Higher Incidence of Strongly Evaporative Days Drives Severe Water Deficit for Ombrotrophic Peatlands

Abstract

The capacity of peatlands to sequester and store atmospheric carbon is coupled to their hydrological functioning but is threatened by increases in the frequency and severity of extreme weather. The hydrological functioning of a near-intact water-shedding ombrotrophic blanket bog is characterised here using a decade-long (2010–2021) hydro-meteorological series. This period includes the substantial drawdown of water tables during the 2018 UK summer heatwave. Annual peatland water balances were calculated for three consecutive hydrological years (2017/2018, 2018/2019 and 2019/2020) and comprised, on average, 930 mm of precipitation (P), 335 mm evapotranspiration (ET), 330 mm runoff and 0 mm change in water storage (ΔS). Average annual water table depth (WTD) relates primarily to available energy (net radiation − soil heat flux), while monthly average WTD is driven mainly by the vapour pressure deficit (VPD). Summer water availability (P − ET) is controlled more by precipitation than ET and drives much of summer changes in ΔS. Deeper summer WTD patterns are associated with a greater incidence of warm, highly evaporative days, and the 2018 summer drawdown (−427 mm) reflects both low water availability (P − ET) and high incidence of evaporative days. By winter 2018/2019, the water balance had recovered, demonstrating the resilience of this near-intact blanket bog to hydrological extremes. Over recent decades, the site has experienced a trend towards milder, drier winters and wetter summers. While increased summer rainfall and air saturation may help offset potential temperature-driven increases in winter ET, the growing risk of summer heatwaves, enhanced winter ET losses and reduced winter–spring precipitation are likely to impact interannual hydrological regimes, in particular the extent of winter recharge and summer water table drawdown, potentially compromising the long-term stability of similar peatlands.