Heterogeneity in sensitivity and response to drought in a headwater catchment

Soils are of critical importance in modulating the response of catchment water storage and flux dynamics under changing hydrological conditions. We explored the short term impacts of an extreme drought on the water storage and transmission dynamics of different hydropedological units, and the role of their spatial organisation on the runoff generation at the integrated catchment scale in a northern environment. Soil types included poorly drained histosols in riparian zones and freely draining podzols on steeper hillslopes in a northern headwater catchment (3.2 km²) in the Scottish Highlands. We employed an integrated monitoring approach of hydrometric data and stable water isotopes in precipitation, stream, soil, and groundwater, to characterise the spatio-temporal storage and runoff dynamics before, during and after a drought period.

We found high spatial variability in the storage resistance and resilience to drought impacts, associated with the distribution of the different soil types, and exacerbated by land use. For example, storage changes in the riparian histosols were remarkably small (<40 mm) during the dry period, compared to those in hillslope moorland (~100 mm) and forest (~200mm) covered podzols. However, results showed that during events, there was consistent threshold behaviour in all soil units and their integrated response at the catchment scale, which appeared not to be affected by relative drying or wetting conditions. The data analyses indicated that during dry periods, large parts of the catchment were disconnected from the river network and runoff was generated mainly from the permanently wet riparian histosols. However, during events there was a quasi-continuous connection of the upper hillslopes that recharged the wetland and stream, which did not appear to have been affected by the drought. This caused a strong recovery and resilience of the catchment in its integrated runoff response. Nevertheless, as future climate projections for northern environments suggest that prolonged warm and dry periods are likely to become more frequent, further work is needed on the potential cumulative or carry over effects of consequative dry periods and the role of the spatial organisation of soil and vegetation characteristics therein.