Application date as a controlling factor of pesticide transfers to surface water during runoff events Laurie Boithias a,b, ⁎ ,1 , Sabine Sauvage a,b , Raghavan Srinivasan c , Odile Leccia d , José-Miguel Sánchez-Pérez a,b, ⁎ a University of Toulouse, INPT, UPS, Laboratoire Ecologie Fonctionnelle et Environnement (EcoLab), Avenue de l'Agrobiopole, 31326 Castanet Tolosan Cedex, France b CNRS, EcoLab, 31326 Castanet Tolosan Cedex, France c Spatial Sciences Laboratory, Texas A&M University, College Station, TX 76502, USA d Irstea, Adbx Research Unit, 50 avenue de Verdun, 33612 Gazinet-Cestas Cedex, France abstract article info Article history: Received 2 September 2013 Received in revised form 30 January 2014 Accepted 18 March 2014 Available online 13 April 2014 Keywords: Application timing Sorption properties Metolachlor Aclonifen SWAT model Save river In agricultural watersheds, pesticide contamination in surface water mostly occurs during stormflow events. When modelling pesticide fate for risks assessment, the application timing input is one of the main uncertainty sources among all the parameters involved in the river network contaminations process. We therefore aimed to assess the sensitivity of the river network pesticide concentration patterns to application timing shifts within a plausible range of application dates, considering two pre-emergence herbicides (metolachlor and aclonifen) characterised by two different octanol/water partition coefficients (K ow ). The Soil and Water Assessment Tool (SWAT) was applied in the 1110 km 2 agricultural watershed of the river Save (south-western France), where wheat, maize, sorghum and sunflower are intensively grown. The pesticide application date was changed within a one-month interval and the pesticide concentration at catchment outlet was simulated from March to June 2010. Total metolachlor concentration prediction could be improved by an application timing shift to 3 days later (Daily R 2 = 0.22 and PBIAS = -57%). By testing the behaviour of the two molecules, it was shown that sorption processes were influencing the control of application timing on the transfer to surface water: metolachlor concentration in the channel depended on both discharge and delay between application date and first stormflow event whereas the transfer of aclonifen depended on rainfall intensity for exportation with suspended sediments through surface runoff. At last, the study discusses the potential implications of the sensi- tivity in terms of regional agricultural management practice design. © 2014 Elsevier B.V. All rights reserved. 1. Introduction The detrimental effect of intensive agriculture on surface water and groundwater quality has been shown by various authors (Burt, 2001; Cullum, 2009; Ulrich et al., 2013; Zalidis et al., 2002; Zeiger and Fohrer, 2009). The transfer of excessive pesticide loading from cultivat- ed land to surrounding surface water, either dissolved or sorbed onto particles, may be harmful to terrestrial and aquatic ecosystems (Martin et al., 2011; Niemi et al., 2009; Polard et al., 2011). The partition between both dissolved and particulate fractions controls the bioavail- ability of the chemical for living organisms' contamination. Pesticide ex- portations, from either point losses (e.g. through leaking tools) or diffuse sources (i.e. mostly through runoff and droplet drift) (Holvoet et al., 2005; Müller et al., 2003), may make stream water and groundwater unfit for human consumption. Drinking water quality European Maximum Permissible Level (MPL) is of 0.1 μgL -1 for an in- dividual pesticide concentration and 0.5 μgL -1 for all pesticide concen- tration (EC, 1998). Recent studies showed the role of one-off and intense events, such as floods, on water quality degradation regarding pesticides, including in the south-western France area (Boithias et al., 2011, 2014a; Taghavi et al., 2010, 2011). Intensity and timing of rain and irrigation were shown to be the main inducers of pesticide transfers (Chiovarou and Siewicki, 2008; Vryzas et al., 2009). Short-term (5-day) precipitation and antecedent soil water deficit were identified as the two most important explanatory variables for maximum pesticide con- centrations in drainflow (Lewan et al., 2009). Reichenberger et al. (2007) listed the shift of the pesticide application to an earlier or later date as an efficient mitigation strategy. Modelling studies corroborated observations for runoff incidence on pesticide exportation (Boithias et al., 2011; Chu and Mariño, 2004; Zhang and Zhang, 2011) and for ap- plication timing role at seasonal scale (Luo et al., 2008) and at rainfall event scale (Fohrer et al., 2014; Holvoet et al., 2005; Neitsch et al., 2002; Vazquez-Amabile et al., 2006). Dubus et al. (2003) highlighted the uncertainties inherent in pesticide fate modelling, including applica- tion timing, which depends on the farmer and varies from year to year Catena 119 (2014) 97–103 ⁎ Corresponding authors at: EcoLab, Avenue de l'Agrobiopole, 31326 Castanet Tolosan Cedex, France. Tel.: +33 5 34 32 39 20; fax: +33 5 34 32 39 01. E-mail addresses: l.boithias@gmail.com (L. Boithias), jose-miguel.sanchez-perez@univ-tlse3.fr (J.-M. Sánchez-Pérez). 1 Author's present address: Catalan Institute of Water Research, Emili Grahit 101, Scientific and Technological Park of the University of Girona, 17003 Girona, Spain http://dx.doi.org/10.1016/j.catena.2014.03.013 0341-8162/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Catena journal homepage: www.elsevier.com/locate/catena