3 Relationships between concentration and discharge of storm flow components
3.1 Save catchment
3.1.1 Relationship with surface plus subsurface flows
3.1.1.1 Relationship with each storm flow component discharge
Hydrograph separation enables us to estimate the contribution of each stream flow component during every flood event. It is also a means to determine the main route of transport of each one of the element by plotting the relationship between the reverine concentration and the discharge of each stream flow components (figure 55).
Figure 55- Relationships between TSM, POC and DOC riverin content and the contribution of surface or subsurface runoff to Save river discharge.
0
The findings of this interaction show the particulate fractions such as TSM and POC are correlated with the proportion of surface runoff in the total discharge and more exportation occurs when the contribution of surface runoff is higher than 60%.
However, the soluble fraction is correlated with discharge of subsurface water.
As illustrated above in the case of TSM and POC, generally the maximum concentration of pesticide is appearing during peak of discharge.
The discharge peak of April storm and the first discharge peak of June 2008 storm are responsible for transporting of aclonifen with concentrations higher than 0.6 (µg.L-1).
Other sampling points show concentrations inferior to 0.3 (µg.L-1), whereas, the contribution of superficial flow is higher than 80% of the total flow for some sampling points. Linuron shows concentrations higher than 0.6 (µg.L-1) for 4 sampling points. Two of them are the sampling points during the peak of storms and the others correspond to the sampling points after (in April) and before (in June) of the peak discharge. Out of these points the concentration of linuron remains relatively stable and less than 0.3 (µg.L-1) (figure 56).
Figure 56- Relationships beteen aclonifen and linuron riverine concentration and the contribution (%) of surface plus subsurface flows in the Save river for 4 storm events.
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
0 20 40 60 80 100
% (Surface + Subsurface) runoff aclonifen (µg.L-1 )
April June December Jaunary Aclonifen
Peak June 2 Peak April Peak June 1
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
0 20 40 60 80 100
% (Surface + Subsurface) runoff linuron (µg.L-1 )
April June December Jaunary
Peak April
Peak June 1
Prak June 2 Linuron
Among all the molecules studied in the course of our research only aclonifen, a molecule with a low degree of solubility, and linuron, a moderately soluble molecule, produced a pattern of relationship with each stream flow with a high degree of concentration for both molecules when the storm reaches its highest level. Despite the existence of a pattern of relationship between a proportion of surface runoff with aclonifen and linuron concentration, this pattern of relation is not statistically significant (rs2 = 0.139, p = 0.087 and rs2 = 0.071, p = 0.20) respectively for aclonifen and linuron. But, the interaction of these molecules shows the important role of surface runoff in exportation of these pesticides especially during peak of April 2008 storm and the first peak of June 2008 storm as a representative of complex flood.
Figure 57 shows that the peak of April storm and the first peak of storm during the complex storm of June in (2008) show the maximum exportation of aclonifen and linuron.
Figure 57- Relationships between aclonifen and linuron riverine concentration and the contribution (%) of surface runoff to the Save river discharge during 4 storm events.
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
0 10 20 30 40 50 60 70 80
% Surface runoff linuron (µg.L-1 )
April June December Jaunary
Peak April
Peak June 1
Peak June 2 Linuron
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
0 10 20 30 40 50 60 70 80
% Surface runoff aclonifen (µg.L-1 )
April June December Jaunary Aclonifen
Peak June 1
Peak April
Peak June 2
A single storm case study: April 2008
Our effort has focussed on identifying the mechanism of pesticide transport and its controlling factors. To do so, storm of April was singled out since:
o during this period the contribution of surface and subsurface water is relatively similar. This similarity would facilitate the comparison and to discover the differences between them (table X).
o this period is coinciding with the application period and, moreover, our sampling points are nearly covered all of the storm phases (rising and falling limbs).
Table X- Contribution (%) of each stream flow components to the total volume of Save river discharge during the different storm events.
The relationships between TSM, POC, pesticides concentration and the contribution (%) of surface plus subsurface flows during the storm event of April 2008 in the Save river is depicted in figure 58.
0 500 1000 1500 2000 2500 3000 3500
0 10 20 30 40 50 60 70 80 90
% (Surface + Subsurface) runoff TSM (mg.L-1) - POC x 100 (mg.L-1)
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
Concentration (µg.L-1 ) TSM POC aclonifen linuron
Figure 58- Relationships between TSM, POC, and pesticides concentration and the contribution (%) of surface plus subsurface flows during the storm event of April 2008 in the Save river.
