TRENDS AND DIRECTIONS IN CLIMATE RESEARCH The Estimation of Probable Maximum Precipitation The Case of Catalonia M. Carmen Casas, a Ra ¨ ul Rodr´ ıguez, a Raquel Nieto, b and Angel Reda ˜ no c a Department of Physics and Nuclear Engineering, Polytechnic University of Catalonia, Vilanova i la Geltr ´ u, Spain b Department of Applied Physics, University of Vigo, Ourense, Spain c Department of Astronomy and Meteorology, University of Barcelona, Barcelona, Spain A brief overview of the different techniques used to estimate the probable maximum precipitation (PMP) is presented. As a particular case, the 1-day PMP over Catalonia has been calculated and mapped with a high spatial resolution. For this purpose, the annual maximum daily rainfall series from 145 pluviometric stations of the Instituto Nacional de Meteorolog´ ıa (Spanish Weather Service) in Catalonia have been analyzed. In order to obtain values of PMP, an enveloping frequency factor curve based on the actual rainfall data of stations in the region has been developed. This enveloping curve has been used to estimate 1-day PMP values of all the 145 stations. Applying the Cressman method, the spatial analysis of these values has been achieved. Monthly precipitation climatological data, obtained from the application of Geographic Information Systems techniques, have been used as the initial field for the analysis. The 1-day PMP at 1 km 2 spatial resolution over Catalonia has been objectively determined, varying from 200 to 550 mm. Structures with wavelength longer than approximately 35 km can be identified and, despite their general concordance, the obtained 1-day PMP spatial distribution shows remarkable differences compared to the annual mean precipitation arrangement over Catalonia. Key words: probable maximum precipitation; spatial rainfall distribution; objective analysis Conceptual Definition of Probable Maximum Precipitation Probable maximum precipitation (PMP) has been defined as “the greatest depth of precip- itation for a given duration meteorologically possible for a given size storm area at a partic- ular time of year, with no allowance made for long-term climatic trends.” 1 Hydrologists use the PMP magnitude and its spatial and tempo- ral distributions to estimate the probable max- imum flood (PMF), which is one of a range of Address for correspondence: M. Carmen Casas, EPSEVG. UPC. Avgda. V´ ıctor Balaguer, s/n. 08800 Vilanova i la Geltr´ u, Barcelona, Spain. Voice: +34938967786; fax: +34938967700. m.carmen.casas@upc.edu conceptual flood events used in the design of hydrologic structures for maximum reliability and safety. Typically, PMF is estimated for a dam catchment in order to design a spillway to minimize the risk of overflowing. Prior to the 1950s, the concept of an up- per limit to precipitation potential was known as maximum possible precipitation. The name was changed to PMP, reflecting the uncer- tainty surrounding any estimate of maximum precipitation. 2 Quoting Benson, 3 “The ‘prob- able maximum’ concept began as ‘maximum possible’ because it was considered that maxi- mum limits exist for all the elements that act together to produce rainfall, and that these lim- its could be defined by a study of the natural Trends and Directions in Climate Research: Ann. N.Y. Acad. Sci. 1146: 291–302 (2008). doi: 10.1196/annals.1446.003 C  2008 New York Academy of Sciences. 291