Some approaches for measuring and modelling riparian shade

The radiation environment of streams is of major ecological importance because it controls stream thermal regime and light availability for photosynthesis. Therefore, methods are needed for measuring stream shade in practical riparian management. The quantity 'diffuse non-interceptance' (difn), defined as the proportion of incident lighting received under a sky of uniform brightness and best estimated from fish-eye images, is useful for general specification of light exposure. For routine measurement of difn along stream reaches we recommend using a matched pair of simple light sensors (e.g. photosynthetically available radiation sensors) under conditions of complete overcast (which has almost uniform brightness). Methods are also needed for predicting future light exposure as riparian plantings grow and increasingly shade the stream. A simple model is outlined for predicting difn at the channel centre as a function of channel dimensions (stream width, w) and riparian plant character (foliage density, canopy height, h). The model reproduces the broad empirical trend of increasing shade with increasing h/w ratio. Future model refinement will aim to quantify the increase in shade moving from channel centre to edge under an overhanging canopy.