Open Journal of Modern Hydrology, 2013, 3, 269-277 http://dx.doi.org/10.4236/ojmh.2013.34030 Published Online October 2013 (http://www.scirp.org/journal/ojmh) Copyright © 2013 SciRes. OJMH 269 On Redefining the Onset of Baseflow Recession on Storm Hydrographs R. Pizarro-Tapia 1 , F. Balocchi-Contreras 1 , P. Garcia-Chevesich 2,3 , K. Macaya-Perez 4 , Per Bro 1 , L. León-Gutiérrez 4 , B. Helwig 1 , R. Valdés-Pineda 1 1 University of Talca, Centro Tecnológico de Hidrología Ambiental, Talca, Chile; 2 Department of Hydrology and Water Resources, University of Arizona, Tucson, Arizona; 3 Instituto Forestal de Chile, Santiago, Chile; 4 Dirección General de Aguas, Santiago, Chile. Email: pablogarciach@gmail.com Received May 24 th , 2013; revised June 24 th , 2013; accepted July 2 nd , 2013 Copyright © 2013 R. Pizarro-Tapia et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. ABSTRACT Two methods that define the point of baseflow recession onset were compared using storm hydrograph data for 27 storm events that occurred between 1982-1995 in the Upeo watershed located in the Andes mountain range in central Chile (Figure 1). Three well-known baseflow recession equations were used to determine whether the method we are proposing here, that defines baseflow recession onset as the third inflection point on the logarithmic graph of the falling limb of the storm hydrograph, more accurately models observed data than the most widely used method that defines baseflow onset as the second inflection point on the same graph. Five time intervals were used to modify the recession coefficient in search of a more accurate fit. Results from the coefficient of determination, standard error, Mann-Whit- ney U test, and Bland-Altman test suggest that redefining baseflow recession onset via the proposed approach more accurately models baseflow recession behavior. Keywords: Baseflow Recession; Hydrograph Separation; Hydrologic Modeling; Recession Analysis; Baseflow Onset 1. Introduction Predicting the rate of baseflow recession is important to water resource management for areas with Mediterranean climates; as the rate of baseflow decrease (recession) varies little year to year in regions with an extended dry season, recession flow analyses are used to study ground- water systems [1], whose characteristics largely deter- mine the feasibility of land use where options are limited by the availability of water resources (Ponce, 1989). As direct runoff and baseflow recede at different rates, it is required to model them separately; hydrologists of- ten use surface and subsurface flow models to accom- plish such an objective [2]. Hydrograph separation me- thods are used to determine whether the stream flow present in a channel during a storm event derives from direct runoff or baseflow [3]. However, hydrograph se- paration itself can be considered arbitrary as there is no real basis for the division between surface and subsurface contributions at any given time, as the definition of the hydrograph components themselves (surface, subsurface, and baseflow contributions) are also arbitrarily defined [4, 5]. Regardless, baseflow recession characteristics may still reliably estimate watershed-scale hydrogeological properties [1] and hence justify further study. Baseflow recession models are used to portray the behavior of baseflow and determine minimum water yields and de- pletion rates [6]. Despite their importance, there are sev- eral viewpoints on the effectiveness of baseflow reces- sion models, which often do not accurately model ob- served data. Several studies worldwide have focused on improving the prediction of baseflow recession. Chapman [7] inves- tigated various algorithms describing baseflow during the precipitation-runoff process and determined that prob- lems arose during the course of hydrograph separation itself. Vogel and Kroll [8] tested six estimators of the baseflow recession constant derived from data for thou- sands of recession hydrographs pertaining to 23 sites in Massachusetts, in the process highlighting how certain assumptions made regarding model error structure af- fected model accuracy. In this paper the definition of the point of baseflow recession onset was analyzed; comparing the most