Characterisation of lowland streams using a single‐station diurnal curve analysis model with continuous monitoring data for dissolved oxygen and temperature

Abstract

Twenty‐three lowland streams throughout the Waikato region, New Zealand, in catchments having a wide range of land uses and intensities, were monitored continuously over 3–4‐day periods for changes in dissolved oxygen (DO) and temperature. A single‐station diurnal curve model, DOFLO (Dissolved Oxygen at Low Flow), was used to produce reach‐averaged values for: K 2(20) , the reaeration coefficient at 20°C; P max. , the maximum daily rate of photosynthetic production of oxygen; R 20 , the daily respiration rate at 20°C; and Q 10 , the ratio of respiration rates 10°C apart. In addition, 24‐h average values for the ratio P/R were calculated and maximum and minimum values of DO and temperature tabulated for each site. Values of K 2(20) (0.05–40 d ‐1 , median 6.0 d ‐1 ) were in broad agreement with values calculated using a modified form of the O'Connor‐Dobbins equation. Values of gross primary production in daylight (0.5–29.2 g m ‐2 d ‐1 ) calculated from P max . (1.75–86.5 g m ‐3 d ‐1 ) were similar to data reported for other streams in agriculturally developed catchments in New Zealand and North America. Respiration rates (3.50–55.0 g m ‐3 d ‐1 ) were generally larger than values reported in the literature, and P/R ratios were mostly well below 1.0, indicative of heterotrophic respiration associated with decaying vegetation and other organic inputs and consistent with diurnal DO excursions of 40 to ≥120% saturation being commonly observed. Maximum daily temperatures up to 25.7°C (median 20.5°C) were weakly negatively correlated with DO minima. Five groupings of streams were identified from DOFLO parameter values, with K 2(20) being most critical in regulating average DO deficits.

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