Rain occurrences and Po River flows during two centuries

In the Po Valley in Northern Italy, historical rainfall series have been available for almost 200 years, and the discharges of the river Po in its section at Pontelagoscuro have been recorded since 1807.The trend of the ratio between these two can be used as an index of the urbanization and of the evolution of land use in the catchment. Some of the historical rain gages with the longest data series in Europe are at work in this region. At the Brera Observatory in Milan, daily air temperatures and meteoric precipitations have been recorded since 1764 and in the Po River watershed six rain gages started monitoring data before the year 1850. To have a uniform historical series, only the data from these six stations have been used to calculate the total monthly rainfall over the whole Po watershed, applying a Thiessen grid for the extension of local measurements. This procedure is at a first level approximation, but has shown to be satisfactory when used to compare the yearly rainfall with the total runoff of the watershed. The Po drainage basin, in the section of Pontelagoscuro (the last section of the river before the delta region), is about 70090 km2. The annual runoff coefficient of the Po River, defined as the ratio of runoff over rainfall over the time period of a year, has been studied to evaluate a possible trend during these two centuries. The layout of the basin at a latitude of 45° North, along with the fact that the river runs exactly from West to East, and finally its surface, not too small, are all important characteristics that make this drainage basin one of particular interest in the study of the hydrologic change in the Northern Hemisphere.