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Long-term flow records for the Upper Swale catchment simply do not exist and without such records the long-term hydrological trends in the flow regime are difficult to determine. That having been said, the data available shows that the flashy response of the Upper Swale catchment is consistent with the runoff regimes of other upland peat-covered catchments in the Northern Pennines. As such the annual hydrographs (river flow data, see below) for the Upper Swale are typical of peatland-covered catchments, which have high surface runoff, but minimal baseflow (that is the lowest river flow). The magnitude and seasonal timings of peak floods on the Upper Swale are similar to the timings of peak floods for other upland headwater catchments. In the Upper Swale catchment, it is likely that large peak flows in the winter months are often due to the influence of snowmelt.
From the table below it appears that there have been some quite significant changes in the catchment hydrology of the Upper Swale when the two periods of record from the Environment Agency gauging station at Richmond and Catterick Bridge are compared.
* = Mean annual rainfall and mean annual losses from Institute of Hydrology data, other data for 1961 to 1980 at Richmond and for 1993 to 2000 at Catterick Bridge from Environment Agency and calculations undertaken as part of MGeog dissertation at Leeds University. There is a great deal of difficulty in determining what may have caused these changes and whether these changes can be attributed, for instance, to the moorland-gripping carried out in the headwaters of the Swale from the 1940s to the 1980s. This is because of the data limitations, measurement at different sites (even though geographically close) and also the problems with differentiating between changes in climate and changes in catchment land use practice. These two factors may both affect the hydrological regime of the Upper Swale catchment.
The mean annual precipitation for the area are 1211mm for Richmond
for 1961 to 1980 and 1171mm for Catterick Bridge for 1993 to 1995.
River Run-off The gauging station at Richmond shows the highest mean daily flow,
for the period 1966 to 1980, is 277.500 m3/s-1 in 1968 and the lowest
65.990 m3/s-1 in 1973. However the maximum peak flows for the period
at Richmond are generally between 75 m3/s-1 and 150 m3/s-1, see
figure 4.2 below. The mean daily flow graph (figure 4.11) for Catterick Bridge shows
more clearly than figure 4.2 the nature of the streamflow response
to precipitation and the significance of baseflow in the flow regime.
The highest flood peak 317.632 m2/s-1 occurred in 1995 and considerable
flood peaks to note also include the two flood peaks in 2000 of
180.319 m3/s-1 and 177.187 m3/s-1, which are the third and fourth
highest flood peaks of the period of record. The highest flood peak
for Catterick Bridge is larger (39.5 m3/s-1 higher) than the highest
peak flow for the period of record at Richmond for 1966 to 1980.
This could however, be a reflection of the difference in catchment
area between Catterick Bridge and Richmond. The pattern of the flood
peaks for 2000 shows a significant increase in the frequency of
the flood peaks, as there are considerably higher flood peaks distributed
throughout the year compared to other years, for which data is available. Flood Peaks For both gauging stations there is a lot of variation in flood
peaks with notable flood peaks occurring in the years, 1968, 1971
and 1979 at Richmond and 1993, 1995 and 2000 at Catterick Bridge
(please note data records are not complete).
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