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Home / Programs / HYDRUS (2D/3D) / FAQ / HYDRUS: 21-30

## FAQ 21-30

### 21. The Use of Decimal Points and/or Decimal Commas on PCs with Different Regional Settings

Windows customizes your PC to suit different national conventions, such as the formatting of dates, currency values, and the use of **Decimal Symbols** (points or commas). HYDRUS, which has been originally developed in United States (where the decimal point is the standard decimal symbol), requires by default decimal points everywhere. However, for certain regional settings (such as in many countries in Europe, where the decimal comma is the standard decimal symbol), while the **Edit Boxes** in HYDRUS still require you to use decimal points, the **Tables**, such as those in the *Water Flow Parameters* or the *Time-Variable Boundary Conditions* windows, require you to use decimal commas. You can adjust these setting easily as described below:

**Windows 7**: Start / Control Panel / Regional and Language / Additional Settings / Decimal Symbol (select the decimal point).

**Windows XP**: Start / Control Panel / Date, Time, Language and Regional Options / Regional and Language Options / Customize Tab / Decimal Symbol (select the decimal point).

**Windows 98**: Start / Settings / Control Panel / Regional Settings / Number Tab / Decimal Symbol (select the decimal point).

### 22. Notes on units in HP1

The geometrical [Lw, Ls] and concentration [M/Lw^3] units used in HP1 (and HP2) are often in conflict since it is common to use meters or centimeters for geometrical units (HYDRUS actually does not support decimeters, or 10 cm), while concentration units in HP1 are expressed in [mol/liter, or mol/dm^3]. HP1 users have to then take that into account and interpret the reported **solute fluxes** accordingly. For example:

If the length unit in Hydrus is cm: *qC* = X cm/time * mol/dm^3 = X/1000 mol/time/cm^2

If the length unit in Hydrus is m: *qC* = X m/time * mol/dm^3 = X/0.001 mol/time/m^2

Similar adjustments need to be done for **mass of solute** in the domain:

If the length unit in Hydrus is cm: *M*=*Lc*= X cm * mol/dm^3 = X/1000 mol/time/cm^2

If the length unit in Hydrus is m: *M*=*Lc*= X cm * mol/dm^3 = X/1000 mol/time/cm^2

### 23. Can HYDRUS consider surface runoff or overland flow?

The standard version of HYDRUS assumes that once the potential surface flux (precipitation or irrigation) exceeds the actual surface flux (infiltration), all excess water is instantaneously removed by surface runoff (and reported as such). The standard version of HYDRUS does not track this excess water at the land surface. This is a reasonable assumption considering that surface runoff (overland flow) is a much faster process than subsurface flow. However, this obviously does not allow excess water from one part of the domain to runoff at the surface and to infiltrate in another, potentially more conductive, parts.

The HYDRUS users can freely download a (non-standard) module, which considers overland flow for 2D problems (associated with the atmospheric boundary) at http://www.pc-progress.com/en/Default.aspx?h3d-Overland. While this module has been extensively tested (see many examples, as well as a published manuscript), since it is not as numerically stable as the rest of HYDRUS, it has not been made a standard part of HYDRUS and is not subject to a technical support by PC Progress.

### 24. What does HYDRUS-1D do when soil temperatures drop below freezing?

In earlier versions of HYDRUS-1D (before 4.15), we did not consider any effects of temperature on water flow below freezing. In the later versions (starting with 4.16) the hydraulic conductivity is reduced by a factor of 1e-4 when temperatures drop below -1C (basically assuming that there should be no flow when the soil if frozen). This may obviously cause non convergence, if users specify at the same time precipitation in the liquid form, since water cannot infiltrate into the soil due to soil being frozen and conductivity being very low. For such conditions it is necessary to consider "Snow Hydrology" (in the Main Processes window). When temperature drops below zero (see detailed description of the snow model in the manual or help), precipitation comes in the form of snow and stay at the top of the profile. Snow then melts when temperature increases above freezing (users can specify a melting constant) and when water can infiltrate into the soil profile. HYDRUS does consider sublimation from the snow layer (and users can specify a sublimation constant - a reduction of E in the Heat Transport Parameter window).

### 25. Why the numerical solution does not converge for fine-textured soils and how to improve the convergence?

The unsaturated soil hydraulic properties are often described using Mualem–van Genuchten (MVG) type analytical functions. Recent studies suggest several shortcomings of these functions near saturation, notably the lack of second-order continuity of the soil water retention function at saturation and the inability of the hydraulic conductivity function to account for macroporosity. It has been shown by Vogel et al. (1985, 2000) and Schaap and van Genuchten (2006) that a modified MVG formulation with a small but constant air-entry pressure (*hs*) in the water retention curve substantially improves the description of the hydraulic conductivity near saturation. It is thus recommended, especially for fine-textured soils with small values of the *n* parameter (*n* < 1.1) to use the **modified van Genuchen-Mualem model with an air-entry value ( hs) of -2 (or -4) cm**. In addition to improving the description of the hydraulic conductivity function, this modified MVG model also dramatically improves the convergence of the numerical solution. (see also FAQ 26 below)

Schaap, M.G., and M. Th. van Genuchten, A modified Mualem–van Genuchten formulation for improved description of the hydraulic conductivity near saturation, *Vadose Zone Journal*, *5*, 27–34, 2006.

