CHARACTERIZATION OF LOWLAND STREAMS USING A SINGLE-STATION DIURNAL CURVE ANALYSIS MODEL WITH CONTINUOUS MONITORING DATA FOR DISSOLVED-OXYGEN AND TEMPERATURE

Twenty-three lowland streams throughout the Waikato region, New Zealan d, in catchments having a wide range of land uses and intensities, wer e monitored continuously over 3-4-day periods for changes in dissolved oxygen (DO) and temperature. A single-station diurnal curve model, DO FLO (Dissolved Oxygen at Low Flow), was used to produce reach-averaged values for: K-2(20), the reaeration coefficient at 20 degrees C; P-ma x, the maximum daily rate of photosynthetic production of oxygen; R-20 , the daily respiration rate at 20 degrees C; and Q(10), the ratio of respiration rates 10 degrees C apart. In addition, 24-h average values for the ratio P/R were calculated and maximum and minimum values of D O 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 calculate d using a modified form of the O'Connor-Dobbins equation. Values of gr oss 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 f or other streams in agriculturally developed catchments in New Zealand and North America. Respiration rates (3.50-55.0 g m(-3) d(-1)) were g enerally larger than values reported in the literature, and P/R ratios were mostly well below 1.0, indicative of heterotrophic respiration a ssociated with decaying vegetation and other organic inputs and consis tent with diurnal DO excursions of 40 to greater than or equal to 120% saturation being commonly observed. Maximum daily temperatures up to 25.7 degrees C (median 20.5 degrees C) were weakly negatively correlat ed with DO minima. Five groupings of streams were identified from DOFL O parameter values, with K-2(20) being most critical in regulating ave rage DO deficits.