Peter R. Waylen

Changes in extreme rainfall events in South Africa

Extreme rainfall events can have severe impacts on society, so possible long-term changes in the intensity of extreme events are of concern. Testing for long-term changes in the intensity of extreme events is complicated by data inhomogeneities resulting from site and instrumentation changes. Using rainfall data from stations in South Africa that have not involved site relocations, but which have not been tested for inhomogeneities resulting from changes in instrumentation, a method of testing for changes in the intensity of extreme events is adopted. Significant increases in the intensity of extreme rainfall events between 1931–1960 and 1961–1990 are identified over about 70% of the country. The intensity of the 10-year high rainfall events has increased by over 10% over large areas of the country, except in parts of the north-east, north-west and in the winter rainfall region of the south-west. Percentage increases in the intensity of high rainfall events are largest for the most extreme events. While some inhomogeneities remain in the data used, the observed changes in the intensity of extreme rainfall events over South Africa are thought to be at least partly real.

Interannual variability of monthly precipitation in Costa Rica

Abstract The varied monthly precipitation data of stations across Costa Rica are assigned to one of five regions, each of which displays a distinct precipitation regime. Peaks and troughs in each regional regime reflect the relative contribution and timing of the major precipitation-bearing winds flowing from the Pacific, the Caribbean, and continental North America. Lag cross-correlations with a Southern Oscillation index reveal different regional associations, and at differing times, with fluctuating atmosphere-ocean conditions in the tropical Pacific. Patterns of correlations are coherent within each region, but display interregional differences that are more marked than previously suggested. The Pacific coast experiences drought shortly before and during the onset of an ENSO event. The Caribbean coast receives increased summer precipitation but decreased winter rains during these episodes. The interior regions of the country display a mixed response to these events. In the years following a warm phase...

A stochastic model of flow duration curves

The estimation of flow quantiles is a major focus of current hydrologic research. Probabilistic representations of a few individual quantiles, notably the annual flood and low flow, have been devised, and recently a few of the larger quantiles have been modeled. This research describes and demonstrates the use of a probability model for all flow quantiles in a river. A flow duration curve represents the annual flow frequency characteristics of rivers by depicting the cumulative frequencies for average ranked flows in a river. A model requiring only five parameters is developed, by combining the principles of order statistics and traditional flow frequency analyses, and applied to flow duration curves for rivers in the province of British Columbia, Canada. Streamflows in British Columbia are generated from a number of distinct physical processes operating in highly variable environments. The model presented incorporates the physical generating processes of streamflow in both the statistical representation of flow duration curves and their interpretation.

Annual and seasonal fluctuations of precipitation and streamflow in the Aconcagua River basin, Chile

Abstract The El Nino-Southern Oscillation (ENSO) phenomenon has been shown to influence dramatically precipitation and streamflow in tropical western South America. The statistical properties of annual and winter precipitation totals and streamflow characteristics in the Aconcagua River basin, in temperate central Chile, are investigated in such a way as to permit the identification of flood- and drought-generating processes and their possible linkages to upset behavior in the tropical Pacific. Despite the considerable distance to those regions generally associated with ENSO events, the phenomenon produces marked effects upon the various physical processes which govern the surface hydrometeorology of the study area. El Nino years result in significant increases in annual and winter precipitation, particularly along the coastal margin. The likelihood of rain or rain-on-snow flooding, in the succeeding winter, increases, as does the size of spring snowmelt in the southern summer, 1 year after the upset conditions in the tropical region. Annual low flows are of higher magnitude and occur earlier in the year than is usual.

El Niño and annual floods on the north Peruvian littoral

Abstract A three-component mixed Gumbel distribution satisfactorily models the observed annual flood frequencies of rivers in northern Peru which display highly variable annual peak-flood characteristics corresponding to three sets of ocean-atmosphere conditions. Physical support for the “a priori” subdivision of the series is provided by consideration of the regional offshore oceanic-atmospheric conditions. Exceptionally warm waters (El Nino) cause extensive heavy rains; unusually cold waters (anti-El Nino) restrict both the quantity and distribution of precipitation. Model parameters reveal marked spatial trends in the severity and extent of flooding during any set of offshore conditions and geographic shifts in the regional flood-frequency characteristics between such conditions.

