**1. Introduction**

Water is a peculiar liquid [1], critical for human existence and all life as we know it. Accurately understanding and being able to predict water conditions changes is among humanity's greatest needs and challenges [2,3]. This does not only apply to the major ocean part of Earth's hydrosphere but also to the essential freshwater on land. Freshwater changes on land interact with other Earth System changes, including changes in climate and in the landscape [4,5] and in both the ecosystems [6] and the societal systems [7–9] of the latter. In recognition of these interactions and critical open questions related to them, new hydrological research directions have developed such as the Panta Rhei community initiative [10] and the fields of ecohydrology [11] and sociohydrology [12].

The freshwater system itself includes various landscape manifestations and formations (streams, lakes, wetlands, water in biota, soil water, groundwater, glaciers, permafrost), with different aspects of variability and change interacting and propagating across them [13–16]. This propagation also involves interactions with different human uses of freshwater and structures for managing these, and all these freshwater interactions prevail and are organized within (the surface and subsurface parts of) hydrological catchments of different scales.

Furthermore, each hydrological catchment of any scale is subject to overarching water balance. In this, the total area-integrated water-fluxes of precipitation ( *P*) and evapotranspiration (*ET*) and the

concurrent water-storage changes occurring over the catchment are balanced with each other and the total runoff flux ( *R*; area-normalized total discharge) from the catchment [17]. This emergen<sup>t</sup> [18] catchment water balance is then also in simultaneous alignment with (i.e., both depends on and determines) and as such couples the different local water manifestations, formations, uses and managemen<sup>t</sup> structures and the conditions of and changes in these over each catchment [19]. However, our ability to understand, interpret and predict these conditions and changes is currently fragmented among various research fields, associated with the different (liquid, frozen) freshwater manifestations and formations (e.g., surface water hydrology, hydrogeology, hydro-meteorology, ecohydrology, limnology, cryospheric science), or the different human uses of and managemen<sup>t</sup> structures for freshwater (e.g., water resources engineering, hydraulic engineering, agricultural water management, urban water management, sociohydrology).

On the one hand, such freshwater fragmentation may be needed for in-depth process study and understanding. On the other hand, it may lead to important knowledge gaps and uncertainties between the fragments. Decrease of these gaps and uncertainties require ability to synthesize and jointly interpret various types of data, related to different freshwater aspects, which interact and coevolve with each other and other landscape and climate changes over hydrological catchments of different scales. For such synthesis and joint interpretation of different data for assessment of overall freshwater conditions and changes in a catchment of any scale, various studies have proposed [6,20] or indicated [4,5,19] that the constraints implied by catchment water balance may decrease the combined result uncertainties compared to the uncertainties associated with different underlying data and aspects.

In this paper, we put this indication to further testing for a case of particularly large underlying uncertainties [21]: Greece and five nested catchments of different scales across it. For these catchments, we investigate freshwater conditions and their changes over the time period 1930–2009, for which relevant data are available. Using the constraints implied by overarching water balance in each catchment, we develop, apply and test a general approach to synthesize, assess and interpret the available data for freshwater changes over the study period, the possible drivers of these changes and associated input-data and result uncertainties. Through the development and specific case quantification and testing of this approach, this study also outlines and uses advancements made in recent research on such long-term, large-scale freshwater changes.
