Relevance of inflows on the thermodynamic structure and on themodeling of a deep subalpine lake (Lake Maggiore, NorthernItaly/Southern Switzerland)

tAtmospheric exchanges largely dominate the heat budget of deep lakes in temperate regions. Heat importand export by through-flows is of much lower entity and has been neglected or simplified in many numer-ical thermodynamic models of lakes. This is often due either to the unavailability of data for inflows andoutflows, or to the difficulties in forecasting the evolution of their discharge and temperature in cli-mate change studies. While disregarding through-flows may seem correct, riverine intrusions can bringwarmer water than the deep hypolimnetic one to the lower metalimnion and upper hypolimnion, wheresunlight does not penetrate and mixing is poor. For holomictic lakes with significant inflow contribu-tions, this can affect the thermal structure at intermediate depths, hampering any numerical modelwhich neglects through-flows. This study focuses on a relevant basin under such aspect, Lake Maggiore(Northern Italy/Southern Switzerland), which drains the rainiest watershed of the Southern Alps. First, wequantify to what extent a one-dimensional fixed-level model ignoring through-flows is able to predict theobserved evolution of the thermal structure of the lake and the improvements resulting from reproducingthe main inflows and outflows. Then, we directly discuss the influence of through-flows on the thermo-dynamic structure of Lake Maggiore. The General Lake Model (GLM) was here adopted, reproducing the1998–2014 period, spanning years with different meteorological and hydrological features. Results showthat a calibrated enclosed-lake model can give satisfactory results only if it employs an unrealisticallylow light extinction coefficient to allow heating of the deep metalimnion and hypolimnion, whose realwarming strongly depends on interflows.