Context and challenges

In southern Europe, climate change (CC) is leading to an increase in the demand for irrigation water for crops and, at the same time, a dwindling of the resource. Various levers have been implemented to save irrigation water, including improved water application technologies such as subsurface drip irrigation. A paradigm shift is to improve water efficiency (rainfall and irrigation) not only by optimising irrigation actions themselves, but through agro-ecological practices, such as conservation agriculture (CA). CA aims to preserve both soil quality and water resources and is based on (1) long and diversified crop rotations, (2) permanent soil cover and (3) simplified or zero tillage (direct seeding). It increases carbon storage in the soil and reduces CO2 emissions, thus contributing to CC mitigation.

In addition, under certain conditions, CA has a favourable impact on the chemical, physical and biological properties of soil and thus on the fate of water in agro-systems. It significantly increases the volume of water retained by the soil, which can reduce drainage losses. Infiltration capacities are generally improved and more stable over time, which reduces surface and sub-surface runoff. Soil evaporation is limited due to the mulch effect and rainwater use efficiency is improved. Thus, although very few references are currently available on the subject, CA could make it possible to limit water withdrawals for irrigation and thus appears to be a potential lever for adaptation to CC that merits a closer look.

Subsurface drip irrigation, whereby ducts are buried between 30 and 40 cm deep, is reputed to be a water-efficient technique but not widely used on French farms because of its costly installation and high level of technical skill it requires. In arable farming, this technique is only deployed on an experimental basis. Subsurface drip irrigation scores very well in terms of efficiency (90% on average) because it eliminates irrigation water losses due to direct evaporation (linked to solar radiation and high temperatures), drift (linked to wind) and evaporation from the soil surface. Subsurface drip irrigation is doubly beneficial in CA because it is not likely to be damaged by ploughing. Furthermore, it could limit weed development since the soil surface is not moist. However, it generates a specific water distribution in the soil that could influence the performance of a CA system.

Very few studies have been carried out on specific irrigation water flows and quantitative irrigation management in CA field crop systems, especially in the Mediterranean context, and even fewer in CA systems with subsurface drip irrigation. The following questions arise:

• Can we expect irrigation water savings in CA-subsurface drip irrigation systems compared to a traditional system (conventional technical itinerary and sprinkler irrigation) or to a CA system irrigated by sprinklers? If so, by how much and at what time scale (a cropping season, a year, a rotation)?
• What are the agronomic and environmental performances (in relation to the water resource) of CA-subsurface drip irrigation systems?
• Are these systems realistic from an economic and financial point of view (for the general interest and farmers respectively)?

Goals

The objective of the COMIC'EAU project is to study an innovative irrigated cropping system based on the combination of two complementary approaches to water saving: CA and subsurface drip irrigation. The aim is to evaluate the agro-environmental and economic performance of this combination in field crops, in order to see if it can reduce water input on a rotational scale, while achieving the same level of performance. In the framework of the present project, an initial state of the system will first be assessed, followed by another evaluation during the first two years after its implementation (i.e. the first two years of the transition phase). At the end of the project, the aim will be to continue evaluating the system until performance has stabilised.

The project targets the Mediterranean context because, on the one hand, it is already facing ever-dwindling water resources and, on the other hand, it foreshadows a large part of France when it comes to climate change.

INRAE units involved

• G-EAU, Montpellier
• AGIR, Toulouse

External partners : Arvalis, Lycée agricole de Valabre, Société du Canal de Provence (SCP)