Empirical modeling of light availability in rivers

While the influence of hydrology and geomorphology on ecosystem‐limiting factors in rivers has been well studied, particularly habitat availability and nutrient cycling, the more fundamental limitation of light availability has received much less attention. Characterizing light regimes in rivers is optically complex and requires consideration of five hydrogeomorphic controls: topography, riparian vegetation, channel geometry, optical water quality, and hydrologic regime. To generalize and quantify these hydrogeomorphic controls, we developed an empirical model that predicts both spatial and temporal variability of photosynthetically active radiation reaching the riverbed (benthic PAR). We applied this benthic light availability model (BLAM) to two dissimilar systems: a large, turbid river and a small, optically clear river. Comparisons between the two systems revealed that the dominant control on temporal light availability for the large river was discharge, which accounted for 90% of the variation. A dominant temporal control for the small river did not emerge, but instead was found to be a function of both above‐canopy PAR and discharge. Spatially, water depth accounted for 99% of the variation in benthic PAR for the large river, and riparian shading accounted for 93% of the variation for the small river. Channel orientation also had a major influence, where an E‐W configuration increased benthic PAR by as much as 108% relative to a N‐S configuration. BLAM predictions agreed well with measured benthic PAR, within 39% on average over a 9‐d period. BLAM is the first model to quantify benthic PAR using all five hydrogeomorphic controls, and thus provides a new tool for investigating the role of light in fluvial ecosystem dynamics and for establishing light availability targets in water resource management.