Actual RWU equals Potential RWU
Actual RWU equals Potential RWU
Hi All,
I am modeling a hillslope with root water uptake enabled. I have supplied Hydrus with Tpot, Surface length associated with Transpiration, and the root distribution function (beta). The water uptake reduction model is set to Feddes. Feddes' parameters are to PO=10 cm, POpt=25 cm P2H=600 cm, P2L=1200 cm, P3=15000 cm.
After running Hydrus, the Actual Root Water Uptake is equal to the Potential Root Water Uptake despite the rooting zone pressure head often lying outside the "optimal" RWU pressure head range and falling below the wilting point at times. This result does not seem correct to me. The reduction model never seems to have no influence on root water uptake and Hydrus is only using the supplied Tpot to define root water uptake. Has anyone run into this before? Maybe I am missing something obvious?
Thanks,
Mark
I am modeling a hillslope with root water uptake enabled. I have supplied Hydrus with Tpot, Surface length associated with Transpiration, and the root distribution function (beta). The water uptake reduction model is set to Feddes. Feddes' parameters are to PO=10 cm, POpt=25 cm P2H=600 cm, P2L=1200 cm, P3=15000 cm.
After running Hydrus, the Actual Root Water Uptake is equal to the Potential Root Water Uptake despite the rooting zone pressure head often lying outside the "optimal" RWU pressure head range and falling below the wilting point at times. This result does not seem correct to me. The reduction model never seems to have no influence on root water uptake and Hydrus is only using the supplied Tpot to define root water uptake. Has anyone run into this before? Maybe I am missing something obvious?
Thanks,
Mark
Re: Actual RWU equals Potential RWU
That’s not possible unless you use the compensated root water uptake (a critical stress index smaller than 1). Then, it is exactly what should happen. J.
Re: Actual RWU equals Potential RWU
Hi Dr. Jirka,
Can we provide a critical stress index equals to 0? In my Hydrus1D simulation for soil moisture, I have 7 materials types along with a 120cm soil profile. I tried to run the model for fully compensated (index=0) RWU and I also provided constant and linear root distribution patterns. My model's initial condition is in terms of water content, the upper boundary is atmospheric BC with runoff and the lower boundary is free drainage.
But, the problem is I haven't seen any compensation even if the top layers getting stressed. Did I do any mistakes? I went through Simunek and Hopsman 2009 paper and examples. There is a very easy and simple example. Do you have any example of Hydrus1D simulation with different materials (Heterogenous soil with multiple layers) along with the soil profile?
Can we provide a critical stress index equals to 0? In my Hydrus1D simulation for soil moisture, I have 7 materials types along with a 120cm soil profile. I tried to run the model for fully compensated (index=0) RWU and I also provided constant and linear root distribution patterns. My model's initial condition is in terms of water content, the upper boundary is atmospheric BC with runoff and the lower boundary is free drainage.
But, the problem is I haven't seen any compensation even if the top layers getting stressed. Did I do any mistakes? I went through Simunek and Hopsman 2009 paper and examples. There is a very easy and simple example. Do you have any example of Hydrus1D simulation with different materials (Heterogenous soil with multiple layers) along with the soil profile?
Re: Actual RWU equals Potential RWU
The critical value of the water stress index ωc must be always larger than zero. If it is zero, you would have a division by zero in eq. 2.25 (see the manual), which clearly cannot happen. J.
Re: Actual RWU equals Potential RWU
Thank you, Dr. Jirka. I have one more question. I ran the model using a critical stress index of 0.5. I am a little bit confused about the result because the simulation showed more water extraction from the upper layer after running the compensation RWU scenario. Is it possible that more water extraction occurred from the upper layer after providing wc<1? if yes, then in which condition it is possible and if no, how the simulation showed this kind of result? I have attached an example of a simulation. Thank you!
 Attachments

 example of simulation
 example of hydrus.JPG (102.47 KiB) Viewed 2221 times
Re: Actual RWU equals Potential RWU
As you can see from eq. 2.25, compensation will increase root water uptake from any soil layer where water is available. J.
Re: Actual RWU equals Potential RWU
Thank you, Dr. Jirka.
Re: Actual RWU equals Potential RWU
Dr. Jirka,
I was reading Hydrus manual and I have seen that "when ωc is equal to zero we obtain fully compensated uptake" in last line of page 21 (after equation 2.25). But, as you said before the ωc should be always greater than zero (referring eq 2.25). I am quite confused because we are running fully compensatory model. According to manual, we are using ωc=0 for fully compensation. Could you please tell me that the written in last line of page 21 is a mistake or not? Could you please explain for me, Dr. Jirka. Thank you
Re: Actual RWU equals Potential RWU
Ful compensation is possible only theoretically, but not practically. As I have written before, OmegaC cannot be equal to zero as then you would have division by zero in eq. 2.25, which the program clearly cannot allow (since that would lead to crash). Compensation will occur only as long as Omega is larger than zero (eq. 2.24). Once it drops to zero, it will stop. J.
Re: Actual RWU equals Potential RWU
Thank you so much for your quick response.
Dr. Jirka, I got your point. just one more follow up question.
When I used Omega_c = 0, the model worked without stopping or crashing. And, actually, I could easily see the differences in RWU output for different Omega_c values ( e.g. Omega_c = 0, 0.1, 0.5, and 1). Did I do any mistakes that's why the model worked? Thank you!
Dr. Jirka, I got your point. just one more follow up question.
When I used Omega_c = 0, the model worked without stopping or crashing. And, actually, I could easily see the differences in RWU output for different Omega_c values ( e.g. Omega_c = 0, 0.1, 0.5, and 1). Did I do any mistakes that's why the model worked? Thank you!
Last edited by mmehata on Mon Mar 01, 2021 9:01 pm, edited 1 time in total.
Re: Actual RWU equals Potential RWU
I have no more comments. The model is fully described in the manual and in Simunek and Hopmans (2009). J.