jump in the value of theta

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Mitra
Posts: 16
Joined: Thu Dec 06, 2018 3:44 pm
Location: Switzerland

jump in the value of theta

Post by Mitra » Mon Dec 09, 2019 9:38 am

Hello,

I have run a simulation (homogeneous soil column) with a variable atmospheric boundary condition at the top and constant pressure head at the bottom. I consider hysteresis effects for saturation. I have several observation nodes in the column. It seems for only one of the nodes there is a jump in water content from a value of 0.3 to 0.4 (saturated water content) at one single time during the simulation. Can I have your opinion on why this is happening?

Thanks.

Jirka
Posts: 4982
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

Re: jump in the value of theta

Post by Jirka » Mon Dec 09, 2019 6:00 pm

You probably encountered some numerical instability. Perhaps use a different model of hysteresis (e.g., Lenhard et al.). J.

Mitra
Posts: 16
Joined: Thu Dec 06, 2018 3:44 pm
Location: Switzerland

Re: jump in the value of theta

Post by Mitra » Tue Dec 10, 2019 1:39 pm

Thanks for the reply,
I had several additional questions when filling in parameters of hysteresis the following parameters exist (with van Genuchten-Mualem model):
(Qr,Qs,Alpha,n,Ks), I, Qm,(QsW,AlphaW, and KsW).
    According to the manual of HYDRUS1D, it is only parameters (Qr,Qs,Alpha,n,Ks) which are required to be known for the basic van Genuchten-Mualem curve, so why HYDRUS1D includes the parameter Qm when I choose the option to include hysteresis?
      The (QsW,AlphaW, and KsW) refers to main wetting curve parameters, but what about the main drying curve parameters, does (Qr,Qs,Alpha,n,Ks) refer to the drying curve parameters? If not, where can I change the drying curve parameters?
        Does HYDRUS1D always use scanning curves? If I set the parameters Qr and Qs to be the same for the main drying and wetting curve, do scanning curves overlap with main drying and wetting curves? Is there any way that scanning curves overlap with the main drying and wetting curves?

        Jirka
        Posts: 4982
        Joined: Sat Mar 16, 2002 3:47 pm
        Location: USA
        Location: Riverside, CA

        Re: jump in the value of theta

        Post by Jirka » Tue Dec 10, 2019 3:32 pm

        First, if you have not actually measured both wetting and drying curves, I do not think that you should be using the hysteresis option in HYDRUS.

        Second, your other questions:
        1. Please, read the manual, where all the models are discribed in detail.
        2. Parameter Qm expands the flexibility of the model by allowing for an air-entry value (e.g., the model with -2cm air-entry).
        3. Parameters Qr,Qs,Alpha,n,and Ks are for the drying curves, and QsW,AlphaW, and KsW for wetting curves.
        4. If you set Qr and Qs the same for the main drying and wetting curves, while having different AlphaW and AlphaD, the scanning curves will be between the main drying and wetting curves.

        See also the description of the model in:
        Šimůnek, J., R. Kodešová, M. M. Gribb, and M. Th. van Genuchten, Estimating hysteresis in the soil water retention function from cone permeameter experiments, Water Resources Research, 35(5), 1329-1345, 1999.

        J.

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