Ground water Subsurface runoff Surface runoff Total water Storm period
m3 % m3 % m3.s-1 % m3
March 2008 5.8x106 59 1.9x106 19 2.2x106 22 9.9x106
April 2008 7.5x106 58 3.2x106 25 2.1x106 17 12.8x106
June 2008 10.1x106 32 7.2x106 23 14x106 45 31.3x106
November 2008 4.9x106 55 2.7x106 30 1.3x106 15 8.9x106
December 2008 10.5x106 65 3.7x106 23 1.9x106 12 16.1x106
January 2008 23.6x106 33 17.7x106 24 31x106 43 72.3x106
The interaction between the contribution of superficial flow (surface plus subsurface) with the concentration of controlling factors and concentration of pesticide shows the maximum concentration of all of the elements observed when the contribution of surface plus subsurface discharge is more than 80 % of the total river flow (figure 58).
The different equations adjusted to the relationships between TSM, POC and pesticides concentration and the contribution (%) of the surface plus subsurface flows to the total Save river discharge are depicted in table XI.
Table XI- Equations adjusted to the relationships between TSM, POC and pesticides concentration and the contribution (%) of the surface plus subsurface flows to the total Save river discharge.
y- factors Equation rs2 p-value
TSM (mg.L-1) y = 10.915e0.053x 0.893 <0.01 POC (mg.L-1) y = 43.699e0.0453x 0.826 <0.01 Aclonifen (µg.L-1) y = 0.0112e0.039x 0.599 <0.01 Linuron (µg.L-1) y = 0.0163e0.0471x 0.597 <0.01
By plotting the relationships between the concentration of aclonifen and linuron with each stream flow component, the main exporting pathway could be traced. The result shows a good relationship between the concentration of aclonifen, linuron and the ratio of surface water to the total flow. This finding highlights the important contribution of surface water to the displacement of these pesticides. The most noticeable relation was also recorded with the ratio of surface water on the total discharge for TSM and POC as it is depicted in figure 59.
Figure 59- Relationships between TSM, POC (mg.L-1) and pesticide (left) concentration and the contribution (%) of the surface runoff to the total Save river discharge during the storm event of April 2008.
y = 0.0093x - 0.027
The relationships between controlling factors (TSM-POC) and the concentrations of pesticides (figure 60) are clearly indicating the important role of POC and TSM in aclonifen and linuron transport. Linuron also shows a positive relation with the total discharge of stream flows (r2=0.440, p-value=0.026), which explains the fact that TSM and POC are not the only influencing factors in transporting of linuron.
Figure 60- Relationships between the concentrations of pesticides (aclonifen, linuron) and the controlling factors (TSM, POC).
In storm of April 2008, the relationship of metolachlor as a soluble molecule and DOC in filtered water, with each stream flow was verified. The result shows a significant relationship between the discharges of surface and subsurface runoff separately with metolachlor, moreover the contribution (%) of surface and subsurface runoffs to the total river discharge also shows a positive relationship with concentration of metolachlor. Though, this relation is more significant with subsurface runoff ( table XII).
Table XII- Coefficient of the relationships between metolachlor concentrations and the streamflow components.
Pesticides Pearson
coefficient Surface runoff Subsurface runoff Surface plus Subsurface runoff
Our observation reveals that subsurface water is the most important pathway for transporting of metolachlor and DOC. Nervertheless, our findings also demonstrated the role of other stream flow components in transporting of these elements especially in the case of metolachlor. Since, total river discharge of stream flow also indicates a positive relationship with metolachlor (r2=0.636, p-value=0.003).
Figure 61- Relation between the concentration of metolachlor and DOC and the discharge of subsurface runoff during storm of April 2008 for the Save river.
Plotting the interaction of metolachlor and DOC illustrated a positive correlation between them as shown in figure 62.
o DOC plays an important role in metolachlor’s displacement ,
o Metolachlor and DOC are transported together by complexation processes.
Figure 62- Relationship between metolachlor and DOC contents in filtered water during storm of April 2008 for the Save river.
y = 0.3444x + 2.8719