General Information

TOXSWA is the acronym of TOXic substances in Surface WAters. It is a quasi-two-dimensional numerical model of pesticide behaviour in a small surface water system, incl. its sediment. It was developed by Alterra, in co-operation with W!SL, the Wageningen Software Labs, both located in Wageningen, the Netherlands.

The TOXSWA development team consists of (in alphabetical order):
Arjan de Jong (W!SL)
Erik Van den Berg (Alterra)
Mechteld ter Horst (Alterra)
Paulien Adriaanse (Alterra)
Wim Beltman (Alterra)

TOXSWA web site : (select products, models)

Short Model Description

Summary of process descriptions

TOXSWA describes the behaviour of pesticides in a water body at the edge-of-field scale, i.e. a ditch, pond or stream adjacent to a single field. It calculates pesticide concentrations in the water layer in horizontal direction only and in the sediment layer in both horizontal and vertical directions. TOXSWA considers four processes: (i) transport, (ii) transformation, (iii) sorption and (iv) volatilisation. In the water layer, pesticides are transported by advection and dispersion, while in the sediment, diffusion is included as well. The transformation rate covers the combined effects of hydrolysis, photolysis and biodegradation and it is a function of temperature. It does not simulate formation of metabolites. Sorption to suspended solids and to sediment is described by the Freundlich equation. Sorption to macrophytes is described by a linear sorption isotherm but this feature is not used for the FOCUS scenarios. Pesticides are transported across the water-sediment interface by diffusion and by advection (upward or downward seepage, zero for FOCUS scenarios). See also following figure.

FOCUS-TOXSWA 1.1.1 handles transient hydrology and pesticide fluxes resulting from surface runoff, erosion and drainage as well as instantaneous entries via spray drift deposition, as shown in following figure.

In order to simulate the flow dynamics in an edge-of-field water body in a realistic way, the field-scale system is defined as the downstream part of a small catchment basin. Water flow is described with the aid of a simple water balance, accounting for all major incoming and outgoing water fluxes. The water level is a function of time, but assumed to be constant in the water body system considered. Water levels are calculated either by assuming uniform flow conditions (Chézy-Manning equation), or by assuming a backwater curve in front of a weir.

TOXSWA uses an explicit finite-difference scheme to solve the water balance and mass conservation equations. Distances between the nodes in the water and sediment layers are in the order of magnitude of metres and millimetres, respectively.

Short description of the TOXSWA software package

TOXSWA simulates water flow and pesticide behaviour in small surface waters; water and pesticide entries into the water body system are calculated by other tools or models. Spray drift deposition numbers are calculated by the FOCUS Drift Calculator and put into TOXSWA input files by SWASH, the FOCUS Step 3 software shell. Drainage water and pesticide fluxes are read from a MACRO output file and runoff/erosion fluxes from a PRZM output file.

For Dutch purposes the TOXSWA User Interface offers access to the Dutch IMAG Drift Calculator. In the near future it will be coupled to the Dutch PEARL model for drainage entries.

The TOXSWA User Interface was developed as a user-friendly environment for running the FOCUS scenarios. It is an integrated environment for data storage, data retrieval, model control and viewing of output data.

The user interface is linked to the SWASH/TOXSWA database, a relational database for easy data access. From there it receives project, run, pesticide properties and loadings data. It generates the two TOXSWA input files, writes the path and name to the lateral entry input file and calls the TOXSWA model. Succint summary output can be accessed from the user interface, as well as standard graphs showing more comprehensive model outputs. The FOCUS input is stored in the SWASH/TOXSWA database in a locked way, so that users cannot change the scenario data.

Life story of TOXSWA model

The TOXSWA model was released in 1996 and TOXSWA version 1.2 is applied in the Dutch registration procedure since July 1999. Version 1.2 can only handle constant water depths and discharges and therefore, it is only suited for simulating spray drift entries in a water body system. FOCUS-TOXSWA 1.1.1 was developed in view of the FOCUS Surface Water Scenarios and this version is able to simulate varying water levels and discharges, that occur when drainage or runoff fluxes enter the water body system.

