Snow Hydrology Discrepancies

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dg34
Posts: 5
Joined: Wed Nov 28, 2018 6:21 pm
Location: Chile

Snow Hydrology Discrepancies

Post by dg34 » Thu May 14, 2020 3:46 pm

Dear all,

We are using HYDRUS to simulate water flow through an unsaturated, single material soil profile. There is no vegetation. Our upper BC is defined as Atmospheric with Surface Runoff and the bottom BC is Free Drainage. Thus, the only possible fluxes are evaporation, infiltration and flux across the bottom surface. We are, however, considering Snow Hydrology.

Our model run is for 500 years, using daily values of temperature and preciptation, and calculating PE using Penman-Montieth.

It is our understanding that HYDRUS treats all precipitation on days with mean temperature above +2 degrees as liquid, all precipitation on days with temp. below -2 degrees as snow, and that a simple linear transition exists between these two. Thus, on a 0 degree day, for example, 50% of precipitation would be snow, and 50% would be liquid.

We have two main questions.
1) How does HYDRUS report (if it does) sublimation from snow? Is this included in the evaporation output? In comparing results of the model between a simulation that considers Snow Hydrology and one that does not, all fluxes are accounted for in the mass balance when Snow Hydrology is not considered, but are not in the model with snowfall.

2) When observing the snow layer output in the T_Level.out file, HYDRUS produces variable percentages of precipitation falling as snow for the same daily temperature. For example, we have multiple days with a mean temperature of -1.565, and observe a range of around 15 to 77% of precipitation fallling on these days being added to the Snow Layer. Do you know where this discrepancy may be coming from? The percentage of precipitation recorded as snow decreases with decreasing amounts of precipitation, and some of the precipitation events are quite small.

Thanks!

Dan

Jirka
Posts: 4922
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

Re: Snow Hydrology Discrepancies

Post by Jirka » Thu May 14, 2020 5:50 pm

HYDRUS never reports any average fluxes over some period, such as daily. It always reports actual values at a particular time.

Thus, if temperature is allowed to vary during the day, the processes may be rather complex. The ratio of water applied as liquid and solid (snow) will vary during the day correspondingly to temperature. Similarly, if temperature varies during the day and exceeds zero, snow melting can occur at that time. If you specify evaporation during the presence of snow, sublimation will occur. The thickness of the snow layer thus reflects all these simultaneous processes.

J.

dg34
Posts: 5
Joined: Wed Nov 28, 2018 6:21 pm
Location: Chile

Re: Snow Hydrology Discrepancies

Post by dg34 » Thu May 14, 2020 9:59 pm

Thanks for the answer.

I should've been clearer. In our case, daily values do not vary throughout the day. Each day has one Tmin and Tmax value. But I was able to calculate sublimation myself, and that aligned with our global mass balance.

Thanks again.

Dan

Jirka
Posts: 4922
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

Re: Snow Hydrology Discrepancies

Post by Jirka » Thu May 14, 2020 10:26 pm

If there is one couple of values T_min and T_max for each day, then temperature varies between these two values following a sine wave. If T_min and T_max are the same, then indeed, there is only one temperature for the entire day. J.

dg34
Posts: 5
Joined: Wed Nov 28, 2018 6:21 pm
Location: Chile

Re: Snow Hydrology Discrepancies

Post by dg34 » Fri May 15, 2020 1:43 pm

I thought that was only the case when the "Daily Variations of Meteo Data During Day Generated by HYDRUS" option is selected in the Time Information dialogue (DailyVar = true in the ATMOSPH.IN file), no? Or does HYDRUS generate those variations automatically?

Thanks,

Dan

Jirka
Posts: 4922
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

Re: Snow Hydrology Discrepancies

Post by Jirka » Fri May 15, 2020 5:26 pm

No, it does not do it for metro variables if you do not select this option (in the Time Information window). However, it does it for temperatures if you specify "Interval for Temperature cycle" larger than zero in the Heat Transport Parameters window and Ampl in Time Variable Condition window. J.

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