Vogel, T., M. Cislerova, and M. Sir, Reliability of indirect estimation of soil hydraulic conductivity, *Vodohospodarsky Casopis*, *33*(2), 204–224, 1985

Vogel, T., M. Th. van Genuchten, and M. Cislerova, Effect of the shape of the soil hydraulic functions near saturation on variably-saturated flow predictions, *Adv. Water Resour.*, *24*(2), 133–144, 2000.

### 26. -2 cm air entry option in the van Genuchten-Mualem soil hydraulic functions

(download a PDF text)

Questions often arise about the meaning and importance of the -2 cm option when selecting the van-Genuchten-Mualem (VGM) hydraulic function in the HYDRUS codes. The option is identified in the “Soil Hydraulic Property Model” window under “van Genuchten-Mualem” and then the subheading “With Air-Entry Value of -2 cm”. This option is recommended only for relatively fine-textured soils when the VGM parameter *n* becomes very close to its lower limit of 1.0 (e.g., less than 1.1 or 1.2). Selecting this option will introduce a small correction (like an air-entry value) in the water retention function to force the slope of the retention function (d*θ*/d*h*) to become zero when approaching saturation at *h*=0.

The correction has absolutely no effect on the macroscopic description of the water retention function of fine-textured soils. But it substantially changes the shape of the hydraulic conduction function, *K*(*h*), as predicted with the VGM statistical pore-size distribution model. The predicted *K*(*h*) is then far less nonlinear and much more accurate for fine-textured soils (typically clays and silty clays). It may also avoid numerical problems when modeling ponded infiltration or other variably-saturated flow problems. The -2 cm correction was first suggested by Vogel and Cislerová (1988) and worked out further by Vogel et al. (2001). Schaap et al. (2006) later showed that a -4 cm correction would be slightly better than -2 cm, but the difference is very minimal.

The -2 cm correction should not be used for medium-, and especially coarse-textured, soils when the value of VGM parameter *α* becomes relatively large (say >0.02 or 0.05 cm). This rule applies unless *n* is again less than about 1.1 or 1.2, which generally is not the case for medium- and coarse-textured soils. The -2 cm should then be avoided since it introduces a Brooks and Corey (1964) type air-entry correction into the retention function to make the function less accurate for many medium- and coarse-textured soils (Schaap and van Genuchten, 2006).

**References:**

Brooks, R. H., and A. T. Corey, Hydraulic Properties of Porous media, Hydrology Paper No. 3, Colorado State University, Fort Collins, CO, 1964.

Schaap, M. G., and M. Th. van Genuchten, A modified Mualem-van Genuchten formulation for improved description of the hydraulic conductivity near saturation, *Vadose Zone Journal*, *5*, 27-34, 2006.

van Genuchten, M. Th., A closed-form equation for predicting the hydraulic conductivity of unsaturated soils, *Soil Science Society of America Journal*, *44*, 892-898, 1980.

Vogel, T., and M. Císlerová, On the reliability of unsaturated hydraulic conductivity calculated from the moisture retention curve, *Transport in Porous Media*, *3*, 1-15, 1988.

Vogel T., M. Th. van Genuchten, and M. Cislerová, Effect of the shape of the soil hydraulic functions near saturation on variably saturated flow predictions, *Advances in Water Resources*, *24*(2), 133 144, 2001.

### 27. When and how to use the Internal Interpolation Tables?

At the beginning of a numerical simulation, HYDRUS generates for each soil type in the flow domain a table of water contents, hydraulic conductivities, and specific water capacities from the specified set of hydraulic parameters. Values of the hydraulic properties are then computed during the iterative solution process using linear interpolation between entries in the table. If the pressure head h at some node falls outside the prescribed interval (ha, hb), the hydraulic characteristics at that node are evaluated directly from the hydraulic functions (i.e., without interpolation). The above interpolation technique was found to be much faster computationally than direct evaluation of the hydraulic functions over the entire range of pressure heads. Interpolation using tables can be avoided by setting ha and hb both to zero. Then the soil hydraulic properties are always evaluated directly from the hydraulic functions (i.e., without interpolation). Output graphs of the soil hydraulic properties will be given also for the interval (ha, hb).

Users are advised to use the default values of the lower and upper limits of the tension interval. The lower limit of the tension interval is usually selected to be a very small number (e.g., 1.e-3 to 1.e-6 cm). The upper limit can be modified so that it encompasses most pressure heads encountered during the simulation (e.g., hCritA or the wilting point).

To avoid problems with the numerical solution when the Brooks and Corey model of soil hydraulic properties is used, the lower limit should be larger (in absolute value) than 1/alpha (i.e., the air-entry value).

To avoid problems with the numerical solution the internal interpolation tables should not be used when the Durner's dual-porosity model is used to describe the soil hydraulic properties, since this model does not have a monotonous hydraulic capacity function.

### 28. Simulation with the Brooks & Corey function does not converge.

When the Brooks and Correy functions are used, either disable the internal interpolation tables or set the lower limit above the air-entry value.

### 29. How to run the computational module of Hydrus-1D from Matlab?

Stathis Diamantopoulos has prepared a tutorial demonstrating the use of Matlab to run the computational module of HYDRUS-1D. Download the text of the tutorial and the test example.

### 30. Maximum number of optimized parameters

The maximum allowed number of optimized parameters (**NpaD**) is currently set at 15. However, please, do not optimize that many parameters simultaneously. The unsaturated flow problems are inherently ill posed, and thus not too many parameters can be optimized simultaneously. I have never successfully optimized more then 5 to 6 parameters. Then the problems usually become nonunique. The problems of unsaturated flow are different than those of the saturated flow, where I know that it is common to optimize many more parameters simultaneously.