Use of streamflow indices in hydrologic modeling

Abstract

A perennial streamflow hydrograph, when measured at the outlet of a basin, continuously and without interruption, can be considered as an integral measure of hydrologic responses. Some of the theoretical and practical aspects of treating streamflow hydrographs as integral indicators of basin properties are addressed in this dissertation. This dissertation is divided into two parts. In the first part, a framework to identify and evaluate whether a streamflow variable is consistent and distinguishable in a given time scale and therefore can be considered as a streamflow index, is developed. The suggested framework is evaluated using as an example two streamflow variables that describe some aspects of the hydrograph shape. In the second part of the dissertation, the utilization of these streamflow indices in hydrologic model parameter estimation is demonstrated. It is assumed that streamflow indices that are evaluated on long streamflow records include large variability of climatic scenarios. Therefore, regardless of climate variability, the consistency and distinguishability are maintained the indices are more related to physical properties of a basin. Consequently, the problem of estimating model parameters that are related to basin properties can be approached by a comparison of indices between the observed and simulated streamflow. Three case studies are presented: the first demonstrates that using the streamflow index which describes the shape of the hydrograph in the parameter estimation processes improves consistency of prediction skill of the 5-parameter HYMOD model in the Leaf River, Mississippi. The second case study explores an important property of the shape descriptors as being relatively insensitive to errors in the data. Such property can be potentially used to identify key sources of uncertainty and to select model parameters that are less affected by data errors. In the final case study, the shape descriptors were used to derive the parameters of the gamma function as a model for the basin's Instantaneous Unit Hydrograph (IUH).