Estimating landscape-level water storage potential as a tool to mitigate floods and nutrient losses

Extreme weather events, which are becoming more frequent due to climate change, result in intensified nutrient losses and may override effects of agricultural mitigation measures. Using a landscape connectivity approach, we study the potential of water flow attenuation in upstream forest areas to prevent or mitigate waterlogging and flooding of downstream arable land, thereby contributing to reduced phosphorus (P) losses. We use soil distribution maps, high-resolution elevation data, land use maps and distributed modelling to quantify water storage potential and possible P reductions. In three out of four study catchments (119-915 km²), calculated storage potential of detention basins located in upstream wetland and forest areas was sufficient to retain water volumes (0.7-3.6 million m³) corresponding to discharge volumes produced during an episode with a 50 -year return period. Furthermore, it was estimated that 9-57 % of targeted annual P load reductions from arable land could be reached if stored water volumes bypass arable land without causing waterlogging/flooding leading to P mobilization and transport. As water storage potential is site-specific, prioritization and selection criteria need to be developed in collaboration with relevant stakeholders to achieve cost-efficient implementation of these measures. The methodology and results of this study have significant potential for application in landscapes with a mixture of forests and arable agriculture to aid land owners and managers to secure food production and improve water quality.