Application of the SWAT (Soil and Water Assessment Tool) model in the Ronnea catchment of Sweden

The subject of this article is the estimation of quantitative (hydrological) and qualitative
parameters in the catchment of Ronnea (1800 Km2, located in south western Sweden)
through the application of the Soil and Water Assessment Tool (SWAT). SWAT is a river
basin model that was developed for the U.S.D.A. Agricultural Research Service, by the
Blackland Research Center in Texas. The SWAT model is a widely known tool that has
been used in several cases world-wide. It has the ability to predict the impact of land
management practices on water, sediment and agricultural chemical yield in large
complex watersheds. The present work investigates certain capabilities of the SWAT
model which have not identified up to now. More in specific, the main targets of the work
carried out are the following:
• Identification of the existing hydrological and qualitative conditions
• Preparation - Processing of data required to be used as input data of the model
• Hydrological calibration - validation of the model, in 7 subbasins of the Catchment
of Ronnea
• Estimation and evaluation of the simulated qualitative parameters of the model
All available data were offered by the relevant Institutes of Sweden, in the framework of
the European program EUROHARP. The existing conditions in the catchment of Ronnea,
are described in detail including topography, land uses, soil types, pollution sources,
agricultural management practices, precipitation, temperature, wind speed, humidity,
solar radiation as well as observed discharges and Nitrogen and Phosphorus substances
concentrations. Most of the above data were used as input data for the application of
SWAT model. Adequate methods were also used to complete missing values in time
series and estimate additional parameters (such as soil parameters) required by the
model.
Hydrological calibration and validation took place for each outlet of the 7 subbasins of
Ronnea catchment in an annual, monthly and daily step. The calibration was achieved by
estimating parameters related to ground water movement and evaluating convergence
between simulated and observed discharges by using mainly the Nash & Sutcliffe
coefficient (NTD). Through the sensitivity analysis, main parameters of the hydrological
simulation, were detected. According to the outputs of the SWAT model, the water
balance of Ronnea catchment was also estimated.
Hydrological calibration and validation is generally considered sufficient in an annual and
monthly step. Hydrological calibration – validation in daily step, generally does not lead to
high values of the NTD indicator. However, when compared to results obtained by the
use of SWAT in Greece, a relatively high value of NTD is achieved in one subbasin.
Finally, a comparison between the simulated and observed concentrations of total
Phosphorus and Nitrogen was carried out.