Comparative study about farming practicals impacts on water quality in the Aunis waterground catchment area using the two SYST’N® and STICS© models.

Jean-Philippe BERNARD – Chamber of agriculture of la -Maritime, Head of programme Innovation –

Keywords : Nitrates leaching, Nitrogen management, SYSTN, STICS, catchment area.

The three catchment areas – « Varaize, « Fraise » and « Anais » - in Aunis land, whose main crops products are cereals, oilseeds and protein crops, provide a fraction of water resource for La Rochelle city. This important resource is the subject of a contaminants monitoring program, including agri- cultural nitrates.

Since more than ten years, La Rochelle city and the Chamber of agriculture of the Charente- Maritime build action plans to support farmers of Aunis to avoid nitrogen leaching. No- wadays, we need to give the partners of this operation more precise informations to improve farming practices in relation with water quality. Therefore, it was decided to use together two modeling tools, SYST’N® et STICS©, to get an overview of nitrogen flows between soil, crops and water throughout the crop develop- ment cycle

Situation of catchment areas of La Rochelle city Source : Géosignal –City of La Rochelle, 2016

Aunis land is an area of calcaric soils with pebbles, more or less deeper, locally called "Groies". A analysis work made with GIS on maps covering the catchment areas from the IGCS library with scale to 1 : 100 000 or 1 : 200 000 (IGCS, 1995 and 2000) et 1/250 000ème (CAM, 2012) shows that: More than 90% of the agricultural area could be defined with only five kinds of soil of which four ones belong to calcaric soils family ; two soil profiles of the "calcaric cambisol" type, a calcaric cambisol of 60 cm deep and a calcaric cambisol of 40 cm deep cover more than 55% of agricultural area.

This first characterization work allowed us to describe baseline soil profiles to begin our modeling work since June 2017 with five farm cases .

Proposal for a soils map of catchment areas of La Rochelle city Source : SCAN 25®, IGN 2016 Comparative analysis of the STICS and SYST'N results with the example of cropping succession of an Aunis farm In a pedagogical way, we presented the SYST'N tool - 1.3 version - to farmers of the catchment areas with a real case of cropping succession on a plot - "Les Roches" - of a farm in the commune of BOUHET (17). We used the both most probable reference soil profiles and the climatological data of the Météo-France station of La Rochelle : the comparative presentation of the results for the two soil profiles enabled us to demonstrate the importance of the properties of the soil to understand the water and nitrogen flows. We also used STICS - version 8.41 of modeling engine - to confirm the SYST'N results and to specify the assessments of water and nitrogen fluxes.

Harvest results for plot "Les Roches" given by farmer and compared to results simulated with STICS

Data from farmers Calcaric cambisol 60 cm Calcaric cambisol 40 cm Succession Harvest year Yield Yield Yield Harvest date Harvest date Harvest date (q/ha) (q/ha) (q/ha) Previous crop : Winter wheat 2010 66 24/07/2009 60 04/07/2010 51 04/07/2009

Spring pea 2011 31 17/06/2011 49 17/06/2011 33 17/06/2011

Durum wheat 2012 81 12/07/2012 88 09/07/2012 69 09/07/2012

Sunflower 2013 27 16/09/2013 17 16/09/2013 18 14/09/2013 Winter wheat 2014 91 04/07/2014 87 01/07/2014 54 01/07/2014 Rapeseed 2015 32 03/07/2015 61 13/06/2015 51 14/06/2015 Winter wheat 2016 77 16/07/2016 93 10/07/2016 57 10/07/2016

The results given by the model are usually overestimated in relation to the real results, because the model does not take into account all the limiting factors. The production results obtained with the "Calcaric cambisol 60 cm" profile are closer to the field results than those of the "Calcaric cambisol 40 cm" profile.

Modeling of percolation and leaching by SYST'N for the two soil profiles : Calcaric cambisol – 60 cm (A) et Calcaric cambisol – 40 cm (B) A

B

The results show a fairly close rhythm of flux between the two soil profiles but the absolute leaching and percolation values are higher for the "calcaric cambisol 40 cm" profile. The most sensitive period according to the model corresponds to the late autumn, before development of a winter crop. The simulated values of percolation and leaching for the "Calcaric cambisol 60 cm" profile are comparable to those obtained after lysimetric researches on the ARVALIS station of Magneraud - SAINT PIERRE d'AMILLY (17) – made between 1984 and 1994, with percolation values from 60 to 355 mm and leaching values from 15 to 70 kg N / ha per cropping season (BOUTHIER and BONNIFET, 1996).

Modeling of percolation and leaching by STICS for the two soil profiles : Calcaric cambisol – 60 cm and Calcaric cambisol – 40 cm

STICS allows to get daily values. They were summed by month in order to bring more informations than SYST'N results: Percolation in "Calcaric cambisol 40 cm" profile begins earlier but from saturation in water, the outflows are similar for the both profiles ; the monthly values of STICS show periods with greater leaching for the "Calcaric cambisol 60 cm" profile, which does not appear with the quarterly values SYST'N

The STICS use allow to make more accurate differences between behaviors of the both soil profiles.

Conclusion The use of the SYST'N model makes it possible to illustrate in a rather clear and pedagogical way the situations of percolation or leaching during a cultural succession However, the reliability of the results depends in particular on the (very) good knowledge of the biological and hydraulic characteristics of the soil. This is why field investiga- tions are necessary to ensure a sufficiently accurate description of the profile.

However, the use of the reference profiles obtained by the cartographic analysis, although less precise, will make it possible to look the correspondences between water and nitrogen flows and nitrate contents measured in the water at the scale of the catchment area.

Bibliography CARTE PEDOLOGIQUE DE FRANCE : FEUILLE LA-ROCHELLE - F15 - CARACTERISATION DES SOLS, échelle : 1/100 000, 1995 ; IGCS. CARTE PEDOLOGIQUE DE FRANCE : FEUILLE ST-JEAN-D'ANGELY - G15 - CARACTERISATION DES SOLS, échelle : 1/100 000, 2000; IGCS. CAM C., 2012. Livret simplifié de la carte des pédopaysages de la Charente-Maritime . Document de la Chambre régionale d’agriculture de -. BOUTHIER A. et BONNIFET J.P., 1996. Lysimètres du Magneraud : bilan de 15 années de suivi in Trente ans de lysimétrie en France 1960 - 1990. Collec. « Un point sur … ». Ed. INRA, COMIFER.