INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 27: 123–138 (2007) Published online 26 June 2006 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/joc.1369 Spatial and temporal daily rainfall regime in Catalonia (NE Spain) derived from four precipitation indices, years 1950–2000 M. D. Mart´ ınez, a X. Lana, b, * A. Burgue˜ no c and C. Serra b a Departament de F´ ısica Aplicada, Universitat Polit` ecnica de Catalunya, Av. Diagonal 649, 08028 Barcelona, Spain b Departament de F´ ısica i Enginyeria Nuclear, Universitat Polit` ecnica de Catalunya, Av. Diagonal 647, 08028 Barcelona, Spain c Departament d’Astronomia i Meteorologia, Facultat de F´ ısica, Universitat de Barcelona, C. Mart´ ı i Franqu` es 1, 08028 Barcelona, Spain Abstract: Annual rainfall amount, P , number of rainy days per year, N , average daily intensity in a year, I , and relevance, R, of the contribution of a rainfall class interval to the annual amount are used as pluviometric indices to deeply analyse the description of the complex behaviour of the daily rainfall regime of Catalonia (NE Spain). For this analysis, daily amounts of rainfall from 75 rain gauges for the period 1950–2000 have been considered. Indices N and I are analysed by distinguishing five percentiles (25th, 50th, 75th, 90th and 95th) of the daily rainfall amounts. Index R is evaluated taking into account <25%, 25–50%, 50–75%, 75–90%, ≥90% and ≥95% class intervals. All these indices are described by their mean annual values, standard deviations and consecutive temporal irregularities. Besides the complex orography of the country, effects of the influence of the Mediterranean regime and remoteness of the Iberian Peninsula to the Atlantic coast are detected in the diversity of spatial patterns of the indices. The influence of the Atlantic regime is also found at some places in the Pyrenees, especially those that faced north. All these features are described through a Principal Component Analysis (PCA), which is applied to average annual values of the pluviometric indices, and the subsequent clustering process. Additionally, time trends of the annual indices are analysed for a selected set of 39 rain gauges with an optimum recording continuity. Trends are derived from linear regression, and local statistical significances at the 95% confidence level are established using the Mann–Kendall test. Field significant trends are investigated by means of Monte Carlo simulations. It is worthy of mention that field significant trends have been detected in the number of rainy days for percentiles up to the 75th, all local trends being negative. Field significant trends are also detected in daily intensity, whatever the threshold level, with positive and negative local trends. The relevance index R depicts field significant trends for the first three class intervals, with a predominance of positive local trends in the first two, thus indicating an increasing contribution of light and moderate daily episodes to the annual amounts. Copyright  2006 Royal Meteorological Society KEY WORDS daily pluviometric indices; spatial clustering; time trends; Mediterranean and Atlantic regimes; Catalonia (NE Spain) Received 23 November 2005; Revised 16 March 2006; Accepted 30 April 2006 1. INTRODUCTION While precipitation statistics at a planetary scale have merited the attention of climatologists for a long time, regional scale precipitation still deserves to be studied with interest in order to obtain new views to describe its particular characteristics. This is the case of Catalonia, NE Spain, where the complex orography and proximity to the Mediterranean Sea (Figure 1) are added to the pluviometric features of a mean latitude region. The spatial variability of the pluviometric regime has recently been analysed through different statistical techniques applied to the average daily rainfall for each month and the corresponding standard deviation and coefficient of * Correspondence to: X. Lana, Departament de F´ ısica i Enginyeria Nuclear, Universitat Polit` ecnica de Catalunya, Av. Diagonal 647, 08028 Barcelona, Spain. E-mail: Francisco.Javier.lana@upc.edu variation (Lana et al., 2004). This last research, together with previous studies on daily and monthly amounts for the same geographic domain (Periago et al., 1991; Fern´ andez-Mills et al., 1994; Lana et al., 1995; Serra et al., 1996, 1998; Lana and Burgue˜ no, 1998a,b,c; Lana et al., 2003), has thrown some light on the dynamic, geographic and orographic patterns governing the Catalan precipitation regime. The daily rainfall is analysed now by means of four precipitation indices, which refer to annual amounts, number of rainy days, daily rain intensities and the contri- bution of several rainfall classes to the total amount. Both the set of all rainy days and the sets of daily amounts reaching or exceeding values associated with the 25th, 50th, 75th, 90th and 95th percentiles are considered. The last percentile is of special interest, as the correspond- ing indices permit a deep analysis of the contribution Copyright  2006 Royal Meteorological Society