Divergent future drought projections in UK river flows and groundwater levels

Abstract

Abstract. Hydrological drought is a serious issue globally which is likely to be amplified by 21st century climate change. In the UK, the impacts of changes in river flow and groundwater drought severity in a future of climate change and higher water demand are potentially severe. Recent publication of a new nationally-consistent set of river flow and groundwater level projections based on state-of-the-art UKCP18 climate projections offers a unique opportunity to quantitatively assess future UK hydrological drought susceptibility. The dataset includes a transient, multi-model ensemble of hydrological projections driven by a single regional climate model (RCM) for 200 catchments and 54 boreholes spanning a period from 1961 to 2080. Assessment of a baseline period (1989–2018) shows that the RCM-driven projections adequately reproduce observed river flow and groundwater level regimes, improving our confidence in using these models for assessment of future drought. Across all hydrological models and most catchments, future low river flows are projected to decline consistently out to 2080. Drought durations, intensities and severities are all projected to increase in most UK catchments. However, the trajectory of low groundwater levels and groundwater drought characteristics diverge from those of river flows. Whilst groundwater levels at most boreholes are projected to decline (consistent with river flows), the majority of boreholes show <10 % reduction in transient low groundwater levels by 2080 and eight show moderate increases. Groundwater drought characteristics in the far future (2050–2079) are often similar to those of the baseline (1989–2018), and in some instances droughts are projected to be most prolonged and severe in the near future (2020–2049). A number of explanatory factors for this divergence are discussed. The sensitivity to seasonal changes in precipitation and potential evapotranspiration is proposed as a principal driver of divergence because low river flows are more influenced by shorter-term rainfall deficits in the summer half-year, whilst groundwater drought appears to be offset somewhat by the wetter winter signal in the RCM projections. Our results have fundamental importance for water management, demonstrating a widespread increase in river flow drought severity and diminishing low flows that could have profound societal and environmental impacts unless mitigated. Furthermore, the divergence in projections of drought in river flows and groundwater levels brings into question the balance between surface and subsurface water resources. The projected contrast in fortunes of surface and subsurface water resources identified for the UK may be replicated in other parts of the world where climate projections suggest a shift towards drier summers and wetter winters.

Propagation and Characteristics of Hydrometeorological Drought Under Changing Climate in Irish Catchments

“…A similar finding was reported for UK catchments by Parry et al. (2023) whereby changes in drought were more modest for groundwater relative to runoff. They also highlight more modest changes by the end of century relative to mid‐century using an independent set of climate scenarios and hydrological models.…”

“…Notably the probability of drought propagation from SPI to SBI are not as high for the 2080s, indicating the importance of increases in winter and spring precipitation in offsetting drought propagation to baseflow. A similar finding was reported for UK catchments by Parry et al (2023) whereby changes in drought were more modest for groundwater relative to runoff. They also highlight more modest changes by the end of century relative to mid-century using an independent set of climate scenarios and hydrological models.…”

“…As we do here, Parry et al. (2023) highlight the role of wetter winters toward the end of the century as a key driver of this finding. Increases in the probability of drought propagation have been found in other contexts (e.g., Jehanzaib et al., 2020 in South Korea).…”

“…Future work should explore the sensitivity of results to different hydrological model structure, although we note that Parry et al. (2023) find overall consensus between four different hydrological model structures in terms of simulation of drought characteristics in the UK. Exploring model structural uncertainty is particularly important in assessing the role of baseflow contributions and the influence of increased precipitation in winter/spring seasons on drought magnitude, frequency and propagation in summer.…”

An appraisal of the severity of the 2022 drought and its impacts

“…This is entirely reasonable, as the latest climate change projections suggest future increases in drought severity, and diminishing low flows (e.g. Parry et al ., 2023, based on the latest eFLaG projections driven by UKCP18).…”