## 1 Introduction  
Continuous civilisational development directly contributes to the transformation of the natural environment, and its particular components respond to the process in different ways. The transformation is evident in reference to the hydrosphere due to the properties of water, with a high capacity for the accumulation and transfer of energy and matter. As a consequence, it is important in reference to water resources and water quality. One of the most elementary water properties is its temperature, which considerably determines the course of all processes that occur in water. Considering the currently observed [1, 2] as well as forecasted climate changes [3, 4], as well as the close relationship between air and water temperature [5, 6], knowledge on these relationships is particularly important. Numerous studies point to an increase in the temperature of inland waters [7, 8], which plays and will continue to play an important role in the transformation of their properties. The proper assessment of the distribution of its characteristics, and particularly the direction and rate of changes, is possible based on an appropriately organised monitoring system. This adopts various forms, both in reference to measurement methods and frequency [9, 10, 11]. In recent years, the role of remote sensing techniques has been increasing, offering, among others, possibilities of capturing thermal differentiation [12, 13, 14]. The role of traditional field measurements, however, is still of particular importance [15]--both in the context of their accuracy and providing the basis for the validation of satellite images. Considering the above, the lack or loss of data referring to water temperature in rivers and lakes can be of high importance for the management of these ecosystems--particularly in reference to biodiversity, oxygen conditions (and consequently for their self-cleaning capacity), and finally their quality. The loss of data can have different causes, both instrumental and administrative. Sensors collecting data in situ are susceptible to technical errors, resulting in failures to record observations and/or anomalies, and therefore in gaps in data [16]. In many parts of the globe, primarily due to the shortage of financial resources, the information collected on water temperature and the monitoring of other quality properties is limited [17; 18]. Economic conditions have been the cause of changes in the observation network in Poland. In the case of rivers, records of water temperature were discontinued in some of the observation sites supervised by the Institute of Meteorology and Water Management--National Research Institute at the end of 2014. The decision was determined by limitations of sufficient financial resources for the functioning of the hydrological-meteorological measurement observation service in the scope of the institution's activity. As a consequence, many rivers have lost valuable data that had been recorded in detail for at least several decades. The situation is particularly unfavorable in the context of further transformations of these ecosystems. Based on research conducted in Mackenzie and Yukon Basins, [PERSON] and [PERSON] [19] determined that mean monthly water temperature shows an incoherence between sites located in the upper and lower courses of the rivers. This specifically results from the differences in the data collection systems. As emphasised by [PERSON] et al. [20], the understanding of trends of changes in river temperature depending on the climate is limited by the data alone, and future improvements in forecasting changes will require the development of a network of monitoring sites offering better spatial and temporal representativeness. Due to the above, an increase in the density of the current observation sites should be expected instead of their complete liquidation. Based on such an assumption, with consideration of the maintenance of the current state of information, research has been undertaken towards the reconstruction of records lost to date.