Effects of scale on land use and water quality relationships: A longitudinal basin-wide perspective

Human land use is a major source of change in catchments in developing areas. To better anticipate the long-term effects of growth, land use planning requires estimates of how changes in land use will affect ecosystem processes and patterns across multiple scales of space and time. The complexity of biogeochemical and hydrologic interactions within a basin makes it difficult to scale up from process-based studies of individual reaches to watershed scales over multiple decades. Empirical models relating land use/land cover (LULC) to water quality can be useful in longterm planning, but require an understanding of the effects of scale on apparent land use-water quality relationships. We empirically determined how apparent relationships between water quality and LULC data change at different scales, using LULC data from the Willapa Bay watershed (Washington) and water quality data collected along the Willapa and North Rivers. Spatial scales examined ranged from the local riparian scale to total upstream catchment. The strength of the correlations between LULC data and longitudinal water quality trends varied with scale. Different water quality parameters also varied in their response to changes in scale. Intermediate scales of land use data generally were better predictors than local riparian or total catchment scales. Additional data from the stream network did not increase the strength of relationships significantly. Because of the likelihood of scale-induced artifacts, studies quantifying land use-water quality relationships performed at single scales should be viewed with great caution.