It is expected that the constant flow option as implemented in the TOXSWA 1.2 will become operational in FOCUS-TOXSWA 1.1.1 in the near future and so, that TOXSWA 1.2 and FOCUS-TOXSWA1.1.1 will merge into one version.

Role and Place in FOCUS scenarios calculation

TOXSWA is used for pesticide exposure assessment in the EU evaluation process. It is the surface water fate model, used in Step 3 FOCUS calculations. It calculates exposure in water and in sediment at the downstream end of a ditch, stream or pond neighbouring a treated field in the 10 FOCUS Surface Water Scenarios. TOXSWA runs should be set up via the SWASH shell for all scenario and water body combinations. After having finalisedthe MACRO and PRZM simulations, the TOXSWA runs can be done.

Current use outside FOCUS

TOXSWA 1.2 is used in Dutch pesticide registration for first and higher tier assessments. Higher tier assessments include the interpretation of field studies for pesticide registration as well as the interpretation of water-sediment studies to determine transformation rates in water and in sediment. TOXSWA 1.2 can be downloaded from the TOXSWA site of the Alterra website: (select products, models)

Excerpt of existing model documentation

  • Adriaanse, P.I., 1996. Fate of pesticides in field ditches: the TOXSWA simulation model. SC-DLO Report 90, DLO Winand Staring Centre for Integrated Land, Soil and Water Research, Wageningen, the Netherlands, 241 pp.
  • Adriaanse, P.I., 1997. Exposure assessment of pesticides in field ditches: the TOXSWA model. Extended summary SCI Pesticide Group Meeting Ecotoxicology of Organic Compounds in the Aquatic Environment. Pestic. Sci. 49, 210-212.
  • Adriaanse, P.I., 1997. A proposed policy for differentiated hazard evaluation of pesticides in surface waters. Exposure concentrations simulated by TOXSWA and ecotoxicological hazards of pesticides in field ditches and main watercourses. SC-DLO Report 141, DLO Winand Staring Centre for Integrated Land, Soil and Water Research, Wageningen, the Netherlands, 303 pp.
  • Adriaanse, P.I., J.P.M. Vink, W.W.M. Brouwer, M. Leistra, J.W. Tas, J.B.H.J. Linders and J.W. Pol, 2002. Estimating transformation rates of pesticides, to be used in the TOXSWA model, from standardized water-sediment studies. Alterra Report 23, Alterra, Wageningen, the Netherlands, 129 pp.
  • Beltman, W.H.J.and P.I. Adriaanse, 1999. User's manual TOXSWA 1.2. Simulation of pesticide fate in small surface waters. SC-DLO Technical Document 54, DLO Winand Staring Centre for Integrated Land, Soil and Water Research, Wageningen, the Netherlands, 112 pp.
  • Beltman, W.H.J.and P.I. Adriaanse. 1999. Proposed standard scenarios for a surface water model in the Dutch authorization procedure of pesticides. Method to define standard scenarios determining exposure concentrations simulated by the TOXSWA model. SC-DLO report 161, DLO Winand Staring Centre for Integrated Land, Soil and Water Research, Wageningen, the Netherlands, 90 pp.
  • Horst, M.M.S. ter, P.I. Adriaanse, W.H.J. Beltman and F. Van den Berg, 2003. User Manual of TOXSWA, FOCUS version 1.1.1. Alterra Report 586, Alterra, Wageningen, the Netherlands.
  • Westein, E., M.J.W. Jansen, P.I. Adriaanse and W.H.J. Beltman, 1998. Sensitivity analysis of the TOXSWA model simulating fate of pesticides in surface waters. SC-DLO report 154, DLO Winand Staring Centre for Integrated Land, Soil and Water Research, Wageningen, the Netherlands, 119 pp. 

