Climatic tendencies on the Trasimeno lake area and effects on the lake system

This work presents an analysis of climatic tendencies in the Trasimeno lake area, (Umbria, Central Italy) and of their influence on the lake system. The Trasimeno Lake is one of the largest Italian lakes by surface area; its catchment is mainly occupied by turbidite formations belonging to the Tuscan and Umbria-Marche units, the permeability of which can be considered, in overall, from low to very low. In some areas, turbidite deposits are overlaid by Holocene alluvial deposits hosting low transmissivity aquifers draining towards the lake. The analysis of a temperature data set (1963 - 2014) showed that annual temperature is increasing, in accordance with what is known for Central Italy and the Mediterranean area, with a gradient around 0.03 °C/year. However, during the last 15 years there is no significant temperature increase. Five rainfall time series (1963-2014), from stations located within the lake catchment or in its surroundings, were analyzed. Climatic tendencies were defined on such stations, and data were used to generate a time series representing the areal rainfall over the catchment. Although it is known that, in the Mediterranean, area, rainfall is generally decreasing, only one of the analyzed stations has a significant negative linear annual rainfall trend (-3.7 mm/year). This trend is due to rainfall reduction in winter and summer. The SPI index, computed on a monthly scale on all the rainfall data-sets and on the series representing the areal rainfall over the catchment, showed that both frequency and duration of extreme and severe draughts have increased in the last decades. Frequency and duration of unusually wet periods are also increasing. Correlations between rainfall and SPI values and both lake and aquifers levels were studied. The analyzed data have shown that there is a good linear relationship between annual rainfall and annual lake level variations. Lake levels variations were also correlated with SPI values, computed on different time-scales. Relationships between rainfall, SPI and groundwater levels are unclear. Apart from uncertainties, the results show that, in the framework of a changing climate, even in the case of long term rainfall without a trend, a change of the variance can have strong impacts on a water system.