Landscape controls on spatiotemporal variability of specific discharge in a boreal region

Abstract:

Spatial and temporal variability of specific discharge is rarely measured at the small-catchment-scale and is commonly ignored by most studies which instead assume spatially uniform specific discharge. This assumption is convenient but can lead to fundamentally wrong results, e.g., when calculating solute fluxes at the catchment scale. Pioneering work on 14, partly nested, sub-catchments in a boreal meso-scale (67 km²) catchment in Northern Sweden revealed substantial spatial and temporal variations in both the magnitude and timing of specific discharge. We explore the structure of this variability and its connection to the landscape characteristics using a 5-year gap filled time series of continuous flow records.

For the long term (5 years) flow magnitudes for the various sub-catchments varied between 73 % and 132 % relative to the flow at the main outlet, with higher flows from wetland dominated catchments (Spearman rank correlation R=0.81). Looking at seasonal and short term flows, both more pronounced variability and stronger links to different landscape properties are seen. Spring flood magnitudes were correlated (R=0.61 to 0.80) with wet areas, while summer flows were negatively correlated to catchment tree volumes (R=-0.61 to -0.75) and potential evaporation (R=-0.48 to -0.78).

On shorter timescales, from daily to monthly, also other catchment properties explained the observed spatial discharge variability. During dry periods of low summer baseflows, sub-catchments with deep fluvial deposits maintained a higher discharge than catchments with shallow soils. This pattern was reversed during summer stormflow events.

The results show that there is a spatial structure in the specific discharge and that this structure is temporally variable. Different landscape characteristics influence the flows at different time scales, and the spatiotemporal discharge variability depends on seasonal climatic variability. The observed structure does not only influence mass balance calculations, but also provides insights into spatial and temporal variability of different runoff generation processes.