Effects of patchy shade on stream water temperature: how quickly do small streams heat and cool?
J. Christopher Rutherford A B G , Nicholas A. Marsh C D , Peter M. Davies E and Stuart E. Bunn C F
A National Institute of Water & Atmospheric Research, PO Box 11-115, Hamilton, NZ.
B Present address: CSIRO Land & Water, GPO Box 1666, Canberra, ACT 2601, Australia.
C Griffith University, Nathan, QLD 4111, Australia.
D CRC Catchment Hydrology.
E Natural Resource Management, University of Western Australia, 444 Albany Highway, Albany, WA 6330, Australia.
F CRC Freshwater Ecology.
G Corresponding author. Email: firstname.lastname@example.org
Marine and Freshwater Research 55(8) 737-748 http://dx.doi.org/10.1071/MF04120
Submitted: 16 June 2004 Accepted: 10 September 2004 Published: 16 November 2004
Summer field observations in five 2nd order streams (width 1–2 m, depth 5–15 cm, velocity 5–10 cm s–1) in Western Australia and south-east Queensland showed that daily maximum temperatures changed by ±4°C over distances of 600–960 m (travel time 2–3 h) immediately downstream from 40–70% step changes in riparian shade. There was a strong linear relationship between the rate of change of daily maximum temperature and the change of shade such that downstream from a 100% change of shade the heating/cooling rates are ±4°C h–1 and ±10°C km–1 (upper bound ±6°C h–1 and ±15°C km–1) respectively. These high rates only apply over short distances and travel times because downstream water temperatures adjust to the new level of shade and reach a dynamic equilibrium. Shade was too patchy in the study streams to measure how long water takes to reach equilibrium, however, using an existing computer model, we estimate that this occurs after ~1200 m (travel time 4 h). Further modelling work is desirable to predict equilibrium temperatures under given meteorological, flow and shade conditions. Nevertheless, landowners and regulators can use this information to determine whether the presence/absence of certain lengths of bankside shade are likely to cause desirable/undesirable temperature decreases/increases.
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