Download TOXSWA

Download TOXSWA

Latest version 4.4.3

Package (22/May2015, 1 file )
Getting Started You need to install SWASH before TOXSWA. The default directory for TOXSWA is C:\SWASH\TOXSWA. In case you selected  another drive than C for SWASH,
                                 the TOXSWA application should also be installed on that drive on subdirectory of the SWASH directory. For example if you have installed SWASH on
                                 D:\SWASH then TOXSWA should be installed at:
                                 -    D:\SWASH\TOXSWA

22/May/2015:The tool facilitates the calculation of metabolite PEC as described in  Generic guidance for Surface Water Scenarios, version 1.4 in addition to facilitating the active substance PEC calculations.

04/Aug/2015: In the summary file, section “Substance properties and substance loadings”, the values given for “Metabolite formed in water” and for “Metabolite formed in sediment” are not correct. These values are specified in the input file as molar fractions formed. TOXSWA converts these to mass fractions and uses these for the calculation, which is correct. However, in the summary file the formation fractions are presented as mass fractions formed so do not correspond to the value in the input file.  The mass fraction formed can be calculated back to molar fraction formed by multiplying the mass fraction formed by the quotient molar mass parent / molar mass metabolite.

30/Nov/2015 Warning: Viewing report file. The FOCUS_TOXSWA shell gives an error when after finalising a run its report is requested for the crops 'pome/stone fruit, early applns',  'pome/stone fruit, late appln' and 'grass/alfalfa'. The forward slash in the crop name is misinterpreted by the html editor that makes the report. For these three crops the results of the run have to be viewed in the summary file, which can be obtained via the menu bar via 'View' and 'Summary file'.

03/Feb/2016: Clarification of the types of concentrations contained in the FOCUS_TOXSWA 4.4.3 output files Runid.SUM and Runid.OUT which are different to earlier versions of TOXSWA. Click here for the details.

25/Jul/2016: Six ‘bugs’ have been identified in FOCUS_TOXSWA version 4.4.3. Click here for the details. The bugs will be repaired in the next version of FOCUS_TOXSWA

Previous : FOCUS_TOXSWA_3.3.1

Package (24/Nov/2009, 1 file, 45,1 MB)
Test Results  
Getting Started with TOXSWA_3.3.1(doc)
24/Nov/2009 :
Differences between TOXSWA_3.3.1 and TOXSWA_2.2.1: Differences document).

Older versions: FOCUS_TOXSWA

Package (21/Dec/2005, 1 file, 30 MB)
Test Results HTML, Word
Getting Started with FOCUS_TOXSWA_2.2.1(doc)
30/Giu/2008 Warning:
TOXSWA stops with error message: "Substance name in m2t or p2t file not equal to substance name in TOXSWA txw file".

FOCUS_TOXSWA 2.2.1 stops at the start of a metabolite run when the metabolite run is part of the first set of runs in a project (parent and metabolite), with e.g. runID 000009, and the preceding run is the last run of another project with a runID 1 number lower, e . g. runID 000008. The error only occurs for drainage scenarios , and only if the two runs (i.e. the run from the current project with runID 000009 and the preceding project with runID 000008 in the given example) are for the same location and for the same waterbody type. The error message shown is "Substance name in m2t or p2t file not equal to substance name in TOXSWA txw file". However, the m2t file is correct. But in the TOXSWA input file the wrong substance is selected. A work around is to copy the project in the TOXSWA UI, next select the correct substance in the run and then continue according to the usual procedure.

19/Dec/2005 :
Differences between TOXSWA_2.2.1 and TOXSWA_1.1.1: Bugs of TOXSWA_1.1.1 have been repaired in TOXSWA_2.2.1, and some functionalities have been added (See the Differences document).

19/Dec/2005 Warning :
(1) Zero application rates cannot be handled by TOXSWA. When a zero application rate is entered, the simulation starts, showing the DOS-box shortly, but stops then. The error file does not report the cause of the failure. When TOXSWA is executed from DOS, "error <65>: floating invalid" is shown. The lowest non-zero application rate that the TOXSWA GUI accepts is 0.1 g/ha. It is advised to accept 0.1 g/ha as a proxy to zero (which should usually be close enough for aquatic risk assessments).

