Accounting for temporal variation in soil hydrological properties when simulating surface runoff on tilled plots Nane ´e Chahinian, Roger Moussa * , Patrick Andrieux, Marc Voltz Laboratoire d’Etude des Interactions Sol–Agrosyste `me–Hydrosyste `me, Institut National de la Recherche Agronomique, U.M.R. LISAH Agro.M/INRA/IRD, 2 Place Pierre Viala, 34060 Montpellier Cedex 1, France Received 7 July 2004; revised 24 October 2005; accepted 25 October 2005 Abstract Tillage operations are known to greatly influence local overland flow, infiltration and depressional storage by altering soil hydraulic properties and soil surface roughness. The calibration of runoff models for tilled fields is not identical to that of untilled fields, as it has to take into consideration the temporal variability of parameters due to the transient nature of surface crusts. In this paper, we seek the application of a rainfall-runoff model and the development of a calibration methodology to take into account the impact of tillage on overland flow simulation at the scale of a tilled plot (3240 m 2 ) located in southern France. The selected model couples the (Morel-Seytoux, H.J., 1978. Derivation of equations for variable rainfall infiltration. Water Resources Research. 14(4), 561-568). Infiltration equation to a transfer function based on the diffusive wave equation. The parameters to be calibrated are the hydraulic conductivity at natural saturation K s , the surface detention S d and the lag time u.A two-step calibration procedure is presented. First, eleven rainfall-runoff events are calibrated individually and the variability of the calibrated parameters are analysed. The individually calibrated K s values decrease monotonously according to the total amount of rainfall since tillage. No clear relationship is observed between the two parameters S d and u, and the date of tillage. However, the lag time u increases inversely with the peakflow of the events. Fairly good agreement is observed between the simulated and measured hydrographs of the calibration set. Simple mathematical laws describing the evolution of K s and u are selected, while S d is considered constant. The second step involves the collective calibration of the law of evolution of each parameter on the whole calibration set. This procedure is calibrated on 11 events and validated on ten runoff inducing and four non-runoff inducing rainfall events. The suggested calibration methodology seems robust and can be transposed to other gauged sites. q 2005 Elsevier B.V. All rights reserved. Keywords: Runoff model; Flood event; Infiltration; Plot scale; Calibration; Tillage; Stormflow generation; Farmed catchments 1. Introduction Farming operations greatly influence local surface runoff, infiltration and surface storage by altering soil hydraulic properties and soil surface roughness (e.g. Larson, 1964; Mwendera and Feyen, 1993, 1994; Journal of Hydrology 326 (2006) 135–152 www.elsevier.com/locate/jhydrol 0022-1694/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2005.10.038 * Corresponding author. Tel.: C33 4 99 61 24 56; fax: C33 4 67 63 26 14. E-mail address: moussa@ensam.inra.fr (R. Moussa).