### Mass balance check: Balance.out vs. T_Level.out

Posted:

**Thu Jun 30, 2005 12:14 am**Gentlemen,

I am using HYDRUS-1D (thanks for the improvements!) to estimate daily net infiltration rates (either + or -) at ground surface using daily precip and PE rates via the atmospheric boundary condition (0 surface layer and precip rates are too low to generate runoff). The results are to be used as inputs to a 3D variably-saturated model that does not offer an atmospheric boundary condition, but will use the same 60 year sequence of daily precip and PE values.

My first approach was to use the sum(vTop), sum(vBot), and Storage values, as presented in the T-Level.out file, to estimate a change in vTop, vBot, and Storage for each day. That is, I assumed that the change in Storage between day i and day i-1 = vTop(i) + vBot(i), all of which can be calculated from the T-Level.out rows corresponding to days i and i-1. However, I found that the discrepancies between the change in Storage and the incremnetal fluxes through the top and bottom boundaries between day i and i-1 are often 1-4% and can reach 12-38%, depending on the hydraulic properties used. The Balance.out WatBalR entries for those days are routinely < 0.01% and I believe spatial and temporal discretization are more than adequate. Why do the change in storage and change in influx+outflux, calculated for each day, differ by so much more (0.5-38%) than the WatBalR values (<0.01%) for the same days?

Another approach is to use the top flux entry from a Balance.out report for each day. However, I noticed the top flux for day i in the Balance.out is usually significantly less than the difference in sum(vTop) for days i and i-1 except after several continuous days of no rain (only evap). The disparities are greatest for rainy days and the drying days immediately following the precip events.

I am puzzled by these disparities in water mass fluxes between the T-Level.out and the Balance.out files. I shall be very grateful for your advice about which, if any, of these estimates I should use to specify net infiltration at the surface.

Thanks and regards,

John Sigda, Ph.D.

I am using HYDRUS-1D (thanks for the improvements!) to estimate daily net infiltration rates (either + or -) at ground surface using daily precip and PE rates via the atmospheric boundary condition (0 surface layer and precip rates are too low to generate runoff). The results are to be used as inputs to a 3D variably-saturated model that does not offer an atmospheric boundary condition, but will use the same 60 year sequence of daily precip and PE values.

My first approach was to use the sum(vTop), sum(vBot), and Storage values, as presented in the T-Level.out file, to estimate a change in vTop, vBot, and Storage for each day. That is, I assumed that the change in Storage between day i and day i-1 = vTop(i) + vBot(i), all of which can be calculated from the T-Level.out rows corresponding to days i and i-1. However, I found that the discrepancies between the change in Storage and the incremnetal fluxes through the top and bottom boundaries between day i and i-1 are often 1-4% and can reach 12-38%, depending on the hydraulic properties used. The Balance.out WatBalR entries for those days are routinely < 0.01% and I believe spatial and temporal discretization are more than adequate. Why do the change in storage and change in influx+outflux, calculated for each day, differ by so much more (0.5-38%) than the WatBalR values (<0.01%) for the same days?

Another approach is to use the top flux entry from a Balance.out report for each day. However, I noticed the top flux for day i in the Balance.out is usually significantly less than the difference in sum(vTop) for days i and i-1 except after several continuous days of no rain (only evap). The disparities are greatest for rainy days and the drying days immediately following the precip events.

I am puzzled by these disparities in water mass fluxes between the T-Level.out and the Balance.out files. I shall be very grateful for your advice about which, if any, of these estimates I should use to specify net infiltration at the surface.

Thanks and regards,

John Sigda, Ph.D.