Using Hydrograph Recession Analysis for Linking Hydrology to Chemistry: A Flow Regime Approach

Hydrograph recession analysis is of great utility for the interpretation of water quality data, particularly when examining the hydrological processes responsible for runoff generation and impacts of changing land use on water quality. The theoretical basis for hydrograph recession analysis is well known, however the technique has traditionally been under-utilized for studies in which water quality is linked to runoff. Here, a 3-step approach is outlined for such investigations. First, long-term flow recessions are compiled in order to construct a Master Recession Curve (MRC) at a particular point of discharge (e.g., a stream reach, spring, or weir), taking any major seasonal effects into account. Second, the MRC is subsequently divided into flow regimes (from peak flow to baseflow) based upon changes in the recession coefficients that arise from the hydrograph recession analysis. Finally, observed chemical data that fall within the identified flow regimes are statistically described and reduced through principle components analysis (PCA) or other means, allowing for more focused interpretation of water quality variability and prediction according to flow conditions. Characterization of the flow regimes is the crucial link between discharge and water quality for it grants the hydrologist the ability to look for patterns and trends within a reduced set of flow-dependent water quality variables. In conjunction with an end-member mixing model that utilizes solute and isotope tracer data, hydrograph separation and end-member mixing analysis (EMMA) can be performed individually on the identified flow regimes. This approach can provide quantitative information about contributions to discharge derived from chemically distinct water sources in accordance with the flow regimes of the system. Examples from a forested headwater watershed and spring flow from a karst region are presented.