Rainfall, runoff and erosion in the upper Tywi catchment
PublisherUniversity College Swansea
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The study examines the pattern of rainfall, runoff and erosion in a small upland catchment in Central Wales, and attempts to assess the effect that a recent major afforestation programme has had upon the development of runoff within the area and upon the erosional stability of the catchment. The setting up of the instrumentation network in the area is briefly outlined and the practical problems encountered in designing and running the network are considered. The rainfall of the area is then summarised and the uniformity of rainfall within the catchment indicated. A classification of all storm events according to their pattern and intensity is undertaken but although there is a marked concentration of the larger storms in the autumn-winter period, no definite relationship between seasonality and storm type is apparent. The most noteworthy falls are examined in more detail, and doubt is cast on previous return period values for large falls in this area in view of the values obtained from the catchment. The pattern of runoff within the field area as a whole, and within individual subcatchments is then compared and substantial differences in their monthly and annual runoff totals identified. Considerable contrasts are also present in the concentration time, recession time and overall length of the storm pulse from one subcatchment to another. These differences are compared and linked primarily with the amount of land drainage and furrow ploughing carried out in individual subcatchments during the afforestation of the area. An examination of some of the gullies in the area that have developed from main forestry drains is carried out. These (iv) gullies are then classified according to their mode of origin and their stages of development are outlined. The rapidity with which erosion is taking place in some of these gullies is discussed, and the principal factor'in their development identified as the large volumes of water that they frequently have to carry. Methods of gully prevention are briefly outlined, z. nd a critical value of gully discharge above which erosion is almost certain to occur is tentatively identified. Suspended sediment, bank erosion and bed-load are also briefly considered, and catchment variations are related primarily to the disturbing effect that the recent major land use changes have had on the runoff pattern and also on the availability of sediment sources. The length of time these changes are likely to last is considered, and their possible implications for Central Wales where a large amount of upland moorland and rough pasture has been converted to forest are discussed.