Estimating river low flows statistics in ungauged sites

The analysis of low flow indices is carried out on the flow data of 65 consistent hydrometric stations located in Tuscany region, in Central Italy, recorded from 1949 to 2008. The area is subdivided into different regions using the L-moments method applied to indices derived from the flow duration curve (Q70 annual series), to the 7-day annual minimum (Q7,T) series and to the annual SQI, Standardized Discharge Index. The division into subregions is validated using discordancy and heterogeneity tests. Several subdivisions are tested, starting from previous studies on different hydrological extreme values and introducing some hydrological features. For every river section of interest the catchment area is identified and an appropriate set of catchment physiographic and climatic characteristics is defined. A physiographical space-based method is used to relate the duration and annual minimum indices of low flow to the rivers basins characteristics. The new space is built as a power correlation of the catchment geomorphologic and climatic characteristics. In this space several interpolation techniques, either deterministic or geostatistical, such as Inverse Distance, Thiessen polygon methods and Kriging, are applied. The results are valuated using the jack-knife method. Different error measurement (mean square error, mean relative error…) are also assessed to compare the results, to quantify the accuracy of the different techniques and to define the most suitable procedure for low flow regionalization.