The effects over time of an arterial drainage scheme on the rainfall-runoff transformation in the Brosna catchment

The earliest arterial drainage schemes (ADSs) implemented on Irish rivers date back to the beginning of the 19th century. Such ADSs mainly involved the deepening and widening of the river channels in order to increase their discharge-carrying capacity, thereby affecting the hydrological response of these catchments to rainfall. Earlier studies carried out to assess the effects of such ADSs on the hydrological behaviour of Irish catchments concentrated mainly on comparisons of unit hydrographs (UHs) and identification of empirical relationships between flood peaks of pre- and post-drainage periods. The present preliminary study of the River Brosna catchment, in Ireland, focuses on assessing changes in its rainfall-runoff transformation, using the conceptual soil moisture accounting and routing model (SMAR), one of the models in the ' Galway river flow forecasting system (GFFS)'.The results of the present study show that the catchment response to rainfall in the early post-drainage period differs substantially from that of the pre-drainage period. As expected, the early post-drainage UHs are more ' peaky ' and have quicker recessions than the pre-drainage UHs, thus confirming the findings of the earlier studies. However, consideration of additional recent data suggests that the catchment response to rainfall has not been consistent over the entire extended post-drainage period. In addition to the expected pre/post-drainage change, the results show that the catchment response to rainfall reverted to virtually pre-drainage-like conditions after just one-and-a-half decades but surprisingly changed back again, over the last decade, to early post-drainage-like conditions. Further investigations are currently underway to explain such changes in the behaviour of the Brosna catchment and to establish if similar changes occurred on other Irish catchments that also underwent arterial drainage schemes.