For aquatic biological processes, diurnal and annual cycles of water temperature are very important to plants as well as to animals and microbes living in the water. An existing one-dimensional model has been extended to simulate the temperature profile within a small water body. A year-round outdoor experiment has been conducted to estimate the model input parameters and to verify the model. Both model simulations and measurements show a strong temperature stratification in the water during daytime. Throughout the night, however, a well-mixed layer starting at the water surface develops. Because the water body is relatively small, it appears that the sediment heat flux has a strong effect on the behaviour of the water temperature throughout the seasons. In spring, the water temperature remains relatively low due to the cold surrounding soil, while in autumn the opposite occurs due to the relatively warm soil. It appears that, in small water bodies, the total amount of incoming long wave radiation is sensitive to the sky view factor. In our experiments, the intensity of precipitation also appears to have a small effect on the stratification of the water temperature.