Hydrological climate change impact assessment is generally performed by following a sequence of steps from global and regional climate modelling, through data tailoring (bias-adjustment and downscaling) and hydrological modelling, to analysis and impact assessment. This “climate-hydrology-assessment chain” has been developed with a primary focus on applicability to a medium-sized rural basin, which has been and still is the main type of domain investigated in this context. However, impact assessment is to an increasing degree being performed at scales smaller or larger than the medium-sized rural basin. Small-scale assessment includes e.g., impacts on solute transport and urban hydrology and large-scale assessment includes e.g., climate teleconnections and continental modelling. In both cases, additional complexity is introduced in the process and additional demands are placed on all components involved, i.e., climate and hydrology models, tailoring methods, assessment principles, and tools. In this paper we provide an overview of recent progress with respect to small- and large-scale hydrological climate change impact assessment. In addition, we wish to highlight some key issues that emerged as a consequence of the scale and that need further attention from now on. While we mainly use examples from work performed in Europe for illustration, the progress generally reflects the overall state of the art and the issues considered are of a generic character.

The effects of climatic changes, progressing urbanization and improved environmental controls on the simulated urban stormwater quality in a northern Sweden community were studied. Future scenarios accounting for those changes were developed and their effects simulated with the Storm Water Management Model (SWMM). It was observed that the simulated stormwater quality was highly sensitive to the scenarios, mimicking progressing urbanization with varying catchment imperviousness and area. Thus, land use change was identified as one of the most influential factors and in some scenarios, urban growth caused changes in runoff quantity and quality exceeding those caused by a changing climate. Adaptation measures, including the reduction of directly connected impervious surfaces (DCIS) through the integration of more green spaces into the urban landscape, or disconnection of DCIS were effective in reducing runoff volume and pollutant loads. Furthermore, pollutant source control measures, including material substitution, were effective in reducing pollutant loads and significantly improving stormwater quality.

Implications of three sewer pipe materials (concrete, galvanized corrugated steel, and polyvinyl chloride (PVC)) for stormwater quality were explored in laboratory experiments, in which three types of stormwater, SW1–SW3, were circulated in 0.5 m long sewer pipe sections. SW1 and SW2 represented synthetic rainwater, without and with fine street sediment added (C_TSS = 150 mg/L), respectively, and SW3 was actual stormwater with the same sediment addition as SW2. Following 20-min runs, with an equivalent distance of 500 m travelled by water particles, a number of statistically significant changes in the stormwater chemistry were observed: (i) pH of all the simulated stormwaters increased in the concrete pipe (from 7.0–7.3 to 8.1–9.3), (ii) turbidity decreased in two stormwaters with sediments (SW2 and SW3) in concrete and galvanized corrugated steel pipes (by 50 and 85%, respectively), (iii) the type of stormwater affected the observed copper (Cu) concentrations, with Cu_diss concentrations as high as 25.3 μg/L noted in SW3 passing through the PVC pipe, and (iv) zinc (Zn) concentrations sharply increased (Zn_tot = 759–1,406 μg/L, Zn_diss = 670–1,400 μg/L) due to Zn elution from the galvanized steel pipe by all three stormwaters. Such levels exceeded the applicable environmental guidelines.