5.1. The Effect of Climate Change and Flow Regulation on the Flow Characteristics
In this study, we investigated how the predicted hydro-climatological changes will affect the distribution of fluvial forces and inundated area during typical high-flow situation (MHQ, mean high discharge), and how modifying the regulation standards could alter the climate change impacts in a low-relief river of the Southern Boreal climate area, Southwestern Finland. We focused on typical high flow events, because they repeat regularly and thus may have a great influence on the river morphology at long run [
48,
50]. In the expected future (2050–2079) hydro-climatological conditions, warming is projected to be substantially above the global average in the South Boreal environment [
60]. The warming will and has affected more discernibly wintertime hydro-climatology, especially the earlier flood peaks. In regulated rivers, the climate change impacts may be reduced with suitable changes in the regulation rules. We used hydrologically modeled future flow scenarios and alternative regulation standards developed by the Finnish Environment Institute and ELY Centers [
51]. According to their work, the average discharge in the Kokemäenjoki River will increase from 235 m
3/s at the control period to 242 m
3/s in the climate change period. However, the most common (largest number of days/per year) river discharge of the control period is 100–150 m
3/s while in the climate change period it will be 50–100 m
3/s (
Table 3), and the number of days with very low discharge (less than 100 m
3/s) will increase (
Figure 9). On the other hand, days with very high discharge will increase as well.
In the Southern Boreal area, the timing of the snowmelt peaks will be earlier than before, causing wintertime discharges in the rivers, including the peak values and the total runoff and inflows to the lakes, to increase [
57]. This will affect the regular high flow events in the near future. Based on this study, it seems evident that the climate change-induced increased runoff will expand the inundated areas during typical (MHQ) high flow events and the adaptive regulation rules are not able to eliminate this, even though they could attenuate the impact. The annual mean high discharge may be diminished by up to 4% with the adaptive regulation rules, which will reduce the inundated area by 2–5%, which means up to 320,000 m
2 in our modeling area. The inundated area during an MHQ discharge in the climate change period will be 10% or 13% larger than in the control period if the ECO or REC strategy is used, respectively (compared to 15% with the CURR strategy). Thus, in addition to the ecological aspects, the ECO strategy is also more adaptive to climate change in terms of flooding. In the Southern Boreal climate areas, especially on the flat terrains, the increase in winter temperature zero-crossings, in addition to high discharges and their daily variation due to hydropower production, means more possibilities for frazil ice flood situations and the flood risk will be an increasing concern [
74]. Frazil ice formation in the river can be prevented by lowering the runoff and reducing discharge variation during the freezing period to help ice cover formation [
5,
15,
74]. Thus, the possibility to lower the winter discharges by regulation is essential for that as well, and will be an important adaptive practice in the future in the Southern Boreal climate area. Based on our results, using the ECO strategy would cause smallest MHQ discharges and inundation areas of all the strategies.
Based on our 2D modeling, even though the discharges and inundated areas will increase in the near future, the erosion potential will decrease, with larger differences if the maximum probable water level (MHQ max) is taken into account. The most important factor for reducing the shear forces is the reduced water surface slope. This can be attributed to the change of timing in floods from spring to autumn and winter, when the sea levels during flood peaks is on average higher, which reduces the water surface slope with the backwater effect.
Kämäri et al. [
47] stated that the wintertime riverine sediment loads in the Kokemäenjoki River are expected to double because of increased discharges by 2070–2099. They appraised adaptive actions, such as discharge controlling, could significantly affect the erosional forces. Our results indicate that adaptive regulation (ECO) could reduce the erosional power even more than is expected with the current rules, but its influence is minor and is mainly caused by the reduced discharge. However, as we were studying regular high flow events, the influence of the adaptive regulation may be significant in long term, even though the differences in erosional power were small. Further research is needed on the longer-term effects of different regulation rules and their effects on various flow events.
