Modelling hydrology and water quality in the pre-alpine/alpine Thur watershed using SWAT
Introduction
During the last three decades, in Switzerland, as well as in other European countries, extensive and costly measures have been taken to reduce pollution input by point sources. The measures included installation of advanced waste water treatment and regulations for restricted use of phosphorous (phosphate ban in household detergents) and toxic substances. In 1972, the “National Long-term Monitoring of Swiss Rivers” (NADUF) program was initiated as a cooperative project between what is now the Swiss Federal Office for the Environment (FOEN), and the Swiss Federal Institute of Aquatic Science and Technology (Eawag) (Binderheim-Bankay et al., 2000) (www.naduf.ch). The program monitored the chemical and physical states of Swiss rivers to evaluate the effectiveness of water protection measures undertaken by Swiss national environmental protection agencies. Several NADUF catchments and stations, including the one investigated in this study – the pre-alpine/alpine Thur River basin, which represents a low level of anthropogenic pollution – serve as reference stations for other international water monitoring programs, e.g., International Commission for the Protection of the Rhine (IKSR), the Global Environmental Monitoring System – United Nations Environmental Program/World Health Organization (GEMS-UNEP/WHO) (Jakob et al., 2002). After changing the water protection law in the late 1980s to early 1990s, the positive effects of these measures were reported at several monitoring stations. In the Swiss part of the Rhine watershed, the international target of 50% reduction in the total inputs into surface waters of P and N was achieved for P (reduction of 51%) but not for N (reduction of 23%) (Prasuhn and Sieber, 2005). Also, lead concentration decreased by 80–90% during the same time period. Furthermore, with the adoption of a new “ecologically oriented” agricultural management in 1993, which included animal friendly farming, balanced use of fertilizers, appropriate proportions of ecological compensation areas, suitable crop rotation, soil erosion protection, and measured use of pesticides – decreasing trends of nutrients in Swiss water bodies were reported as well (Jakob et al., 2002, SAEFL, 2002). The total phosphorous and nitrogen concentrations decreased significantly by 28% and 14%, respectively, from 1985 to 2001 (Prasuhn and Sieber, 2005). However, the problem of non-point source pollution still exists and is associated primarily with the agricultural applications of mineral (ammonium, nitrate) and organic (liquid and solid manure) fertilizers. It should be noted that the landuse change during the period of 1980–1995 has been quite insignificant in the Thur region as indicated by the first (from 1979 to 1985) and the second (from 1992 to 1995) landuse maps complied by the Swiss Federal Statistical Office (www.bfs.admin.ch). The latest landuse map shown in Fig. 1 indicates a predominantly agricultural region.
Surface runoff, especially immediately after a storm, is an important medium of transport for non-point source pollution. Runoff from different landuses may be enriched with different kinds of contaminants. For example, runoff from agricultural lands is generally enriched with sediments, nutrients and pesticides, whereas runoff from actively developed urban areas contains heavy metals, hydrocarbons, chloride and other contaminants (Huber, 1993). Due to the significant reduction in the loads from point sources in the past years, the relative significance of diffuse sources of pollution in Swiss waters has increased. Presently in Switzerland, wash-out and runoff from agricultural lands contributes to a greater extent to the impairment of natural waters than it was a few decades ago (Prasuhn and Sieber, 2005).
Inverse modelling (IM) has in recent years become a very popular method for calibration (e.g., Beven and Binley, 1992, Abbaspour et al., 1997, Simunek et al., 1999, Duan et al., 2003, Gupta et al., 2003, Wang et al., 2003). IM is concerned with the problem of making inferences about physical systems from measured output variables of the model (e.g., river discharge, sediment concentration). This is attractive because direct measurement of parameters describing the physical system is time consuming, costly, tedious, and often has limited applicability. Because nearly all measurements are subject to some uncertainty, the inferences are usually statistical in nature. Furthermore, because one can only measure a limited number of (noisy) data and because physical systems are usually modelled by continuum equations, no hydrological inverse problem is really uniquely solvable. In other words, if there is a single model that fits the measurements there will be many of them. Our goal in inverse modelling is then to characterize the set of models, mainly through assigning distributions (uncertainties) to the parameters that fit the data and satisfy our presumptions as well as other prior information.
To make the parameter inferences quantitative, one must consider: (1) the error in the measured data (driving variables such as rainfall and temperature), (2) the error in the measured output variables (e.g., river discharges and sediment concentrations used for calibration), and (3) the error in the conceptual mode (inclusion of all the physics in the model that contributes significantly to the data).
The objective of this research study was to evaluate the application of a mechanistic modelling approach as a complementary technique to the monitoring program in investigating the relative impacts of different types of landuse and agricultural managements on water quality and quantity of the Thur River. A number of simulators such as SWAT (Soil Water Assessment Tool) (Arnold et al., 1998), HSPF (Hydrologic Simulation Program Fortran) (Bicknell et al., 1996), and SHETRAN (Ewen et al., 2000) could have been used in this study. Several comparisons of these models indicated similarly reasonable results in simulating discharge, phosphorous, and sediment (e.g., Singh et al., 2005, Borah and Bera, 2004). We chose SWAT because of its availability and user-friendliness in handling input data. SWAT was evaluated by performing calibration and uncertainty analysis using SUFI-2 (sequential uncertainty fitting ver. 2) algorithm (Abbaspour et al., 2004), which is a semi-automated inverse modelling procedure for a combined calibration-uncertainty analysis. The available time series data on discharge, sediment, nitrate, and total phosphorus loads at the watershed outlet as well as some constraints on sediment and nutrients from different landuses were used to perform calibration and validation studies.
Section snippets
Description of the study site
The Thur watershed with an area of 1700 km2 is situated in north-eastern Switzerland near the border with Germany (Fig. 2). The main river (Thur) has a total length of 127 km. The major tributaries to this river are Murg, Glatt and Sitter rivers. Mean elevation of the watershed is about 774 m above sea level and mean slope is around 7.5°. The lowest point is located at Andelfingen gauging station at 356 m above sea level and the highest point is the Saentis at 2500 m above sea level. Close to 75% of
Results and discussion
Calibration of models at a watershed scale is a challenging task because of the possible uncertainties that may exist in the form of process simplification, processes not accounted for by the model, and processes in the watershed that are unknown to the modeller. Some examples of the above mentioned model uncertainties are: effects of wetlands and reservoirs on hydrology and chemical transport; interaction between surface and groundwater; occurrences of landslides, and large constructions
Conclusions
Given the complexities of a watershed and the large number of interactive processes taking place simultaneously and consecutively at different times and places within a watershed, it is quite remarkable that the simulated results comply with the measurements to the degree that they do. Based on the results obtained in this study, SWAT is assessed to be a reasonable model to use for water quality and water quantity studies in the Thur watershed. On that positive note, however, a careful
Acknowledgments
This study is a part of the NADUF program and supported by the Swiss Federal Office for Environment (FOEN), the Swiss Federal Institute of Aquatic Science and Technology (Eawag), and MeteoSwiss. Special thanks to the graduate students at Eawag for their valuable contribution in collecting and processing environmental data.
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