A REVIEW OF TECHNIQUES FOR ANALYSING BASEFLOW FROM STREAM HYDROGRAPHS

Postgraduate

Abstract

Understanding the groundwater contribution to streams is critical when dealing with a wide range of water management issues. Analysis of the streamflow hydrograph, specifically separating and interpreting baseflow (the longerterm delayed flow from storage) from quickflow (the short-term response to a rainfall event) is a well-established strategy in understanding the magnitude and dynamics of groundwater discharge. A multitude of methods have evolved and these can be conveniently categorised into three basic approaches of baseflow separation, frequency analysis and recession analysis.

 

Baseflow separation uses the time-series record of stream flow to derive the baseflow signature. Graphical separation methods tend to focus on defining the points where baseflow intersects the rising and falling limbs of the quickflow response Filtering methods process the entire stream hydrograph to derive a baseflow hydrograph. Recursive digital filters, which are routine tools in signal analysis, are commonly used to remove the high-frequency quickflow signal to derive a low-frequency baseflow signal. Such filters are simple and robust but the results are very sensitive to the filter parameter, which needs calibration before the results can be considered to be numerically valid. Also, many of the filters have no hydrological basis.

 

Frequency analysis takes a different approach by deriving the relationship between magnitude and frequency of streamflow discharges. In its most common application, a flow duration curve (FDC) is generated showing the percentage of time that a given flow rate is equalled or exceeded. As well as the general shape of the FDC, various indices have been developed to characterise baseflow. Many of these indices are strongly intercorrelated and limited work has been undertaken to link these indices to groundwater processes.

 

Recession analysis focuses on the recession curve which is the specific part of the hydrograph following the stream peak (and rainfall event) when flow diminishes. Recession segments are selected from the hydrographic record and can be individually or collectively analysed to gain an understanding of the processes that influence baseflow. Graphical methods, such as correlation or matching strip techniques involve plotting multiple recession curves to derive a master recession curve representing a composite of baseflow conditions. In analytical methods, equations are applied to fit the recession segments. A storage-outflow model is developed to represent discharge from one or more natural storages during the recession phase. In its simplest form, the classic exponential decay function as used to represent heat flow, diffusion or radioactivity is applied. This assumes a linear relationship between storage and outflow which is commonly not applicable, so more complex functions have had to be developed.

 

Baseflow analysis, with a wide availability of methodologies, is a valuable strategy in understanding the dynamics of groundwater discharge to streams. Streamflow data is commonly collected and made publicly available, so is amenable to desktop analysis prior to any detailed field investigations. However, it is important to remember that the assumption that baseflow equates to groundwater discharge is not always valid. Water can be released into streams over different timeframes from different storages such as connected lakes or wetlands, snow or stream banks. As the hydrographic record represents a net water balance, baseflow is also influenced by any water losses from the stream such as direct evaporation, transpiration from riparian vegetation, or seepage into aquifers along specific reaches. Water use or management activities such as stream regulation, direct water extraction, or nearby groundwater pumping can significantly alter the baseflow component. Hence, careful consideration of the overall water budget and management regime for the stream is required.


INTRODUCTION

Historically, groundwater and surface water in Nigeria have been perceived and managed as isolated resources. There is however growing recognition that rivers can receive groundwater from underlying aquifers, and this can have significant implications for river water quantity and quality. The analysis of groundwater inputs into streams is critical when dealing with issues such as reliability of water supply, water allocation and trading, design of water storages, hydroelectric power generation, ecosystem water requirements, waste dilution, contamination impacts or predicting peak stream salinities.

 

A stream hydrograph is the time-series record of stream conditions (such as water level or flow) at a gauging site.  The hydrograph represents the aggregate of the different water sources that contribute to stream flow. These components can be subdivided into:

(i.)             Quickflow   – the direct response to a rainfall event including overland flow (runoff), lateral movement in the soil profile (interflow) and direct rainfall onto the stream surface (direct precipitation), and; (ii.)     Baseflow – the longer-term discharge derived from natural storages.

 

The relative contributions of quickflow and baseflow components changes through the stream hydrographic record.

The flood or storm hydrograph is the classic response to a rainfall event and consists of three main stages (Figure 1):

(i.)                Prior low-flow conditions in the stream consisting entirely of baseflow at the end of a dry period; 

(ii.)              With rainfall, an increase in streamflow with input of quickflow dominated by runoff and interflow. This initiates the rising limb towards the crest of the flood hydrograph. The rapid rise of the stream level relative to surrounding groundwater levels reduces or can even reverse the hydraulic gradient towards the stream. This is expressed as a reduction in the baseflow component at this stage;

The quickflow component passes, expressed by the falling limb of the flood hydrograph. With declining stream levels timed with the delayed response of a rising watertable from infiltrating rainfall, the hydraulic gradient towards the stream increases.  At this time, the baseflow component starts to increase. At some point along the falling limb, quickflow ceases and streamflow is again entirely baseflow. Over time, baseflow declines as natural storages are gradually drained during the dry period up until the next significant rainfall event.


BASEFLOW ANALYSIS

TECHNIQUES

 

Analysing the baseflow component of the stream hydrograph has had a long history of development since the early theoretical and empirical work of Boussinesq (1904), Maillet (1905) and Horton (1933). Several useful reviews have been written including Hall (1968), Nathan and McMahon (1990), Tallaksen (1995) and Smakhtin (2001) to map this development. The multitude of methods that have evolved can be conveniently categorised into three basic approaches of baseflow separation, frequency analysis and recession analysis.

 

Baseflow Separation

 

Baseflow separation techniques use the time-series record of stream flow to derive the baseflow signature. The common separation methods are either graphical which tend to focus on defining the points where baseflow intersects the rising and falling limbs of the quickflow response, or involve filtering where data processing of the entire stream hydrograph derives a baseflow hydrograph.