Lotsari et al. [
59]) stated that the erosion potential will overall increase in the Kokemäenjoki River (they simulated longer reach, ~43 km), they noticed that the lower river reach, which is mostly affected by the sea, will experience decreasing erosional power in the future, if the maximum relative increase in sea water level will actualize. Their results indicate that the high future sea levels would counteract the impacts of increasing discharges on the erosion potential of the river channel. However, many coastal areas of Finland still experience a relative fall in sea level due to the isostatic land uplift [
25] and it is most probable that, by the year 2079, the isostatic land uplift will still exceed the climate change-induced sea level rise in our study area [
63]. No climate change impacts on the sea level was considered in the WSFS model in this study, but based on the results of this study and the studies by Lotsari et al. [
59] and Pellikka et al. [
63], the sea level change will definitely be an important climate change impact on the area affecting flooding in particular. If the sea level rises according to the highest expected scenario (RCP8.5, rise of 0.63 m by years 2081–2100) [
63], it would increase flooding in the lower Kokemäenjoki River already during the climate change period of this study. Our study indicates that timing of flooding may possibly counteract with the rising sea level in increasing flooding on the area. In addition, the short-term sea level variations and expected changes in sea level extremes, such as weather-induced events, should be considered in addition to the long-term mean sea level changes [
63].
It has also been stated that the hydropower generation could play an important role in the climate change adaptation of water resource availability, but attention is needed to mitigate the environmental and social impacts in the changing climate [
16]. Based on our study, the demands of environment, society, and hydropower generation are not necessarily contradictory in terms of climate change adaptation. Northern Europe belongs to the area of increasing runoff and hydropower generation: an overall increase in river flow and earlier spring peak flows are expected [
22,
53]. The hydropower production is expected to benefit from the increased runoff in Kokemäenjoki River as well. Dubrovin et al. [
51] also estimated the change in hydropower potential (kWh) of 10 hydropower plants in the area with the average climate scenario (
Table 2), considering the different regulation rules. They calculated energy (kWh) as a product of hydraulic head, turbine flow, efficiency coefficient, acceleration of gravidity, water density, and number of hours. Based on their study, compared to the control period with the CURR strategy, hydropower production will increase 0.1–1.9% by 2050–2079 depending on the regulation strategy (
Figure 10). With lower snowmelt flood discharges, the CURR strategy is the most productive during the control period, whereas in 2050–2079, the similar decrease of lake water levels in late winter and early spring will force more water to spill due to early spring floods. On the other hand, the ECO strategy allows for higher water levels in spring, which, along with preparation for winter floods (with lower water levels), will notably reduce the spill over in 2050–2079 compared to other strategies [
51]. Thus, in addition to reduced flood risk, the ECO regulation strategy would enable 0.8% and 1.8% higher energy production in the future compared to the REC and CURR strategies, respectively. In terms of hydropower production, the REC strategy is between the two other strategies in the climate change period. It should be noted that the economic value of hydropower depends not only on the amount of energy but also on the energy need, that is, the timing of the production. In general, the energy price is higher during the most energy-intensive cold season. The short-term (weekly or daily) regulation, which is conducted at the hydropower plants in the study area to correspond to the daily fluctuations in the energy demands, was not included in the model of Dubrovin et al. [
51]. Sources of uncertainty include assumed constants (head and efficiency coefficient) in addition to uncertainties in climate scenario, hydrological model and regulation model.
5.2. Uncertainties in the Methodological Approach
There are several issues in the methodological approach applied in this study, which affect the results and cause uncertainties, and which the reader should be aware of. These include the choice of the emission and climate scenario, the single hydrological scenario used for further analysis, the method used to transfer the climate signal to the hydrological model and the hydrological model structure and parameters [
21,
24]. In addition, the choice of the regulation options and their estimated impacts on ecology and recreation, as well as the hydrodynamic model, contain uncertainties.
