Introduction

The hydrologic response of a watershed is based on interactions between landscape characteristics and climatic characteristics & input; as the soil property descriptors, geomorphologic descriptors, geologic descriptors and land use varies among different watersheds, the watersheds could respond very differently to precipitation (Mohamoud, 2004). The main aim of this coursework is to assess the impact of recent climate change on river flow; to uncover its significance in affecting river flow by analysing and comparing river flow records from National River Flow Archive, and to highlight and assess the differences in the hydrologic response to climate change of the three chosen rivers with contrasting characteristics, located in the UK.

The three rivers chosen to be analysed includes the East Avon, the River Dove and the River Greta, all with natural catchments (natural to within 10% at Q95), with no known major artificial changes to the catchment that would influence the flow of the rivers, in order to attempt to focus solely on the effect of climate change (CEH, n.d.). The catchments examined all similar in size: 83000m² for River Dove at Izaak Walton, located in central England; 86100m² for Greta at Rutherford Bridge, located in north- east England; 85800m² for East Avon at Upavon, located in south- west England (CEH, n.d.).

Figure 1– showing the locations of the three gauging station on three separate maps of the UK (CEH, n.d.)

Apart from the differences in location (figure 1.), there are also contrasting catchment characteristics. East Avon at Upavon predominantly consists of 64.5% of upper greensand and lower chalk of 27%; the remaining 8.5% consists of middle chalk, upper chalk, clay, as well as gault, plus river gravel and alluvium at the bottom of the valley (CEH, n.d.). In comparison, the Greta at Rutherford Bridge catchment is known to be steep, and it mainly consists of millstone grit (CEH, n.d.). Finally, in contrast, Dove at Izzak Walton is known to be long and narrow (CEH, n.d.). It mainly consists of mudstone, millstone grit and sandstone, with underlying carboniferous limestone forming the left hand watershed (CEH, n.d.). When looking at the catchment statistics in relation to the geology, East Avon’s catchment consists of 40.3% of high permeability bedrock, with 59.7% moderate permeability bedrock (CEH, n.d.). In contrast, Greta and Dove’s catchment consists of 100% of moderate permeability bedrock (CEH, n.d.). Besides the obvious difference in geology, the sites also vary differently in terms of their climatic characteristics, as shown in table 1.1, 1.2 & 1.3, and finally, land cover also varies among the three river catchments, as shown in table 2 (Met Office, n.d.; CEH, n.d.).

Table 1.1– averages table showing climate data for the England SE & Central S District, which covers period 1981-2010 (Met Office, n.d.)

Table 1.2– averages table showing climate data for the Midlands District, which covers period 1981-2010(Met Office, n.d.)

Table 1.3– averages table showing climate data for the England E & NE District, which covers period 1981-2010(Met Office, n.d.)

Table 2- Catchment statistics of Land Cover for each of the three catchments (CEH, n.d.).

Methodology

The river flow data obtained from these three gauging stations, between the year of 1973 and 2013 were used for analysis. In order to observe and identify flow pattern for each of the chosen sites, as well as to identify any change in the hydrological regime of the three rivers due to recent climate change at the sites, the three sets of river flow data from National River Flow Archive (NRFA) was first imported on to a spread sheet, where the flow measurement/ reading of each river were sorted in to order, according to the hydrological date of the measured flow. The data was then plotted as follows:

1. Discharge vs. time
2. Monthly flow vs. time
3. A flow duration curve for flow frequency analysis
4. Mean discharge vs. Hydrological year Julian date

Next, visual inspection of the graphs was carried out, and the graphs produced for each river were directly compared to assess how seasonal and time – series patterns of flow differ across the three sites, and to determine whether all three sites showed the same pattern of flow through time.

