Regional Analysis for Rivers Low Flow Statistics Estimation

Estimates of river low flow characteristics are needed for several purposes in water resources and environmental management, including water supply planning, river basin management, drought mitigation, hydropower development and environmental flow availability definition. Low flow regime is closely dependent on the catchment area hydrogeological features but on a practical perspective, although scientifically proven, statistical analysis is widely applied to derive indices to characterize low flow regimes. Indices are commonly evaluated at gauged sites from observed streamflow time series. To improve their reliability, often affected by the lack of observed data and to estimate low flow statistics in ungauged sites it is possible to refer to regional statistical analysis. The method employs catchment area and climatic characteristics, as independent variables, and data from other catchments where stream flow data are recorded. The analysis of low flow indices is carried out on the discharge data of 65 consistent hydrometric stations located in Tuscany region, in Central Italy, from 1949 to 2008. The area is subdivided into different regions using the L-moments method applied to the 7-day annual minima and to the Q70 annual series. The subdivision is tested using discordancy and heterogeneity statistics. For each river section, 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 indices to the investigated territory characteristics. The new space is built as a power combination 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 measurements 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.