Modelling sea level rise impacts and the management options for rice production: the ebro delta as an example

The climate is warming and coastal territories have to cope with increasing risks related to sea level rise (SLR). The world's coastal zones are threatened by the increase of mean and extreme sea levels in a variety of ways, such as flooding of shallow areas, coastal erosion, salinization, wetland loss, damage to property, and decreased agricultural and aquaculture production. Among coastal systems, deltas are thought to be highly vulnerable to SLR because of their low elevation, their land subsidence and their dense population. The Ebro Delta (NE Iberian Peninsula) is one of the most valuable coastal systems in the Western Mediterranean, with a total surface of 320 km2. The Ebro Delta is representative of the vulnerability of coastal areas to SLR, it is a low-lying area characterized by an elevation gradient from a maximum of about 5 m close to the river, down to the coastline, with about 50 % of the total surface below +0.5 m above mean sea level. The delta plain contains a number of ecosystems (e.g. coastal lagoons, sand spits, brackish marshes, and fresh water springs) that provide suitable habitats for a diverse and abundant wildlife, and give a high ecological value to the delta, which is protected as Natura 2000 site of the European Union and as Natural Park and Biosphere Reserve (UNESCO). Freshwater and nutrient inputs from the river allow the development of prosperous fishery and farming activities. Rice cultivation is the main economic activity in the region. Rice fields occupy most of the delta (ca. 210 km2, 65 % of the total surface) and are vulnerable to accelerated SLR and consequent increase in soil salinity, the most important physical factor affecting rice production. However, literature analysing the impacts of SLR in crops is very scarce, and existing studies only consider the potential damage by flooding but not by salt stress, which is the main remaining impact once adaptation measures to avoid inundation (i.e. coastal defences) are implemented. Thus, it is necessary to analyse the main impacts of SLR, mainly flooding and soil salinization, on the Ebro Delta, and to develop appropriate adaptation measures to preserve its ecological integrity and rice production under future SLR scenarios. Within this context, we developed different spatial models (resolution of 1 × 1 m) to predict the impacts of SLR on the Ebro Delta. The developed models include: the identification of areas prone to be flooded, sediment loss due to SLR, soil salinity and rice production loss to SLR. All models were built by coupling data from Geographic Information Systems with Generalized Linear Models and run under present conditions (Reference state) and under different scenarios predicted by the Fifth Assessment Report of the Intergovernmental Panel on Climate Change for 2025 and from 2030 to 2100 with 10-year steps. Finally, we evaluated the feasibility of an innovative nature-based adaptation measure consisting in reintroducing fluvial sediments (retained in the lower basin reservoirs) into the delta plain for balancing the effects of SLR. Different sets of data were obtained from digital imagery, cartography and field data for the lower Ebro River and its delta, and then used to derive new raster datasets. Rice production data were obtained from surveys among farmers. Our models showed that soil salinity was directly related to distances to the river, to the delta inner border, and to the river old mouth, while clay presence, winter river flow and surface elevation were inversely related to it. Surface elevation was the most important variable in explaining soil salinity. Rice production was negatively influenced by soil salinity, thus the models predicted a decrease from higher elevation zones close to the Ebro River to the shoreline. The model predicts a reduction in normalized rice production index (RPI) subject to SLR scenario considered. Therefore, the models predict a RPI reduction from 62.1 % in 2010 to 54.6 % by 2100 in the most conservative scenario (RCP 4.5, SLR = 0.53 m); and to 33.8 % by 2100 in the worst considered scenario (RCP 8.5 modified by Jevrejeva et al. 2014, SLR = 1.8 m), with a decrease of profit up to 300 € per hectare. For the same period and considered SLR scenarios, our flood models predict that between the 36 and the 90 % of the rice fields will be flooded and lost by 2100, under the most conservative and the worst case scenario, respectively, which means a sediment loss ranging between 122 and 418 million tonnes. Among the proposed methods to bypass and transport sediments trapped in the lower basin reservoirs to the delta plain, flushing floods were the most viable in economic terms. The proposed nature-based adaptation measure had a positive effect on rice production, therefore it can be considered as an innovative management option in contrast to classical engineering solution, for maintaining the Ebro Delta ecosystem services although SLR. Our models can be applied to other deltaic areas worldwide, and help rice farmers and stakeholders to identify the most vulnerable areas to SLR impacts, and to develop adequate management plans.