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.

Keywords:

• dissolved oxygen
• temperature
• lowland stream
• agriculture
• diurnal curve analysis