Robust assessments of variations in freshwater availability are essential for current and future water resource management in the Pinios River Basin (PRB), which is one of the most productive basins of Greece in terms of agriculture. To support sustainable water resources management in the PRB, we set up and calibrated the mGROWA hydrological model at a high spatial (100 m) and temporal (daily) resolution for the period 1971–2000, with particular attention given to deriving crop-specific irrigation requirements. We developed and implemented a comprehensive methodological framework to overcome data scarcity constraints in the PRB, thus enabling the derivation of high-resolution spatially continuous estimates of many input variables required for the mGROWA model. We generated estimates of spatiotemporal variations in the water balance components actual evapotranspiration, irrigation requirements, total runoff, and groundwater recharge for the PRB. In addition, through the calculation of indices, such as the potential irrigation to groundwater recharge ratio (PIQR), we demonstrate a way to identify potential unsustainable water use in irrigated agriculture. The established mGROWA model can be used both as a hydrological reference model providing continuous decision support for water resources management, focusing on irrigation water use, and a basis for climate impact studies for the PRB.

The input of nutrients into surface waters and groundwater is directly linked to runoff components. Due to the different physicochemical behaviour of nitrogen and phosphorus compounds, the individual runoff components have different significance as input pathways. Within the scope of the Germany-wide project AGRUM-DE, spatially differentiated runoff components were modelled with the water balance model mGROWA at a resolution of 100 m. The modelled distributed runoff components include total runoff, surface runoff, drainage runoff, natural interflow, direct runoff from urban areas, and groundwater recharge. Although the mGROWA model operates in daily time steps, modelled runoff components can be aggregated to mean long-term hydrologic reference periods—for this study, 1981–2010. We obtained good model agreement through the comparison of measured discharge from 298 river gauges against the spatial means of the modelled runoff components over their corresponding catchment areas. Therefore, the model results provide reliable input for input pathway-specific modelling of actual nutrient inputs as well as scenario analyses expected from the application of nutrient reduction initiatives. This ensures that any differences in the model results stem exclusively from differences in nutrient supply (fertilisation of the soils) and not from climatic effects, such as the influence of wet or dry years.

The concept of water-sensitive cities continues to gain traction globally, as the disruptive effects of urbanisation on local hydrological processes and the potential benefits of green infrastructure become increasingly evident. Despite this, in many planning instances, consideration is only given to how the water balance will be altered and hazard risk reduced from the current urbanised state to the state after implementation of green infrastructure. Why is the understanding of the natural water balance in the pre-urbanisation state often not considered as reference point for planning? If urban green infrastructure should provide hydrological and ecosystem services, should these services be similar to those in the natural condition before urbanisation?For our study, we recreated the daily near-natural water balance for the city of Hamburg to quantify how urbanisation has already affected the water balance, particularly in years of hydrological extremes that represent hydrological hazards. Using the fully-distributed daily water balance model mGROWA, we developed two very high resolution (25 m) models for the city of Hamburg for 1991–2020; one representing the current hydrological situation and one representing a theoretical near-natural situation. To generate the near-natural scenario, the input datasets for topography, soil and land cover were adjusted through the integration of various datasets representing non-anthropogenic conditions, while sealed surfaces and artificially drained areas were removed from the datasets. As expected, due to the lack of runoff from sealed surfaces the actual evapotranspiration is much higher (+40%) in the near-natural scenario than in the current one. Groundwater recharge was also higher in the near-natural scenario (+27%), mainly due to the lack of surface sealing. We then compared the water balance components for the two models against the SPEI meteorological drought index to assess differences in the extremely wet and extremely dry periods that represent potential hydrological hazards. This revealed an increasing divergence in some water balance components between the scenarios for the extreme conditions, quantifying the extent to which the urbanisation of the city has exacerbated hydrological hazard risks. Our study presents a transferable methodology for assessing how urbanisation has affected the natural water balance of a region, which can be used as a starting point for defining targeted solutions for green infrastructure, with the aim of achieving water-sensitive cities.