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Comparing the flow patterns of rivers in different regions can be most revealing to the contemporary geographer. As Veissman and Lewis have emphasised, all waterways are influenced by the six components of the hydrological cycle. These are: precipitation levels, infiltration, evaporation, transpiration, surface run-off and ground water flow. Such factors, along with topography, often vary considerably within relatively small geographical boundaries.
For the purposes of this investigation the flow duration curves (FDCs) of the rivers Owenmore, Funshion, Rye Water and Fane, which are located in the Republic of Ireland, shall be subjected to inspection and comparison.
The Flow Duration Curve
As Singh has noted ‘the flow duration curve (FDC) is one of the simplest but most informative tools in hydrology’ and ‘shows the percentage of time during which specified flows were equalled or exceeded during the period of record.’
In essence, the FDC is a model which can be utilised by hydroelectric companies and other agencies in order to determine the energy capacity of a river, normally on an annual basis. The vertical, or Y, axis represents the flow rate (Q) measured in cubic metres per second (mï‚³/s) and the horizontal, or X, axis represents the percentage of time over a specific period in which flow exceeds the value on the Y axis. Thus, the area below the curve shows the potential energy capacity of the river.
The shape of the FDC is influenced by the six components of the hydrological cycle and also the topography and shape of the river’s catchment area. As Ward and Robinson have highlighted, a steeply sloping FDC is typical of ‘highly variable flows with a large quick flow component.’ Such a trend is associated with waterways in the Mediterranean region due to extremes in precipitation levels, high surface run-off and a low storage capacity of the soil. On the contrary, a gently sloping curve indicates a ‘large delayed flow component.’ This is typical of many rivers in temperate climates which continue to receive through-flow during periods of low precipitation from water stored in the surrounding soil and bedrock. The lower end of the FDC is indicative of ‘perennial storage in the basin.’ Thus, if the FDC ‘flattens out,’ the catchment area has a considerable degree of water stored which will flow into the river during dry periods.
A Comparison of Rivers
The Flow Duration Curves of both the rivers Owenmore and Funshion are similar in shape and exhibit a much steeper trend in comparison to the FDCs of rivers Rye Water and Fane. However, the curve of the Funshion flattens out more than that of the Owenmore. Flow rate never decreases below 1.8mï‚³/s on the Funshion whereas on the Owenmore it can be as low as 0.8mï‚³/s. This shows that the Funshion benefits from a greater level of through flow and storage capacity within its drainage basin. Indeed, this is understandable when one considers that the total area of the Funshion’s catchment exceeds that of the Owenmore by 64 Km . Specific run-off is also 0.9 mï‚³/s higher in the area of the Owenmore which accounts for the river having a higher maximum flow rate of 20.90 mï‚³/s. In only 5% of the time, 17.3% of the total annual flow rate passes through its water channel. Mean flow rate of the River Funshion is 8.14 mï‚³/s, the highest of all the rivers. Flow rate exceeded for 95% of the year (Q95) is 0.495 and emphasises that the Funshion has the greatest minimum river flow of all the waterways.
The rivers Rye Water and Fane have a flow range which is less than that of the Owenmore and Funshion. Maximum flow rate is 11.8 mï‚³/s on the Rye Water and 8.0 mï‚³/s on the Fane. This is due in part to the lower drainage area of both basins. The Fane’s is only 234 km and the Rye Water’s is the smallest of the four at 215 km . The flow rate exceeded for 95% of the year (Q95) of the Fane is 0.085 mï‚³/s to the Rye Water’s 0.046 mï‚³/s. The Fane’s catchment exhibit’s a greater degree of storage capacity than that of the Rye Water as flow rate never drops below 0.20 mï‚³/s. However, the Rye Water’s flow rate drops considerably and levels off at only 0.10 mï‚³ /s for 15% of the year, contributing to the lowest mean flow rate of all the waterways at just 2.16 mï‚³/s. The specific run-off rate of the Rye Water is 0.4 mï‚³/s higher than that of the Fane.
Of all the rivers, the Funshion exhibits the highest base flow rate whilst the Rye Water’s is the lowest. The Owenmore has the widest range of flow rate from 20.90 mï‚³ /s to only 0.80 mï‚³/s. The Fane has the gentlest FDC and ranges from 8.00 mï‚³/s at high flow to 0.20 mï‚³/s at low flow.
Geographical Locations and Factors of Influence
The River Owenmore is located in the north of the Republic of Ireland and flows from north central Mayo to Sligo Bay. Precipitation levels are high in this oceanic coastal area and average 1163 mm per year. As Mitchell and Ryan have noted, this zone is also dominated by poorly drained peat and gley soils. Thus, during periods of heavy rainfall, a large amount of surface run-off flows directly to the river channel. High upland plateaus, which dominate the region to heights of up to 2113 feet, contribute to this phenomena. This explains why the flow rate of the River Owenmore can reach 20.90 mï‚³/s during periods of high rainfall. The course of the river traverses terrain utilised for grazing and dairying purposes. Such an expanse of grassland further exacerbates surface run-off during the winter months.
The River Funshion is a tributary to the Blackwater River in south west Ireland. This region is also overlooked by uplands which can rise to heights of 2000 feet at the source of the Blackwater. The steepness of the surrounding topography is a potent factor in generating a top flow rate of 20.20 mï‚³/s. Heavily forested slopes also ensure that a higher degree of subsurface water is retained within the basin than in the sparsely wooded catchment area of the River Owenmore in the north. The Funshion thus receives a higher level of through-flow at times of low precipitation due to this vegetation factor and flow rate never drops below 1.80 mï‚³/s. Rainfall averages 1190mm per year in this catchment area, which is the most of all the four drainage basins.
The rivers Rye Water and Fane experience lower levels of precipitation due to their location further east. The Fane is situated in the drumlin belt close to the town of Monaghan in South Armagh and the Rye Water is a tributary of the Liffey River which traverses the city of Dublin. The terrain influencing both drainage basins is not as dynamic as that of the west and altitudes rarely reach 655 feet. The highest flow rate of the Rye Water is just 11.80mï‚³/s which is testimony to the lower eastern topography. However, urbanisation around the river channel has contributed to increased surface run-off; the river passes through the settlements of Kilcock and Leixlip before entering the Liffey. Orme has noted that the drumlin soils of catchments such as the River Fane are often poorly drained and prone to water logging. This factor explains why the lower end of the river’s FDC does not drop as abruptly as that of the Rye Water. Clearly the Fane has a higher degree of surface storage in its catchment area.
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