Integrated Sediment and Wetland Management for Climate Adaptation in the Ebro Delta, Spain

The interventions combined sediment, water and habitat management across rice fields and constructed wetlands to address land elevation loss, coastal erosion, water pollution and greenhouse gas emissions. The approach was designed around two core natural processes: (i) rebuilding delta elevation through vertical accretion by adding inorganic sediments and increasing organic matter accumulation; and (ii) reducing emissions by managing soil aeration and organic matter decomposition in flooded rice systems.

Two pilot pathways were tested to reintroduce sediments that are currently trapped upstream or removed during water treatment.

1. Reinjecting sediments from the CAT water purification plant into the irrigation network
Sediments removed at the Tarragona Water Consortium (CAT) water purification plant (around 1,000 tonnes per year) were treated as a resource rather than waste. A pilot test demonstrated the feasibility of routing these sediments back into the Ebro Delta via the existing irrigation canal network, so they could be deposited on the delta plain, including rice fields, contributing to soil fertility and land elevation. A key implementation output was CAT’s development of an engineering project to set up a permanent return system to deliver these sediments into the irrigation canals.

2. Injecting sediments into the lower Ebro River to enable downstream transport to the delta
A second pilot tested whether the Ebro River, under current hydrological conditions, can transport injected sediments from the lower river towards the delta plain. This included sediment injection tests and model simulations to quantify the amount and type of sediment that could be transported under different flows, and to calibrate theoretical water and sediment transport models. The work was intended to inform the feasibility and design of a future sediment transfer system from reservoirs to the river, including potential sediment by-pass through the Riba-roja reservoir. Results indicated that sediment loads reaching the river mouth could be significantly increased, by up to around 1 million tonnes/year, particularly under medium to high flow conditions. Simulations referenced controlled flood conditions (e.g., 1,000 m3/s over 10 days) as a basis for estimating transport potential.

Two wetlands were set up over three consecutive years to maximise nutrient and pollutant removal while also promoting carbon sequestration and soil elevation gains. Implementation focused on operational parameters shown to control performance: inflow contaminant load and pH were identified as key drivers of removal efficiency; lower water depth was associated with higher nitrogen removal; and higher vegetation density with higher phosphorus removal. Monitoring also showed that gains in carbon sequestration and soil elevation were greatest in the first wetland cells, where suspended sediments and organic matter preferentially accumulated. Based on these findings, the project concluded that larger volumes of water could be diverted through the wetlands without losing high removal efficiency.

Pilot actions in rice fields tested water-management measures to reduce methane emissions, particularly the Alternate Wetting and Drying (AWD) approach (and also Mid-Season Drainage). The intervention logic was to introduce planned soil aeration periods to suppress methane formation while maintaining production. The pilots showed that large methane reductions were achievable (up to 90%), but also identified a practical constraint: if soil salinity rises too much, yields can fall. This led to the definition of “safety rules” for farmers when applying AWD, to avoid production penalties. The project also assessed seasonal emission patterns to target management: methane emissions peaked during the post-harvest period, and incorporating rice straw after harvest was identified as a dominant contributor to annual emissions (up to 70%); delaying straw incorporation was found to reduce emissions.

In parallel, experimental plots were used to test whether adding sediments from the CAT water purification plant affected rice production or greenhouse gas emissions. The pilot found no negative effects on rice productivity or quality, and no significant changes in greenhouse gas emissions, supporting the practical feasibility of using these sediments in rice-field management.

To guide where adaptation actions should be prioritised, the project produced a high-resolution subsidence susceptibility map using satellite and geological methods. The mapping identified the most vulnerable areas to subsidence and sea-level rise (with reported subsidence rates of 1-2 mm/year, highest near the river mouth). This tool was intended for territorial managers to prioritise areas for measures that reduce flooding and salt intrusion risks.

The pilot results were consolidated into a Climate Action Plan document (“Actions for Climate in the Ebro Delta”), integrating measures proposed in the Catalan Strategy for Climate Change Adaptation with those tested in the pilots. The plan was developed through an extensive stakeholder consultation process involving the rice sector, irrigation communities and nature conservation NGOs, with an emphasis on coordination between administrations and socio-economic actors.

Implementation relied on a partnership combining research, public administration and sector actors. IRTA coordinated this partnership, with key roles for ACA (water agency), CAT (water consortium and treatment plant operator), CRSAE (irrigators and agricultural community), ICGC (mapping), OCCC (climate policy interface) and UCO (university). Farmers, irrigation communities and public administrations were explicitly involved as core stakeholders, particularly for rice-field practice changes and the use of irrigation infrastructure for sediment redistribution.

Key constraints identified during the implementation of the project included uncertainty over sediment transport capacity under present-day river flows (addressed through injection tests, river/canal transport capacity assessments and model calibration); risk of rice yield penalties linked to salinity under AWD (addressed by defining the need for farmer “safety rules” in applying AWD); dependence of wetland pollutant removal on operational conditions (addressed through multi-year optimisation of water depth, inflow characteristics and vegetation density, and by identifying where within wetland cells accumulation and sequestration are greatest).