Analyzing the sensitivity of viticultural practices to weather variability in a climate change perspective: an application to workable-day modelling

摘要

Aims: The study aims at applying to a perennial crop the concept of “workable days” developed for annual crops by G. Kreher in 1955. A workable day is a day that provides conditions agronomically correct for work.Methods and results: A survey among grapevine growers was carried out to collect their perception of practice sensitivity to weather variability. The study provided information on working periods, techniques and machinery involved, weather constraints and thresholds generally used by the vine growers, for 21 annual cropping practices. Pesticide spraying against powdery and downy mildew appeared to be the most sensitive to weather parameters. Therefore, this practice was selected for the purpose of modelling workable days, according to decision rules based on the postulate that all weather parameters should be within the acceptable ranges. The model was run over a period of 13 years (1998-2010). Analyses of the outputs pointed out a great variability in the number of workable days from one year to another as well as in the respective proportion of unfavourable weather factors involved in the reduction of workable days. A validation based on the analysis of the actual spraying practices realized over a period of 12 years (1999-2010) showed that 32 % of the treatments were done on a day characterized as not workable by the model. The errors could be attributed to periods of holidays and risky sanitary situations when decision rules had to be bypassed.Conclusion: The model proved to represent rather well the decision rules use by vine growers to carry out their annual cropping practices in relation with the weather variability, within the area of study. The controversial role of the ‘leaf wetness duration’ weather parameter needs to be clarified; it may help to better parametrize the model. In order to improve its performance, the model should be coupled to a disease propagation model.Significance and impact of the study: In a climate change perspective, modelling of workable days can be run with simulated climatic data. It may prove to be useful to adapt vineyard management strategies in terms of innovative cropping practices and equipment. IntroductionA crop management plan is made up of a series of agricultural practices that a farmer needs to carry out according to a schedule, determined by the plant’s cycle. Common practices include soil tillage, sowing, fertilization, disease control, and harvesting. For each of these practices, the farmer must try to aim for the most favourable timeslot so that he can expect the highest harvest potential. For instance in the case of an annual crop, if sowing is done a week too early or too late, harvest potential can be affected. However, within each timeslot (henceforth called work-period), the number of days that are actually suitable for work depends largely on weather conditions. Tilling, for example, should not be done if the soil is too wet as it would result in a top soil layer too compact for the next crop to be sown. Moreover, the weight of the tractor can cause deep soil compaction, which may jeopardize harvest potential for the following years.In 1955, Kreher formalized this notion under the concept of workable days (Kreher, 1955). A workable day, for a given practice, is a day that provides conditions that are agronomically correct for work. Workability criteria depend on the type of practice considered, since they are not all equally sensitive to weather conditions, and on the type of technique or machinery used (a workhorse does not damage a wet soil as a tractor does).Because of weather variability, the number of workable days for a given work-period varies from one year to the next. This implies that some years the farmer's workforce and machinery will be oversized and other years they will be undersized. Kreher thought that if a farmer could know the frequency of each type of situation, he would be able to adapt his workforce and machinery according to an objective level of risk. However, Kreher did not provide a detailed method to achieve this.Workable days were studied extensively during the 1960's - 1980's in France and in North America. Research focused mainly on soil tillage in arable crops. The first method for determining workable days was introduced by Reboul in the early 1960's (Reboul, 1964). It was based on the direct observation of the farmers' practices over long periods of time (10 years). He considered that “good” farmers (i.e., farmers producing high average yields) are well qualified to determine work-periods and to judge if conditions are agronomically correct. His method aroused much interest and became widely used in France, but its length (10 years) and lack of precision were major drawbacks (Reboul, 1985). Besides, each study was specific to a small area and could not be generalized to other areas with different soil types or climates.Reboul (1985) thought that if a relation could be found between workability and meteorolo