põhivoo

Põhivoo is the portion of a river's discharge that is supplied by groundwater seeping into the channel, continuing even during dry periods. It contrasts with runoff or quickflow, which responds rapidly to rainfall.

It is estimated by hydrograph separation, separating baseflow from stormflow in recorded discharge, using methods such as recursive digital filters (Lyne-Hollick), Eckhardt, or Chapman, and sometimes supported by tracers or groundwater models.

Baseflow is ecologically important, sustaining aquatic habitats, maintaining stream temperature, providing drought resilience, and contributing to nutrient and pollutant dilution. A common metric is the baseflow index (BFI), the ratio of baseflow to total discharge over a period.

In the Estonian context, baseflow is influenced by regional geology and groundwater systems; long-term monitoring of põhivoo helps assess groundwater resources and river health, particularly in temperate climates with seasonal precipitation patterns.

Etymology: the term combines 'põhi' (base, bottom) and 'vool' (flow).