Spatial distribution of unmonitored inland water discharges to the sea

SummaryWe develop a modelling methodology and investigate how spatial data on catchment surface characteristics may be used for quantifying relative coastal discharge contributions of streams and diffuse submarine groundwater discharge along the coastlines of unmonitored near-coastal catchment areas. The methodology is applied to two small Swedish coastal catchments areas, considering also two different ways to estimate evapotranspiration and its spatial variation within the catchments. Despite considerable differences between the characteristics of the two catchment areas, their relative coastal distribution results are similar, with about 80% of the total coastal discharge occurring through focused flows in visible and permanent streams, whereas the remaining 20% is diffuse and may occur through submarine groundwater discharge (SGD), small transient streams or both at different points in time. Evapotranspiration differs between the two used models, which results in relatively uncertain local flux values within the diffuse flow fields. The large focused stream flows and the mean values and total sums of diffuse flows (such as SGD) along some considerable coastline length, however, are constrained by the catchment-scale hydrological balance and considerably more robust and certain to estimate than the spatially variable small local fluxes within the diffuse flow fields.