Estimation of vertical exchange velocity through sediment–water interface (Case study: Ziarat River in Golestan province)

Precise Estimation of water exchange between surface and subsurface flow in hyporheic zone which is habitat of microorganisms is vital. The temperature can be used as a tracer for estimation of water exchange through sediment water interphase. In this study, for the first time in Iran, an instrument was designed and constructed to make the measurement and recording of sediment temperature in the depth of hyporheic zone possible. In this regard, measurements were made in the Ziarat River of Golestan Province by the aforementioned instrument, and surface and subsurface water exchange rates were calculated using an extended conceptual model of heat transfer. For this purpose, in a 40 m interval of the river, 10 cross sections were selected at 4 m intervals, and at each cross section, temperature of 4 different depths of the riverbed (immediately below the bed, 0.25, 0.50, 0.75 m) were recorded in July and January. The results showed that in all seasons there is a constant flow due to the difference of thermal potential between surface and subsurface of the river flow which leads to the transfer of nutrients to micro-organisms and consequently to the permanent self-purification of the river. The average of vertical exchange for July and January was 0.34 mm/day and 0.86 mm/day respectively.