A complete daily precipitation database for northeast Spain: reconstruction, quality control, and homogeneity

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This paper reports the procedure used in creating a homogeneous database of daily precipitation in northeast Spain. The source database comprised 3106 daily precipitation observatories, with data ranging from 1901 to 2002. Firstly, a reconstruction of the series was performed. Data from adjacent observatories were combined to provide long temporal coverage. Data gaps were filled using values from the nearest neighbour observatories. A distance threshold was set to avoid the introduction of spurious information in the series. Secondly, the reconstructed series were subjected to a quality control process. Empirical percentiles corresponding to each precipitation observation were compared to the percentiles corresponding to the closest neighbour observatory, and a threshold difference was set to identify questionable extremes. After careful inspection of each case, 0.1% of the data was rejected and replaced with information from the nearest neighbour. Thirdly, the homogeneity of the series was checked using the standard normal homogeneity test. This allowed detection of inconsistencies present in the original database or introduced by the reconstruction process. Four parameters were assessed at a monthly level: amount of precipitation, number of rainy days, daily maxima, and number of days above the 99th percentile. A total of 43% of the series had some periods of inhomogeneity and were discarded. The final database comprised 828 series with varying time coverages. The greatest number of stations existed during the 1990s, but more than 300 series contained information from the 1960s, and 34 series contained a complete record since 1920. Comparisons of the spatial variability of several parameters describing the daily precipitation characteristics were made. The results showed that the final database had improved spatial coherence. The process described here is proposed as a model for developing a standard procedure for the construction of databases of daily climate data.

This work has been supported by the following projects: CGL2005-04508/BOS (financed by the Spanish Commission of Science and Technology and FEDER), PIP176/2005 (financed by the Aragon Government), and ‘Programa de grupos de investigacion consolidados’ (BOA 48 of 20-04-2005), also financed by the Aragon Government.

Peer reviewed