*2.1. Greece and Its Catchments*

For several reasons, Greece is a relevant and useful case example for the present investigation. The country constitutes in itself the major part of a regional catchment draining into the Ionian and Aegean parts of the eastern Mediterranean Sea (Figure 1). Through Greece, this regional catchment is more or less peninsular and its long coastline facilitates selection of various nested coastal catchments of different scales within it, for comparative multi-catchment investigation using different parts of the available data for the total regional catchment.

Furthermore, previous comparative multi-catchment studies have shown that hydro-climatic changes and associated freshwater interactions with the atmosphere are subject to particularly large uncertainties in Greece [21]. These include: (a) uncertainties due to observation data limitations in terms of both temporal extent and spatial coverage across the country; and (b) climate model uncertainties for freshwater conditions, especially for model estimations of *ET* and, as a consequence, also of *R* and other related freshwater conditions in the Greek landscape for the reference period 1961–1990 [21]. These uncertainties make Greece a particularly important and useful case for investigating a possible robust methodology for catchment-wise data synthesis and interpretation and testing if the constraints implied by catchment water balance decrease the combined uncertainty effects for freshwater conditions and their changes.

Finally, in addition to climate change, Greece has undergone considerable agricultural irrigation developments over the study period 1930–2009, which is chosen due to availability of different types of relevant data for Greece over this period. Previous studies have reported such developments to be important drivers of freshwater changes in different parts of the world [7,8,17,22–24]. The occurrence of such developments over the study period in Greece contributes to making this a relevant case for studying freshwater changes and their interactions and co-evolution with such human-driven developments as well as with climate change.

**Figure 1.** Location and extent of the nested study catchments. These include: the total regional catchment (coloured) and within it the catchments of: Mainland Greece (red outline) and Peloponnese (black outline); and of the Ionian Sea (green area) and the Aegean Sea (light blue area). Data-wise, the Mainland catchment includes three local catchments (blue outlines) with reasonable openly reported runoff data series. The Peloponnese catchment includes three water managemen<sup>t</sup> districts and associated local catchments (blue outlines) of Northern Peloponnese (NP), Western Peloponnese (WP) and Eastern Peloponnese (EP) with available temporal average runoff and other freshwater data.

Overall, freshwater conditions, interactions and changes across Greece are here quantified and interpreted from the available reported data on hydro-climate and irrigated land-use and water-use for the period 1930–2009. To distinguish freshwater changes and associated uncertainties that may have been driven by the atmospheric climate change and/or the irrigation development occurring over Greece during this period, we compare long-term average conditions in relevant water-related variables between two 20-year sub-periods, 1930–1949 and 1990–2009, for five nested catchments of different scales (Figure 1). The two comparative sub-periods are chosen long enough to represent long-term average climate (and not just temporary weather) conditions and with as long time between them as possible for capturing freshwater changes from an early time with relatively small human influences on climate and irrigation to the recent time with expected much greater such influences.

With regard to the five catchments, the largest one is the total regional catchment (coloured in Figure 1; 178,984 km2) that includes the whole of Greece and drains into the eastern part of the Mediterranean Sea through different coastline stretches. In addition to Greece (without its numerous islands, which are not considered in this study), this total catchment also includes some parts in the north that extend into the territories of the former Yugoslav Republic of Macedonia, Bulgaria, Turkey and a minor part of Serbia. This regional catchment is further divided into: the catchment of Mainland Greece (red outline in Figure 1; 157,550 km2) and that of Peloponnese (black outline in Figure 1; 21,434 km2); and the catchment draining into the Ionian Sea (green area in Figure 1; 31,958 km2) and that draining into the Aegean Sea (light blue area in Figure 1; 147,026 km2). These catchments are identified from reported watershed boundaries and associated vectorized river networks at 15 arc second by 15 arc second resolution [25].

Data-wise, the Mainland catchment includes three local catchments (blue outlines within the Mainland catchment in Figure 1) with reasonable openly reported data series of runoff (explaining further what is considered reasonable in the data section below). Moreover, the Peloponnese catchment includes three water managemen<sup>t</sup> districts and associated local catchments (blue outlines within that catchment in Figure 1) of Northern Peloponnese (NP), Western Peloponnese (WP) and Eastern Peloponnese (EP), for which relevant temporal average runoff and other freshwater data are available (as described further below).