Results and Discussion

In order illustrate the seasonal river flow pattern in the three catchments; figure 2 shows hydrographs for the three rivers. The location of these catchments is shown in figure 1, and characteristics are presented in the introduction. Upon inspecting the hydrographs, the following observations were made (points of reference are labelled as A on the hydrographs):

1. East Avon’s mean discharge peaks at 1.17 m³s^-1, on day 130
2. Greta’s mean discharge peaks at 6.81 m³s^-1, on day 69
3. Dove’s mean discharge peaks at 3.35 m³s^-1, on day 82

Figure 2-Hydrographs for three rivers, showing the mean discharge vs Hydrological year Julian date, plus a graph for comparison between the rivers’ mean discharge over days

According to a study on UK river flow regimes, Hannaford et al. (2012) had suggested that UK river flow regimes can be considered temperate precipitation/evapotranspiration dominated, rather than snowmelt dominated. This means that the seasonal cycle will be mainly driven by evapotranspiration, leading to higher flows in winter and lower flows in summer, with the spring and autumn as transition seasons (Hannaford et al., 2012). When referring back to the peak discharge observations above, all three rivers conformed to the same general pattern, as day 82, 69 & 130- the days where the mean discharge has reached the peak for the three river all lies within the winter period, indicating that the flow will be high during winter days. Furthermore, the hydrographs also shows that, for all three rivers, the mean discharge appeared to be relatively low, and have all remained low between day 280 -320 for all three rivers (section B on the hydrographs), during the summer period.

Figure 3- Monthly Discharge vs. Hydrological Year Date graph for all three rivers, with a secondary axis corresponding to the mean monthly discharge curve

In terms of the consistency of the flow, figure 3 shows East Avon’s maximum & minimum curve, and its mean curve look very similar- the curves are almost overlapping one another which shows a low fluctuation in flow. This suggests that the flow of the river is very consistent. In contrast, the other two rivers have less consistency. This can be observed when comparing the max, min and mean curve in Dove’s graph- the general shape of the curves are very similar to one another, yet there are a few points in the graph where there are some very noticeable differences, where the mean curve tend to have a greater fluctuation and peaks at higher discharge points compared to the other two curves, thus showing that it is generally consistent, but the consistency is lower compared to East Avon. Finally, Greta’s corresponding graph displays great fluctuation; although both max and mean curves are both similar and conforms to a similar pattern, it is clear that the min curve looks a lot flatter, with a pattern that is not very similar to the other two curves within the graph. This indicates that Greta’s consistency between years is relatively poor.

Although all three river exhibit similar seasonal flow patterns, there are still notable difference in their response time. The occurrence of lag time and the difference between the response times of the three sites can be explained by the difference in the catchment’s physical characteristics and its underlying geology. When referring back to the peak discharge data, East Avon displays a lagged response, peaking at day 130, as opposed to peaking at days closer to 82 and 69 (days of which Dove and Greta reached its peak). This significant variation can caused by East Avon’s catchment geology, as it consists of 40.3% of high permeability bedrock, with 27% of chalk in the catchment, as opposed to 0% of high permeability bedrock in the other two catchments; the high permeability bedrock and the highly permeable chalk means that groundwater storage plays a significant role in effecting the runoff regime of the East Avon catchment, which lead to East Avon’s discharge peaking at around February, towards the end of the winter period, as opposed to peaking towards the start of the winter period, like the other two rivers have.

Next, in order to illustrate the reason behind Greta’s earlier peak, in comparison to Dove’s later peak at day 82 (figure 2), the physical feature of both catchments must be examined in detail. Both catchments have an identical percentage of moderate permeability bed rock, and both are similar due to the fact that the catchments both consist of Millstone Grit. However, the topography are significant different between the two catchments. Since Greta’s catchment is significantly steeper when compared to the Dove’s catchment, as illustrated in figure 6 and table 3, Greta will have a more responsive regime compared to Dove due to a quick run- off rate of precipitation. This could also provide an explanation to why the mean discharge curve in the Greta hydrograph is subjected to a greater level of daily variation in comparison to the other two sites and their respective hydrographs.

