3 Hydrograph separation
3.3 Separation of storm flow components
The streamflow component separation has been a debatable subject among the researchers for the last few decades. Hewlett and Hibbert (1967) reported the separation of surface runoff from baseflow is one of the most desperate analysis techniques in use in hydrology.
In our study hydrograph separation in different components (direct runoff on ground surface, subsurface or interflow and groundwater) is classically undertaken from a graphical decomposition method used by Barnes (1939). In this method, inspired from Maillet’s work (1905-1906), river discharge during storm flow events in a mixture of three components: superficial runoff (surface plus subsurface) and baseflow contributing to river discharge in variable proportion.
The method which we used in our study is graphical method. The recession limb of a hydrograph is separated into three segments of different slopes, from which the quantity of water contributed to the stream by surface runoff, interflow and baseflow. In fact since Maillet (1905), the exponential function Qt=Q0.e (-αt) has been widely used to describe the base flow recession after a storm event. Qt stands for discharge at time t, Q0 refers to the initial discharge in t=0, and α is the recession constant which can be considered to represent average response time in storage, and depends on physical characteristic of
different reservoirs. In other words recession constant (α), is an index to the storage volume of a runoff component, reffered to as the storage delay time to drain the volume.
The lower the value of α, the longer the storage volume will take to drain.
In fact the graphical separation methods are commonly used to plot the baseflow component of a flood hydrograph event, including the point where the baseflow intersect the falling limb, whereas there are some methods to select the inflection point or intersect point.
In our research we benefited from the technique proposed by Probst (1983, 1985 and 1986) for separation of different components in stream flow but with some modification.
In the hydrograph figure 12, different phases of the river can be observed at different levels in the watershed.
There is usually a time-lag between the moment the watershed received rainfall and its influence on the river. The amount of water runs at time (t) in the river is the result of an earlier phase (t0) in the watershed. The initial discharge corresponding to surface runoff (Q0S), is identified to the peak of the storm (P), at the time t2. It is more difficult to determine the value of Q0SS, the initial discharge of the subsurface water. According to Lambert (1968 and 1975), t0SS is the beginning of the subsurface water discharge and it is determined by the hydrological characteristics of its phase by observation in the field.
In our study Q0SS is the peak of storm (A0) and Q0G is the first intersection point between the slop of surface and subsurface water on the recession phase of hydrograph (B0) at time t3.
In the paragraph above S and SS stand to surface and subsurface water, G is referred to groundwater and t represents time.
However, the study conducted by Muller (2003) shows the choice of the hydrograph separation method doesn’t influence on hydrological classification of pesticide loads.
Figure 12- Separation of streamflow components (modified from Probst (1983, 1985, 1986), P0 (beginning of stormwater), P (initial discharge of surface water), A0
(beginning of subsurface drainage), B0 (beginning of groundwater drainage), A (initial discharge of groundwater), B (end of the storm).
3.3.1 Limitation of hydrograph separation techniques
The idea of the recession limb of a stream flow hydrograph separation is based on an abstract assumption rather than a practical and visible separation to the human eye.
Hence, hydrograph segmentation of different slopes from which the quantity of water contributing to the stream by (i) surface runoff, (ii) interflow and (iii) baseflow is a somewhat arbitrary process.
The absence of the constant intensity and evenly distributed precipitation, on the one hand, and clear cut change lacking in slopes together with heterogeneity of a typical catchment, on the other hand are all quite understandable natural and geological phenomenon.
The accuracy of a hydrograph separation technique has been a controversial issue among a number of researchers. Sklash and Farvoldon (1979), argue that ground water has a significant role in generating of storm and snow-melt runoff in streams than a hydrograph segmentation technique may help to predict.
Hubert (1989) set forth that a graphical separation technique can largely overestimate the contribution of direct runoff to the stream. He argues that if the reconstitution or prediction of the stream flow rate is what we want to know then the problem expressed earlier can not be considered as an important factor, though it becomes important if our aim is reconstitution or the prediction of the stream water quality.
Surface
Subsurface
Groundwater
QG QH
Qs Time
Discharge(m3.s-1)Log Q
t0 t1 t2 t3 t4 P
P0
A
B B0 Qt= Q0e (-at) A0
Surface
Subsurface
Groundwater
QG QH
Qs Time
Discharge(m3.s-1)Log Q
t0 t1 t2 t3 t4 P
P0
A
B B0 Qt= Q0e (-at) A0
Summary
This chapter began with introducing study area and also four families of pesticides all bonds together using 14 different molecules. In this investigation we have opted for the most commonly techniques in use in taking samples and analysis to achieve the objectives we have set in this research. Separation and quantification of the components of the samples were done by Gas Chromatography (GC) and Mass Spectrometery (MS), using a multi-residue approach. Measuring DOC, POC, TSM, pH and EC are used as a tool for controlling factors of distributing pesticides between dissolved and particulate phases. Finally we utilized the graphical method for separation stormwater’s hydrograph to better understand the contribution of different storm flow components in transfer of each pesticide.