26/Jan/2006 Warning:
(2) Non-zero initial concentrations in the water layer cannot be handled by TOXSWA 2.2.1 in FOCUS Step 4 runs. When a non-zero concentration has been entered, TOXSWA crashes and reports a mass balance error. Work-around: Transform the wished initial concentration in the water layer into a corresponding spray drift loading. Add the spray drift loading to the TOXSWA input file (Section 4: increase number of loadings, 'ntldsd', and insert as first loading with dummy date, 'chatldsd', dummy application rate, 'applot', and spray drift loading, 'mldsd'). Add the loading also in the header of the m2t or p2t input file (increase number of applications, and insert first application timed at the first day of the simulation, with dummy application rate) to enable TOXSWA to check and approve the correspondence between the txw and m2t or p2t file. Please take notice that spray drift loadings are default set at 9 a.m. in FOCUS runs, so the time of mass loading will not equal the start of the simulation at midnight (0:00 h). This bug will be repaired in the next release.

FOCUS_TOXSWA_2.2.1 (22/Mar/2007) Warning:
FOCUS_TOXSWA_2.2.1 may crash for runs with relatively low pesticde mass inputs and substances having Freundlich exponents of 0.7 (or close to 0.7). The run then stops stating that more than 1% of the mass is missing in the mass balance of the sediment. A work around is to multiply the loadings with a factor of 10, and divide the resulting PECs by 10. This gives an overestimation of the PECs for the water layer, and an underestimation for the PECs in sediment. It is anticipated that this bug will be solved in the next release.

Package (08/May/2003, 1 file, 14.2 MB)
Test Results HTML, Word
Getting Started with FOCUS_TOXSWA_1.1.1
08/May/2003 : There are a number of known bugs in FOCUS_TOXSWA_1.1.1 : read this note 1

08/July/2003 : Another bug was found in FOCUS_TOXSWA_1.1.1: read note 2

24/Nov/2003 : There is a serious problem with TOXSWA runs for metabolites for D scenarios: read note 3

05/Dec/2003 : Serious bug of 24/Nov/03 in released version of FOCUS_TOXSWA_1.1.1 is now completely described (point 3 added) and a work-around is proposed in note 4

04/Jan/2004 : Message ‘I/O Error 103’ pops up when clicking the ‘Calculate’ button to start the TOXSWA calculations. Hereafter the project is locked : read note 5

19/May/2004 : Update on FOCUS_TOXSWA_1.1.1 Note 5 concerning the Message ‘I/O Error 103’ : note 6

06/Jun/2005 : Updated list of all known bugs for FOCUS_TOXSWA_1.1.1

In this compilation all known bugs described above at this website have been merged and the description sometimes has been slightly changed or improved.

Moreover a number of new bugs have been added. These are numbered from 8 to 13. Your attention is drawn to bug number 11 : This is the only bug (of the 6 new bugs) that may occur without noticing by the user, because there is no crash or error message and still causes erroneous results.

It is expected that this will be the final situation concerning the 1.1.1 version: FOCUS_TOXSWA_2.2.1 is in preparation for official release ; in this new version all known bugs of TOXSA have been repaired.

21/Dec/2005 Updated list 2 of all known bugs for FOCUS_TOXSWA_1.1.1

In this compilation all known bugs described at this website above have been merged and the description has sometimes been slightly changed or improved. Moreover a number of other bugs have been added. These are numbered from 8 to16. Your attention is drawn to bug number 11: This is the only bug (of the 9 new bugs) that may occur without noticing by FOCUS users, because there is no crash or error message and still causes erroneous results.

Compared to the Updated list of 06/Jun/2005 three new bugs have been added to the list, bug numbers 14 to16. Bug 14 is in fact the same bug as bug number 10, but now the consequences of this bug for the PECs have been quantified. Bug number 15 does not affect the calculations for the FOCUS waterbodies (all having rectangular wetted cross-sections). Bug 16 concerns the use of zero application rates.


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