SIMO v1.0: simplified model of the vertical temperature profile in a small, warm, monomictic lake

Abstract

Abstract. A simple 1-D energy budget model (SIMO) for the prediction of the vertical temperature profiles in small, monomictic lakes forced by a reduced number of input meteorological variables is proposed. The model estimates the net heat flux and thermal diffusion using only routinely measured hourly mean meteorological variables (namely, the air temperature, relative humidity, atmospheric pressure, wind speed, and precipitation), hourly mean ultraviolet B radiation (UVB), and climatological yearly mean temperature data. Except for the initial vertical temperature profile, the model does not use any lake-specific variables. The model performance was evaluated against lake temperatures measured continuously during an observational campaign in two lakes belonging to the Plitvice Lakes, Croatia (Lake 1 and Lake 12). Temperatures were measured at 15 and 16 depths ranging from 0.2 to 27 m in Lake 1 (maximum depth of 37.4 m) and 0.2 to 43 m in Lake 12 (maximum depth of 46 m). The model performance was evaluated for simulation lengths from 1 to 30 d. The model performed reasonably well, and it was able to satisfactorily reproduce the vertical temperature profile at the hourly scale, the deepening of the thermocline with time, and the annual variation in the vertical temperature profile, which shows its applicability for short-term prognostic simulations. A yearlong simulation initiated with an approximately constant vertical profile of the lake temperature (≈ 4 ∘C) was able to reproduce the onset of stratification and convective overturn. The epilimnion temperature was somewhat overestimated, especially with the onset of the convective overturn. The upper limit of the metalimnion was well captured, while its thickness was overestimated. Nevertheless, the values of the model performance measures obtained for a yearlong simulation were comparable with those reported for other, more complex models. Thus, the presented model can also be used for long-term simulations and the assessment of the onset and duration of lake stratification periods when water temperature data are unavailable, which can be useful for various lake studies performed in other scientific fields, such as biology, geochemistry, and sedimentology.

Effect of climate change on stratification of a small, monomictic, temperate, karstic lake (Lake Kozjak, Croatia)

“…However, in recent years, no freezing over was observed of the lake (Plitvice Lakes National Park, personal communication). Moreover, continuous observation of the lake water temperature pro le from 2018 to 2020 also indicated that the lake had started to become monomictic [33], most likely as a result of climate change.…”

“…For further details on the model and on calculating the heat ux terms, turbulent thermal conductivity, and light attenuation, please refer to Šarović et al ( 2022) [33]. In that research [33], the model was run without lake-speci c calibration to show its general applicability.…”

“…Additionally, in that research [33], the Lake Kozjak model was discretized using 16 layers corresponding to the measuring depths of the available data, with a vertical resolution that decreased with depth. In this study, the resolution was kept at a minimum of 2 m, even in the deeper layers, to more precisely capture the strati cation process and deepening of the thermocline.…”

“…An altitude difference correction assuming adiabatic lapse was applied to the air temperature values before interpolation. The shortwave radiation data were used for calculating the daily radiation pro le only, while the actual total daily shortwave irradiation was calculated by SIMO v.1.0 as described in previous research [33]. The calculated shortwave radiation used as an input in the water temperature prediction model is shown in Fig.…”

“…For the prediction of the vertical temperature pro le in Lake Kozjak, a simple one-dimensional model for monomictic lakes SIMO v.1.0 was used [33]. This model is based on the one-dimensional energy balance equation, assuming a constant horizontal cross-sectional area of the lake:…”