Modelled effects of channel orientation and tree canopy shape on average shade in streams

ABSTRACT

Replanting riparian vegetation can improve ecological health but there are few guidelines that help stakeholders achieve shade targets cost-effectively. A computer model was extended to calculate average shade to direct and diffuse solar radiation in straight channels. It was used to recommend strategies for achieving shade targets, including the 70% guideline shown to prevent nuisance aquatic plant growths and extreme water temperatures. For similar trees on both banks, 70% shade occurs when the ratio of tree height (H) to stream width (W) lies in the range 1.0 < H/W < 2.6 depending on stream orientation and tree shape (canopy overhang and dripline height). With trees on both banks, shade is lower by up to 34% in east–west (EW) than north–south (NS) streams except for tall trees at the equinox. EW streams planted only on the north bank can have a 30% higher shade than NS streams planted on one bank, but low vegetation on the unplanted bank eliminates this difference. For EW streams in the Southern hemisphere, planting tall vegetation on the north bank is cost-effective, with low vegetation on the south bank providing additional benefits. Further work is planned on channel meandering, gaps between trees and canopy transmission.

KEYWORDS:

• stream shade
• riparian vegetation
• canopy shape
• stream orientation
• solar radiation
• computer model
• water temperature
• nuisance aquatic plants

Acknowledgements

Rob Davies-Colley, NIWA, Hamilton provided useful input to the project.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Animal ethics

No experiments involving animals were conducted as part of this study, and no animal ethics issues arise.

Additional information

Funding

This research was supported by strategic investment funding to NIWA from the NZ Ministry of Business Innovation and Employment under contract [FWRP2002] ‘SMARTer edge-of-field mitigation strategies’. It also received financial support from Hawke’s Bay Regional Council.