How much streamflow is groundwater discharge? - a method for assessment

Abstract

The question of how much streamflow originates from groundwater is relevant to the issue of effects of land use on stream water quality. Whereas phosphorus from agricultural land is transported primarily by overland flow, nitrate is leached from the soil profile and moves to streams via saturated soil-water flow, installed drainage systems, and groundwater flow. The portion that is transported through groundwater is of particular interest because measurements of groundwater age show that average contaminant transport times, even in steep hillslopes (Mike Stewart, GNS Ltd), range from several to many years. This means that the response of nitrate concentration in streams to changes in land use in the catchment can be characterised by a time scale of years. The proportion of streamflow derived from groundwater is also relevant to streamflow quantity. The associated problems of quantifying baseflow separation, low flow recession, and relationships between streamflow and groundwater storage, have been the subjects of mathematical theories for the past 130 years.