Development of an agricultural drain detection method: hydraulic issue of the Saclay plateau

Funded under the 2017 "Innovation" Call for Projects, the DRAIN-ACT project took place over 2 years (2017-2019).

Project leader: Julien TOURNEBIZE (UR HYCAR)

Partners

• in BASC: ECOSYS
• academic outside BASC: UMR METIS (Sorbonne University), Office national d'études et de recherches aérospatiales (ONERA)
• non-academic: Terre & Cité

The Plateau de Saclay is undergoing significant development as part of the Paris-Saclay Cluster. It is essential to ensure that these developments do not disturb the fragile hydraulic and hydrological balance, including agricultural drainage, which is an integral and patrimonial part of the Plateau's history. The objective of the DRAIN-ACT project was to develop a generic method for detecting drainage in the absence of historical plans for general application to the Saclay plateau.

The question of the knowledge of the hydrological functioning and, beyond that, of the functioning of the agricultural system of the Saclay Plateau, of which agricultural drainage is a historical component, contributes to better adapt the future developments of the Plateau for a sustainable activity. Links are to be structured with other BASC projects, for example ASSETS, and Dynamiques.

The documentary approach allows us to retranscribe the hydraulic history of the Plateau de Saclay, which began in 1663-1692, with a network for collecting water from the plateau by means of gutters in order to ensure a volume of water for Versailles. The law of 1856 followed, structuring on a national scale the works of agricultural drainage on a parcel basis. The archives have thus provided valuable information through the requests for tolerance of discharge of drained water into the drainage network until 1935 (49 in number). Surveys conducted by the association Terre et Cité, directly with local farmers, made it possible to gather the drainage plans of 8 farms (1884 to 2000, 998 ha). DDAF documents from 1991 and 1996 complete this archival panorama (total estimated on the whole territory at 2404ha drained). The cross-referencing with pedological data from the Saclay Plateau soil mapping project illustrates the correlation of soil type according to their degree of hydromorphy with the presence of drainage. Thus, on the Saclay plateau (3615ha), 1267ha were identified as drained at a date prior to the current new development work. 

Drainage network detection techniques are of two types:

1. the passive methods for which the detection is based on observation is made by visualization of the drain/interdrain gradients of one or more variables (moisture, temperature, yield,...) in connection with the normal functioning of a drained soil.
2. and active methods, for which the drain/interdrain contrasts are artificially accentuated.

Passive methods include remote sensing image analysis in different spectral ranges, geophysical methods based on ground penetrating radar, seismic, tomographic resistivity, magnetism and electromagnetism. They have not shown any generic potential for clayey-silt soils such as those found on the plateau. The method based on thermography as for the visible is potentially exploitable by airborne system, under certain conditions specified in the studies conducted (height of mowing, threshold of detectable degree, adapted periods). In visible light, 40% of drained surfaces have been identified by this method.

The active method consists in generating a contrast of water content variables artificially either by water or air injection from upstream. These two techniques developed within the framework of the DRAIN-ACT project have been successful, with the exception of the operability of the water injection method (which is considered unfeasible in situ). The air injection method, which creates desaturation around the drain during wet periods, seems more operational, provided that the pump is properly sized according to the size of the network to be explored (Hénine et al., in progress).

===>The project leader explains the project and its RESULS in VIDEO  (LabEx scientific days, December, 2020)