Catchment Agronomy: Processes, Mitigation Measures and Indicators to Protect Water Quality

Keywords: Catchment, landscape, water management, nutrient management, water quality, nitrate.

Rural landscapes are heterogeneous mosaics reflecting interrelations between anthropogenic, technical and environmental processes. Diffuse water pollution due to agriculture is taking place in this mosaic composed of fields and natural areas exchanging water, particles and solutes up to the stream water network. An agricultural catchment is therefore composed of nutrient sources, sinks or bio-reactors. The resulting spatio-temporal patterns contribute to modulate the nutrient export by modifying the buffer capacity of the catchment, i.e. the ratio between nutrient output and input, by diluting and trapping them along the different flow pathways. A new field of knowledge and engineering is emerging, ‘catchment agronomy’, coupling farm and crop management within the catchment area, firstly as the functional level of the landscape hydrology, and secondly, as a level controlling nutrient emissions to stream water. The catchment can no longer be considered as a sum of nutrient plot and farm outputs, but as a level where nutrients can really be management- and nutrient input and output-regulated. The paper presents the basic concepts related to nutrient attenuation and some of the major mitigation measures at catchment level.

While many scientific studies show the influence of agricultural landscape patterns on the water cycle and water quality, only few of these have proposed scientifically-based and operational methods to improve water management. ‘Territ’eau’ is a framework developed to adapt agricultural landscapes to water quality protection, using components such as farmers’ fields, semi-natural areas or human infrastructures. The nitrogen (N) module is presented here. This enables the estimation of nitrate fate scores: i) at plot level, integrating them on the length of the crop rotation, taking into account environmental drivers (soil, climate) easily available by regional data sets; ii) at catchment level, identifying and qualifying functional semi-natural areas in terms of denitrification and dilution, and assessing their limits and functions, finally calculating nitrate fate scores per headwater catchment. This N module is integrated in a holistic method which allows us to objectivise the functions of the landscape components, and constitutes an established approach for adapting them to new environmental constraints. This framework helps in proposing different approaches for changing agricultural landscape, acting on agricultural practices or systems, and/or conserving or re-building semi-natural areas in controversial landscapes.

Chantal Gascuel-Odoux, INRA, UMR 1069, Sol Agro et hydroSystem, F-35000 Rennes, France and Agrocampus Ouest, UMR 1069, Sol Agro et hydroSystem, F-35000 Rennes.

Sylvie Guiet, Chambres d’agriculture de Bretagne, F-35000 Rennes. France.

Thierry Morvan and Françoise Vertès, INRA, UMR 1069, Sol Agro et hydroSystem, F-35000 Rennes, France and Agrocampus Ouest, UMR 1069, Sol Agro et hydroSystem, F-35000 Rennes.

23 pages, 6 figures, 58 references.