The interpretation of non-homogeneous hydrometeorological time series: a case study

Abstract

Long records of hydrological and hydrometeorological variables are of inestimable value for the planning, design and management of water resource systems. Unfortunately, such long series invariably exhibit inconsistencies and non-homogeneities arising from a wide variety of causes - for example, changes in instruments and observation practices, and alterations to the general environment of the instruments themselves. Both non-parametric and parametric statistical tests are available to detect the presence of long-term movements in recorded time series, but the interpretation of results from such testing has often to be carried out in the absence of sufficient station metadata. A case study of precipitation records for 20 stations in Scotland over the period from 1961-99 has demonstrated that the results from statistical testing for inconsistency and non-homogeneity should also be interpreted in the context of prevailing weather systems. In particular, the increases in both annual and winter (Dec-Feb) rainfall totals recorded during this period at sites in western Scotland appear to reflect the changes in the winter atmospheric circulation over the Northern Hemisphere as encapsulated in the North Atlantic Oscillation Index (NAOI). Sustained periods of positive values of the NAOI, albeit less severe than during those of the 1970s and 1980s, are evident in the historical record of this index. The trends in rainfall totals identified could therefore be interpreted as arising from natural variability or even greenhouse gas forcing rather than from any inconsistency and non-homogeneity.