Modelling water stress to urban amenity grass in Manchester UK under climate change and its potential impacts in reducing urban cooling

It is well known that the evapotranspiration of vegetation such as grass can help to reduce the urban heat island. However, the cooling can be reduced or lost in summer droughts, when soils dry out, an effect that is likely to be more pronounced and occur for longer as climate change proceeds. Here we modelled the likely effect of climate change on the droughting of amenity grass in Greater Manchester, UK. We used a simple Bucket model, with data on Greater Manchester's soils, and its current and anticipated precipitation and potential evapotranspiration. This was experimentally validated by measuring the weight loss of ryegrass turves. The results show a dramatic increase in drought, especially in the drier south west of the conurbation, with some areas exhibiting reduced evapotranspiration for 3–5 months by the 2080s, and evapotranspiration reducing by over a half for 1–2 months, in an average year. Such changes could have large effects on the urban heat island, resulting in increases in surface temperatures of up to 15 °C in areas where grass accounts for a large proportion of the surface cover. The problem could be overcome by irrigation of grassland so that it will continue to provide cooling, and it is shown that runoff from large rainfall events could in theory provide adequate irrigation water, particularly in highly built-up areas.