TEMPORAL AND SPATIAL VARIABILITY OF ANNUAL PRECIPITATION IN COSTA RICA AND THE SOUTHERN OSCILLATION

Annual precipitation totals from over 100 stations in Costa Rica are analysed to provide estimates of the nature of their response to El Niiio-Southern Oscillation (ENSO) events. Responses are found to be varied in terms of their signs (droughts or excessive precipitation), magnitudes, and durations. The results of simple interstation correlations and lag cross-correlations with the Southern Oscillation Index suggest a marked difference in response in those areas draining towards the Pacific and those towards the Caribbean, as well as latitudinal variations, particularly along the Pacific coast. The marked regional differences in statistical properties over so small an area are related to complex physical processes of precipitation generation, the differing provenances of the humidity laden winds, and the fluctuations of local atmosphere-ocean interactions in response to ENSO.

Spatial variability of annual precipitation and ENSO events in western Peru.

Abstract The probability distributions of annual rainfall totals at 18 stations from north coastal Peru to the southern highlands and Altiplano are examined. Most stations exhibit high interannual variability which cannot be satisfactorily modelled by a single lognormal distribution. Indices of the El Nino-Southern Oscillation (ENSO) suggest that two precipitation regimes may be identified, the ENSO regime and the non-ENSO regime. Simple descriptive statistics of rainfalls within the two sub-samples indicate that during ENSO years the Altiplano and southern highland regions experience drought, with an average reduction of mean annual precipitation of 18%, while the north coastal region is receiving over 88% above non-ENSO rainfall. A mixed lognormal model of annual precipitation is successfully fitted at all stations. The estimated parameters reflect the contrasting response of the two regions to ENSO events. Pearson product-moment correlation matrices generated between stations for each subsample further...

Seasonal Response of Mean Monthly Streamflow to El Niño/Southern Oscillation in North Central Florida

The southeastern United States, including Florida, has been identified as a region of homogeneous response to the El Nino/Southern Oscillation (ENSO) climatic anomaly, in which mean monthly precipitation and discharge during winter is above or below normal following the onset of the warm (El Nino) or cold (La Nina) phase of ENSO, respectively. However, this understanding of the response is expanded through a study of the effects of the ENSO phenomenon on the probability distributions of mean monthly streamflows of the Santa Fe river. The Santa Fe river basin is situated between one region, which experiences the greatest proportion of annual streamflow during winter, and another where the largest percentage of annual flow occurs during late summer. The basin experiences both winter and summer peaks in precipitation and (subsequent) streamflow and may therefore display responses to ENSO during each season. A two-parameter lognormal distribution is employed to model these streamflows during warm and cold pha...

The changing rainy season climatology of mid-Ghana

Daily rainfall data are examined through the temporal analysis of various definitions of variable temporal units (VTUs) consisting of combinations of various starting dates and durations over mid-Ghana. These VTUs are independent of, yet encompass, the starting dates and durations of the major and minor rainy seasons. Within each VTU, total rainfall and number of rainy days are calculated to describe the rainfall characteristics of the unit. Means and variances of each variable are calculated for each unit over two 20-year periods, 1951–1970 (P1) and 1981–2000 (P2). In P2, the major and minor rainy seasons have undergone varying degrees of desiccation. This reduction in rainfall is, however, not temporally or spatially uniform. The widespread decline of mean rainfall totals and number of rainy days during the minor rainy season, often associated with greater inter-annual variability, is particularly threatening to the production of a second crop.

Areal prediction of annual floods generated by two distinct processes

ABSTRACT The compounded Gumbel distribution is found to provide a good description of flood frequency in areas where the annual peak flows are produced by both snowmelt and rainfall. The spatial variation of the flood generating processes can be represented by trend surfaces fitted to the compounded Gumbel parameters. This method offers a practical tool to predict flood frequencies for ungauged basins in such areas.