done localy, at different depths). Integrating actual root water uptake over the entire rooting depth then gives actual transpiration.
3. If you want to use crop coefficients, you can calculate reference ETo outside of the HYDRUS environment, multiply it by Kc to get ETp, divide ETp into Ep and Tp, and provide as input directly Ep and Tp, similarly as many other HYDRUS users have done in the past.
4. Obviously.

J.

kpan84
Posts: 2
Joined: Tue May 28, 2019 11:28 am
Location: Cyprus