HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Groenendijk, P. Articles by Van Dam, J.C. Search for Related Content PubMed Articles by Groenendijk, P. Articles by Van Dam, J.C. GeoRef GeoRef Citation Agricola Articles by Groenendijk, P. Articles by Van Dam, J.C.

COMMENTS

Comments on "A Set of Analytical Benchmarks to Test Numerical Models of Flow and Transport in Soils"

Wageningen UR, P.O. Box 47, 6700 AA, Wageningen, The Netherlands
piet.groenendijk{at}wur.nl

The increasing demand for detailed information on hydrological processes as they are influenced by land use, soil characteristics, and water management have encouraged the development of dynamic process-oriented models. Currently applied models are not perfect and have to be thoroughly tested and validated. The awareness of the need for good modeling practice protocols is increasing outside the academic community as well. Testing protocols include verification of (numerical) model codes and validation against field experiments. Therefore, we were very pleased with the paper by Vanderborght et al. (2005) on benchmark tests for numerical models that use Richards' equation for water flow and the convection–dispersion equation for solute transport.

For the case with high infiltration into a soil profile with clay above sand, the SWAP model (Van Dam et al., 1997; Kroes and van Dam, 2003) reproduced exactly the water balance but showed deviations in the computed soil water pressure head profile. Vanderborght et al. (2005) attributed these deviations to the use of volumetric water content changes as a stopping criterion in the iteration procedure, instead of soil water pressure head changes.

Further inspection of the model and the SWAP computer code for this case indicated the following:

1. Changes in both the volumetric water content and the pressure head are used to test progress of the iteration cycle. The criterion for the volumetric water content is user-supplied, while the criterion for the change of pressure head is fixed in the model code.
   
2. When progress of the numerical solution is not sufficient within six iterations, the time step is decreased and the iteration cycle starts again. It appeared that the maximum number of six was not sufficient for this case. After increasing this maximum to 20, the model generated a stable pressure head profile, although results were still not accurate.
   
3. The hydraulic conductivity is described as a function of relative saturation (S_e) using the analytical Mualem–van Genuchten function (van Genuchten, 1980) (MVG). Near saturation the SWAP model uses a not well documented linearization of this function: in the range 0.99 < S_e < 1.00, the hydraulic conductivity is linearly interpolated between its value at S_e = 0.99 according to van Genuchten (1980) and the saturated hydraulic conductivity at S_e = 1.00. Figure 1 shows the effect of this linearization. In the benchmark test the final relative saturation of the clay layer was >0.99 and the linearization caused higher conductivity values than would be the case for the analytical MVG model.
   

View larger version (9K): [in this window] [in a new window]   Fig. 1 Conductivity of the clay soil and linearization of this relation near saturation. ksat = 10 (cm d–1); = 0.01 (cm–1); n = 1.1; = 0.5.

While the benchmark tests presented by Vanderborght et al. (2005) are very useful to test model performance, one may question whether they are representative for practical applications. Especially the parameterization used for the clay soil seems to lack any physical basis. Due to the low n value, the difference between k(h) at –1 cm pressure head and k_sat is very large (Fig. 1). At h = –0.1 cm the conductivity is already <25% of k_sat. At this pressure head, the air entry value of clay soils will not yet be reached, and the clay matric is still saturated. Therefore, such a large drop in the hydraulic conductivity is not realistic. Vogel et al. (2001) showed that the shape of the k function near saturation can have a large impact on variably saturated flow predictions. The broken line depicts the linearization employed by the SWAP model. This linearization causes the conductivity at the soil surface to be overestimated by a factor of 8.75 times.

Our main conclusions are that the input options of the SWAP model were insufficient for direct application to the infiltration case of a clay soil on top of coarse sand. While linearization of the MVG relation near saturation proved to be a hidden model option useful for practical field simulations, this also caused deviations from the pressure head profile for the extreme case we tested. To increase user flexibility we will release a new version of SWAP that offers input of k(h) and (h) directly in tabular form in addition to analytical MVG functions. Documentation will be improved and an extensive test-set will be made available for download from our web site (www.swap.alterra.nl [verified 13 Jan. 2006]).

REFERENCES

• Kroes, J.G., and J.C. van Dam (ed.) 2003. Reference manual SWAP version 3.0.3. Alterra-report 773. Alterra, Wageningen, The Netherlands.
• Van Dam, J.C., J. Huygen, J.G. Wesseling, R.A. Feddes, P. Kabat, P.E.V. van Walsum, P. Groenendijk, and C.A. van Diepen. 1997. Theory of SWAP version 2.0. Simulation of water flow, solute transport and plant growth in the Soil-Water-Atmosphere-Plant environment. Rep. 71. Subdep. Water Resources, Wageningen University Technical document 45, Alterra Green World Research, Wageningen, The Netherlands.
• van Genuchten, M.Th. 1980. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Sci. Soc. Am. J. 44:892–898.[Abstract/Free Full Text]
• Vanderborght, J., R. Kasteel, M. Herbst, M. Javaux, D. Thiery, M. Vanclooster, C. Mouvet, and H. Vereecken. 2005. 2005. A set of analytical benchmarks to test numerical models of flow and transport in soils. Available at www.vadosezonejournal.org Vadose Zone J. 4:206–221.[Abstract/Free Full Text]
• Vogel, T., M.Th. van Genuchten, and M. Cislerova. 2001. Effect of the shape of the soil hydraulic functions near saturation on variably-saturated flow predictions. Adv. Water Resour. 24:133–144.[CrossRef]

This Article Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Groenendijk, P. Articles by Van Dam, J.C. Search for Related Content PubMed Articles by Groenendijk, P. Articles by Van Dam, J.C. GeoRef GeoRef Citation Agricola Articles by Groenendijk, P. Articles by Van Dam, J.C.