Figure 4- Flow duration curves for all three sites, with an additional graph (bottom graph) combing the Q* data (Discharge Ratio where Q*= Q/Q₅₀) of three sites for comparison- note that scale of Q* is in Logarithmic Scale (Base:10)

Figure 5- Flow duration curves for all three sites, with an additional graph (bottom graph) combing the Q* data (Discharge Ratio where Q*= Q/Q₅₀) of three sites for comparison- the scale of Q* has been adjusted to go from 0-6 for comparison

Additionally, figure 4 shows that Greta’s curve has the steepest slope, followed by Dove, and then by East Avon with the flattest slope. The observations mirrored those findings above precisely; Greta’s steepest slope indicates a highly variable river, and the flow mainly consists of direct runoff (Searcy, 1959). In contrast, curves with a flatter slope (e.g. East Avon with the flattest curve) which means they have a more constant flow, and can signify the existence of surface and/or groundwater storage – in East Avon’s case, highly permeable chalk acts as storage for water, which equalized the flow of the river (Searcy, 1959). Furthermore, in figure 5, the graph also provides information on the three rivers’ frequencies of very high flows and very low flows. When employing the parameters of Q*=5 for high flow, and Q*=0.2 for low, the curves shows that Greta exhibits a significantly lower proportion of time flow lower than the Q* of 5, whereas the curves for Dove and East Avon are very similar, with a much higher proportion of time flow less than Q* of 5, meaning that high flows occurs a lot less frequently in Dove & Avon in comparison to Greta. In terms of low flow, three rivers are all dissimilar in their frequency of low flow. Greta’s proportion of flow less than 0.5 is ≈0.02, whilst Dove’s proportion is ≈0.16, with East Avon’s proportion is ≈0.34. East Avon’s higher proportion of time flow less than 0.5 means that the occurrence of low flow is more frequent in East Avon, and in comparison, Dove has got a relatively lower frequency of low flow, and Greta with the lowest frequency of low flow over the years within the sample period.

Table 3- Elevation data for Greta and Dove’s catchment (CEH, 2014)

Figure 6-Elevation Map of England. This map shows the significant difference in elevation between the North of England and the South of England. (Windpower Program, n.d)

As seen in figure 7, the flows of all three rivers do seem to conform to a similar pattern over time, with no significant changes in the temporal pattern and frequencies of flood/ droughts. However, upon further inspection, the graph shows that the magnitude of the floods for all three rivers had increased over time; the high flow peaks have seemed to be higher in more recent years. This phenomenon can possibly be explained by global climate change; as global temperature increase, this leads to an increase in water vaporing the atmosphere. As suggested by Milly et al. (2002 cited Das et al, 2013), Kunkel et al. (2013 cited Das et al., 2013) and Trenberth (1999 cited Das et al, 2013), storms are likely to yield more extreme peak precipitation rates, which can lead to more intense floods around the globe (Groisman et al., 2005 cited Das et al, 2013). However, although the trend identified above is consistent with climate change, it is also consistent with variability driven North Atlantic Oscillation (Hannaford, 2013). With the significant knowledge gap in the understanding of long term multi-decadal variability in flow driven by NAO, along with the lack of long term flow data available for this report, it will be premature to attribute specific steam flow trends to anthropogenic climate change (Hannaford, 2013).

Figure 7- Hydrograph showing change in river discharge between 1973- 2012

Summary & Conclusion

In conclusion, river flow regimes of the three assessed rivers are heavily dependent on catchment geological characteristics and climate. Climate plays a major role in effecting the flow, as the relatively temperate climate in England meant that the dominant factor in effecting flow regimes are precipitation/ evapotranspiration, which leads to the occurrence in flow variation between seasons as rate of evapotranspiration varies. In relation to climate, among the three rivers, there were observed changes in peak flow and flood magnitude over time, which global climate change might be responsible for, as it can lead to precipitation extremes, which in turns lead to more run-off and higher river flow. Besides that, variation in geology also contributes to the difference in hydrology of each river, as groundwater storage can affect the rate of run- off, which in turns affects the flow and the response of the three rivers. Finally, anthropogenic influences can affect flow regime of rivers (Schneider et al, 2013). However, there is no significant evidence to show how these had modified the flow of the rivers.

Reference

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