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

Abstract

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.