Optimizing the bioenergy water footprint by selecting SRC willow canopy phenotypes: regional scenario simulations

•Background and Aims Bioenergy is central for the future energy mix to mitigate climate change impacts; however, its intricate link with the water cycle calls for an evaluation of the Carbon-Water nexus in biomass production. The great challenge is to optimise trade-offs between carbon harvest and water use by choosing cultivars which combine low water use with a high productivity. •Methods Regional scenarios were simulated over a range of willow “genotype × environment” interactions for the major UK soil x climate variations with the process-based model LUCASS. Soil available water capacity (SAWC) ranged from 51 to 251 mm and weather represented the North-West (wet-cool), North-East (dry-cool), South-West (wet-warm) and South-East (dry warm) of the UK. Scenario simulations were evaluated for small/open narrow-leaf (NL) versus large/closed broad-leaf (BL) willow canopy phenotypes using “Baseline” (1965-89) and warmer “Recent” (1990-2014) weather data. •Key Results The low productivity under “Baseline” climate in the North could be compensated by choosing BL cultivars (e.g. ‘Endurance’). “Recent” warmer climate increased average productivity by 0.5 to 2.5 t ha-1, especially in the North. The modern NL cultivar ‘Resolution’ had the smallest and most efficient water use. On marginal soils (SAWC <100 mm), yields remained below an economic threshold of 9 t ha-1 more frequently under “Baseline” than “Recent” climate. In the drought-prone South-East, ‘Endurance’ yielded lower than ‘Resolution’, which consumed on average 17 mm yr-1 less water. Assuming a planting area of 10,000 ha, in droughty years between 1.3 to 4.5 ˟ 106 m3 of water could be saved with a small yield penalty for ‘Resolution’. •Conclusions With an increase of air temperature and occasional water scarcities expected with climate change, high yielding NL cultivars should be the preferred choice for sustainable use of marginal lands and reduced competition with agricultural food crops.