Ephemeral rivers are a particular type of dryland river which only flow during and immediately after rain events. Despite their important social and ecological roles in drylands we know very little about the ecosystems or their sensitivity to human activities. This PhD investigates drivers of riparian tree distribution, recruitment and dieback along a dammed ephemeral river; and also investigates wider changes that have occurred to the riparian ecosystem and stakeholders’ perceptions of these changes. A variety of methods are used such as ecological and social surveys, remote sensing and archival searches. The study system is the Swakop River in Namibia; the largest catchment of the country’s westerly flowing ephemeral rivers. In the 1970s two large dams were installed in the upper catchment of the river to supply water to central Namibia, including the capital city of Windhoek. In order of abundance, the riparian woody vegetation community of the Swakop River is composed of Prosopis species (an invasive species introduced in the 1900s), Tamarix usneoides, Faidherbia albida, Salvadora persica, Vachellia erioloba, Euclea pseudebenus and Vachellia tortilis. Although no net change in woodland extent is observed, the woodlands decreased in density by 18%. This reduction in density is consistent with the considerable mortality observed in the riparian trees: 51% of all F. albida, 29% of V. tortilis, 26% of Prosopis spp., 25% of E. pseudebenus and 23% of V. erioloba. The river’s longitudinal profile from the interior of Namibia towards the coast is associated with an increasingly drier climate. This climatic gradient is the most consistently implicated driver of riparian tree occurrence, recruitment and dieback. This study also indicates that tributaries may play an important role in tree population dynamics along ephemeral rivers and may help mitigate the negative effects of damming.

Problems arise with coastal flood modelling because of the short time scale of gauged records and the lack of observed data to constrain the model. Records going back, say 50 years, will not necessarily capture any extreme coastal flood events and even where such an event has been recorded, there are few instances where a comprehensive data set exists to determine the hydraulic boundary conditions for calibration and the flood extent for validation of the model. Whilst the use of historical data, predating gauged records, has been propounded in fluvial hydrology since the NERC 'Flood Studies Report' of 1975, there have been no previous coastal flood modelling studies in the UK which use such historical data to constrain the model. This thesis proposes to evaluate whether historical data can be used in the context of coastal flood modelling and whether the use of multiple validation data sets can offset the problems of imprecision in the data. A chronology of severe storm and coastal inundation events in the Bristol Channel and Severn Estuary is presented. The archival research revealed many more historical sea-floods than had been anticipated. Whilst the 1607 inundation has widely been held as the most severe to have ever occurred in the Severn Estuary, seven other events of similar, if not greater, magnitude have been documented in the last 500 years. The apparent increase in frequency of coastal inundation events can be explained by the increased reporting since the mid-1800s. There is no evidence of increasing extreme tidal elevations. Selected historic floods were researched in depth and those with sufficient information on tidal elevations and flood extent were simulated using the LISFLOOD-FP inundation model. The knowledge gained from using multiple data sets outweighs the inherent uncertainties of individual cases and has enabled a reality-check on the more speculative estimates of depth and extent of past inundations. This thesis demonstrates the value, both of the research into the historical record for data on past flood events, and in the construction of a model to corroborate that data. This approach would be applicable in any project to investigate the historical record for the purpose of coastal risk assessment.

Soil moisture is a key element in the hydrological cycle, regulating evapotranspiration, precipitation, infiltration and overland flow. However, its effective utilisation in hydrological modelling is still in a state of infancy. Generally, hydrological application of soil moisture data requires: 1) soil moisture data relevant to hydrology, and 2) appropriate hydrological model structure compatible with such data. This thesis focuses on tackling those two aspects by enhancing the compatibility between soil moisture observations and operational hydrological models' soil moisture state variable. In this study satellite soil moisture observations are mainly focused on, because of their increasing availability and large-scale global coverage

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The purpose of this research is to investigate some of the factors controlling the rainfall-runoff relationship and consequently flood control in the Ardingly-Goldbridge basin, which is a subject that has given rise to considerable debate. Primarily the thesis is concerned with the study of rainfall and runoff with respect to various hydrometeorological and physical factors. Magnitude/frequency relationships of maximum daily rainfall and for rainfalls with durations of a to 72 hours are studied and the areal distribution of the maximum daily falls for numerous return periods are also investigated. The basis of this part of the study 'is an examination of extreme events using a Gumbel type of analysis. The seasonal distribution of these maximum falls was determined and shows a distinct pattern. Variations in the frequency per year of daily rainfalls over 25 mm. (daily rainburst) is investigated and the results showed that there was a quasi-regular fluctuation in the number of daily rainbursts over the period 1909- 1974. The magnitude/frequency relationship of annual maximum instantaneous discharge is studied and a brief assessment of the seasonal distribution of flood series shows that November was the most likely month for floods, especially those of greater magnitude. Annual maximum rainfall is compared with annual maximum flood and it was ascertained that there was no viable relationship between these two series. The relationship between rainfall and runoff, with regard to several hydrometeoro10gica1 factors, is investigated and an equation to predict the discharge peak determined. The prediction equation illustrates that the maximum peak discharge can be regarded as a function of rainfall intensity, rainfall duration, duration of rainfall up to time of concentration, and the previous discharge. Such analyses are considered vital as a basis for flood warning systems in the area.

Estimation of the probability of occurrence of future flood events at a site is frequently required for the design of bridges, culverts, dams and other river engineering works. This study considers a method for estimating the flood frequency distribution from the physical characteristics of the drainage basin for use in Scotland when adequate records of river discharge are not available. The data base collated includes 3071 station years of annual maximum flood peaks for 168 high quality gauging stations and 12 physical characteristics for each drainage basin. A linear regression model is derived which explains 91% of the variation in the average magnitude of floods using five physical characteristics indexing drainage area, rainfall, stream density, soil type and lake storage. This model appears robust over the range of basin types and shows no improvement when shrinkage or ridge regression is employed. Five physically homogeneous subsets of basins are derived using a clustering algorithm (NORMIX) and the same five characteristics, with the addition of an index of channel slope. For each of subsets 1, 3, 4 and 5, the individual dimensionless flood frequency distributions for each station are not significantly different from a single GEV distribution derived for that subset. Consequently these subsets are considered to be hydrologically homogeneous in addition to their physical homogeneity. Dimensionless regional flood frequency distributions are produced for each subset which allow the estimated average flood magnitude to be scaled to estimate floods of less frequent occurrence. These regional 'growth curves' imply a larger return period for a given magnitude flood than those from the Natural Environment Research Council Flood Studies Report (NERC, 1975). When the floods are described by a lognormal model which allows for cross-correlation between stations the respective return periods are smaller.

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