The parameterisation and calibration of complex hydrological models like SWAT [Arnold, J.G., Srinivasan, R., Muttiah, R.S., and Williams, J.R. 1998. Large-area hydrologic modeling and assessment: part I. Model development. J. American Water Resour. Assoc. 34, 73–89.] is a time consuming process. The use of simple models can shorten this effort substantially because they enable us to identify the important processes in the catchment very fast and thus facilitate the parameterisation of the complex model.

We used the models SWAT and SIMPEL [Hörmann, G., 1997. SIMPEL – Ein einfaches, benutzerfreundliches Bodenwassermodell zum Einsatz in der Ausbildung. Dt. Gewässerkundliche Mitteilungen 41, 67–72.]. SWAT is a complex, meso-scale eco-hydrologic model, SIMPEL is a set of spreadsheets with a one-dimensional soil water model where runoff is calculated with a unit hydrograph. The parameterisation of the SWAT model was very time consuming because many parameters had to be estimated due to the scarce data situation (e.g. of land use data). To avoid long test runs, we first implemented and tested possible solutions in SIMPEL and finally transferred it to SWAT.

The models were applied to the Xiangxi catchment in China which is a tributary of the Yangtse, situated near the Three Gorges dam. Elevation ranges from 150 to 3000 m. The database consists of 8 climate stations, one station with pan evaporation and one gauging station at the basin level. Land use in the valleys is mainly agriculture, the hill slopes are terraced and planted with tea or citrus plants, the remaining area is covered by forest. The water of the river is also used by ca. 39 small hydropower stations.

As an example, we present the estimation of the effect of the power stations in the river valley. After the basic calibration, the modelled low flow of both models in winter was too low compared to the measured values. Possible causes were the terraces and the power stations. By implementing different storage strategies in SIMPEL we finally found out, that the best approximation could be achieved by a storage of about 300 mm with a constant release in the low discharge season. Finally, this strategy was implemented in SWAT and led to an increase of the Nash–Sutcliffe index from 0.27 to 0.75.

Compared to the normal calibration process, the use of SIMPEL as a test bed has shortened the time consuming calibration process of the SWAT model considerably.