The climate scenarios, applied in this study, are based on regional climate model data from the ENSEMBLEs data archive [
77]. These data use SRES (Special Report on Emission Scenario) A1B scenario [
89] The ENSEMBLES data were used since they were available at the time when the hydrological modeling work began, several years ago. Since then, RCM data from EURO-CORDEX using RCP (Representative Concentration Pathway [
90]) scenarios have become available [
91]. The main differences between these data are different emission scenarios, higher resolution (0.11° in EURO-CORDEX and 0.25° in ENSEMBLES) and in part newer versions of the RCMs and/or GCMs [
91]. However, the higher resolution brings the greatest improvements in regions with substantial topographical variability [
92], and therefore in the flat Kokemäenjoki watershed (elevations ~0–200 m) the impact was considered minor. Furthermore, based on the study by Ruosteenoja et al. [
56] there are only minor differences between the climate change predictions in Finland based on RCP and SRES emission scenarios and those differences are mostly in the summer temperatures. The range for temperature and precipitation change in 2050–2079 (from 1985–2014) with the seven RCM scenarios with A1B used in this study was 1.6–3.5 °C and −1.1–13.5%, respectively (
Table 2). The range of ensemble means of the four RCP scenarios in 2050–2079 provided by Ruosteenoja et al. [
56] (with reference period 1981–2010) are 1.9–4.0 °C for temperature and 6–13% for precipitation. The range of the climate scenarios we used is therefore rather similar to the range with ensemble means of the different RCP scenarios. For this study, we selected for further analysis one climate scenario, which was considered to represent the average projection from the range of scenarios. In 2050–2079, this average scenario corresponds rather well to the mean of RCP scenarios 4.5 and 6.0 [
56]. The use of one climate scenario means that the uncertainties of climate change are not modeled in the CDF analysis and the possible range of changes in fluvial forces and inundated areas is larger than projected by our results.
The delta change approach was used to transfer the monthly changes in temperature and precipitation to the hydrological model. This method capability to estimate floods has been criticized, since it does not take into account changes in distributions and hence different changes in extreme precipitation [
44,
79]. However, in this study, we investigated an average high flow event, which is not very extreme. Additionally, the properties of the watershed with large area and many lakes means that the flood events are not typically produced by a single precipitation event, but rather by snowmelt or prolonged periods of heavy precipitation. The direction of changes in floods in Finland with the delta change method and other methods have been found to be mostly similar [
24]. Therefore, the delta change method is likely to provide adequate results for this study.
Further uncertainties for the methodological approach brings the cascade of the different simulations, because of the non-linearity of the climatic, hydrological and geomorphic systems [
93,
94]. The climate model, together with the applied emission scenario, affects the air temperature and precipitation results. These temperature and precipitation further impact on the hydrological model’s discharge and water level outputs, which are also affected by the hydrological model structure and parameter uncertainties. Modeling of each regulation strategy also involves uncertainty: the same set of regulation rules, which is applied every year, is not able to regulate all individual years optimally.
The errors originating from hydrodynamic modeling are mainly related to the quality of the initial and boundary conditions, and the spatial resolution, accuracy, and precision of the topographic and bathymetric data. In addition, the model resolution affects the level of detail. To assess the hydrodynamic model performance, we performed four validation runs with different flow conditions. The mean error of the four validation runs at the calibration data point was 0.045 m and the maximum error was 0.07 m. Considering the relatively low grid resolution (5–15 m in the channel), and the point density of 10 m in the original topographical data (which resulted in an average absolute error of 0.29 m due to interpolation into the computational grid), the model performance was considered adequate in the validation process. Errors caused by the hydrodynamic modeling may have an effect, especially on the inundation area on the flat terrain. However, the effect of error should not be significant, when investigating the relative differences of the high flow events in the reference period and future. In addition, some of these uncertainties produced by the previous simulation steps propagates also to the CFD analyses, where the discharge is applied as the input value of the upstream boundary. However, the uncertainties in climate change impacts are not modeled in the CDF analysis, since results from only the average scenario are used.
In this study, we tried to minimize these cascading uncertainties with the detailed possible calibration of each model applied. In hydrological climate change impact assessments, the uncertainties from choice of GCM are usually found to be larger than uncertainties from hydrological model structure or parameters [
21]. Since the uncertainties and the propagation of uncertainties remain large, the decision makers should be made aware of these uncertainties.