Technical Note: Testing an improved index for analysing storm discharge–concentration hysteresis

Abstract

Abstract. Analysis of hydrochemical behaviour during storm events can provide new insights into the process controls on nutrient transport in catchments. The examination of storm behaviours using hysteresis analysis has increased in recent years, partly due to the increased availability of high temporal resolution data sets for discharge and water quality parameters. A number of these analyses involve the use of an index to describe the characteristics of a hysteresis loop in order to compare storm behaviours both within and between catchments. This technical note reviews the methods for calculation of the hysteresis index (HI) and explores a new more effective methodology. Each method is systematically tested and the impact of the chosen calculation on the results is examined. Recommendations are made regarding the most effective method of calculating a HI which can be used for comparing data between storms and between different water quality parameters and catchments.

High‐frequency monitoring of catchment nutrient exports reveals highly variable storm event responses and dynamic source zone activation

“…This variability was quantified through the application of the HI, which enables robust comparison of data between storm events of different magnitudes [Lloyd et al, 2016a]. Our modeling results suggested that observed hysteresis patterns were driven by a combination of the hydroclimatological conditions of each particular storm event, coupled with the antecedent hydroclimatological conditions in the period leading up to that event [Darwiche-Criado et al, 2015].…”

“…Predictor variables were transformed where necessary and standardized to improve the interpretability of regression coefficients, i.e., effect sizes [Schielzeth, 2010], and the robustness of the statistics. We then fitted all possible subset models of the global model and ranked them using AIC values [Bowes et al, 2005;Chen et al, 2012;Outram et al, 2014;Lloyd et al, 2016b] To better understand the processes controlling nutrient-specific export patterns observed at the catchment outlet (see hypothesis 3), NO 3 -N and DOC were characterized as hysteresis indices (HI) for each storm event using methods developed by Lloyd et al [2016a]. The HI for each storm quantifies the "fatness" and direction of the hysteresis loop, whereby clockwise hysteresis behavior is represented by positive HI values and anti clockwise behavior by negative values.…”

Concentration–discharge relationships describe solute and sediment mobilization, reaction, and transport at event and longer timescales

“…Other studies have used index‐based approaches to assess solute (Butturini, Alvarez, Bernal, Vazquez, & Sabater, ; Lloyd, Freer, Johnes, & Collins, ; M. C. H. Vaughan, Bowden, et al, ) and particulate (Aich, Zimmermann, & Elsenbeer, ; Langlois, Johnson, & Mehuys, ; Lawler, Petts, Foster, & Harper, ; Sherriff et al, ) hysteresis during events. Various quantitative hysteresis indices (HIs) exist that variously compare the C‐Q regression relationships on the rising and falling hydrograph limbs (Langlois et al, ), the ratio of constituent concentrations on the rising and falling limbs (Lawler et al, ), and the size of hysteresis loops (Aich et al, ; Lloyd et al, ; M. C. H. Vaughan, Bowden, et al, ). The HI proposed by Lloyd et al () and modified by M. C. H. Vaughan, Bowden, et al () uses normalized discharge and concentrations to quantify differences on the rising and falling hydrograph limbs.…”

Effect of Water Quality Sampling Approaches on Nitrate Load Predictions of a Prominent Regression-Based Model

“…As is evident from the positive hysteresis observed in this study (Figure 6), there is a dilution of nitrate on the rising limb of each storm event within the Greensboro watershed. Several researches have reported similar response for agricultural watersheds [43][44][45]. Figure 8 shows boxplots of storm nitrate concentration at different flow percentiles for all monitored storm events and demonstrates that median storm nitrate concentration decreases with increases in flow.…”