Available online at www.sciencedirect.com ScienceDirect Procedia Engineering 162 (2016) 145 – 152 International Conference on Efficient & Sustainable Water Systems Management toward Worth Living Development, 2nd EWaS 2016 New method for estimation mean hydrological changes and question of reliability in forecasting future hydrological regimes Dejan Dimkiüa,* a Jaroslav ýerni Institute for the Development of Water Resources, Jaroslava ýernog 80, 12226 Belgrade, Serbia, Abstract Important hydrological changes are observed in Serbia, as well as in many parts of the world Many national and international projects and studies address climate change and its impact on water resources Some focus solely on the impact of climate change, while others also assess the impact of land use changes and/or changes in human use of water Due to differences in climate change and human activities, the different climate and hydrological trends are obtained for the different regions in Serbia This paper presents the past longterm temperature, precipitation and river discharge trends across Serbia Second, more important aim of the research, is to assess and forecast average relationships between an increase in air temperature and changes in river discharges and precipitation This relation could help us to find appropriate regional climate and hydrological models Even within a single catchment, depending on the regional models used, the initial and other assumptions made, and the impacts assessed, the differences between the results can be small but often significant, too The reliability of the projections receives little or no consideration This paper also discusses this topic and attempts to provide some guidelines © by Elsevier Ltd This is an open access article under the CC BY-NC-ND license ©2016 2016Published The Authors Published by Elsevier Ltd (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of the EWaS2 International Conference on Efficient & Sustainable Peer-review underManagement responsibilitytoward of the organizing committee of the EWaS2 International Conference on Efficient & Sustainable Water Systems Worth Living Development Water Systems Management toward Worth Living Development Keywords: Trends; climate and hydrological changes; temperature; precipitation; reliability * Corresponding author Tel.: +381-11-3906-478; fax: + +381-11-3906-481 E-mail address: dejan.dimkic@jcerni.co.rs 1877-7058 © 2016 Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of the EWaS2 International Conference on Efficient & Sustainable Water Systems Management toward Worth Living Development doi:10.1016/j.proeng.2016.11.030 146 Dejan Dimkić / Procedia Engineering 162 (2016) 145 – 152 Introduction Considerable pressures regarding future water supply security, like in many parts of the world [1], [2], are expected in Serbia, given the imminent increase in water demand and decrease in discharge, to a different extent, of all rivers and water resources in the region [3], [4], [5] This paper considers just mean annual values for Temperature (T), Precipitation (P) and River discharge (Q) In 20th and first decade of 21th century, Serbia’s rivers registered a negative trend This led to a large number of projects, studies and papers over the past ten years, which address the problem and future expectations based on certain initial assumptions The question is reliability of such forecastings Analysis of observed changes and some additional investigations, presented here, could help us in finding answer to this question Applied Methodology and Results are related to Serbia, and the same could be done elsewhere To be clearly, second (Methodology) and third (Results) sections of this paper are divided in three parts: x Observed climate and hydrological changes in Serbia x Correlation between air temperature and river discharges and precipitation x The reliability of the Regional climate and hydrology model’s projections Methodology 2.1 Observed climate and hydrological changes in Serbia For assessment of observed average annual changes, it is very important to find the period suitable for analysis Assessment of the spatial distribution of one of the three parameters (T, P, Q) required a sufficient data density (for reducing the stochastic component, which is always present when the number of analyzed stations is small), such that the selected period for analysis in this case was from 1949 to 2006 This period was convenient because it was relatively long (58 years), data were available from numerous monitoring stations, and they exhibited a close similarity to estimated long-term temperature and precipitation trends, and particularly river discharge trends in Serbia [4], [6], [7] To assess past climate and hydrological trends, 26 temperature, 34 precipitation and 18 hydrological stations were selected All the trend charts shown were generated using Surfer software, based on data recorded at analyzed stations, removing the stochastic component by regional averaging [4], [5] Trend for analyzed period (1949-2006) for T and P was adjusted as follows: For each given station, all other available stations at an air distance of up to 100 km were identified Then the distance was caluculated between each of these stations and the given station, for which the climate parameter (temperature or precipitation) trend was to be adjusted Finally, the trend of the given station was determined using the formula: ௌ ܶǤ ൌ ܶ ௌ ή ሺͳ െ ܭሻ భ ೋ భ σ ೋ σ ் ή ήܭ (1) where: Zi – distance of the i-th station from the given station; Ti – (T or P) trend measured at the i-th station; TS – (T or P) trend measured at the given station; K – coefficient dependent on the number and distance of stations within a 100 km radius (0.4 ÷ 0.7) TSkor – adjusted (T or P) trend of the given station Trend for river discharge is not adjusted, it is used as it is Hydrological stations were selected in accordance with written in section 3.1 Dejan Dimkić / Procedia Engineering 162 (2016) 145 – 152 2.2 Correlation between air temperature and river discharges and precipitation Observed hydrological changes told us that there is a downward average annual riverflow trend in Serbia (part 3.1) If temperature continues to increase, what is to be expected with regard to hydrological trends? Common approach (RCM models) try with some scenarios to give us answer In addition to common approach, the good way of arriving at the answer to this question is to analyze what has happened in the past with average annual temperature vs riverflow levels, and it is also useful to establish the same type of correlation between temperature and precipitation [4], [5], [6] The temperature and precipitation stations which are closest to the center of the Catchment Area (C.A.) of a hydrological station were taken as reference stations The analysis included 18 hydrological stations and their associated meteorological stations Relative parameters were calculated for each C.A., for each years during the 1949-2006 period: x Average annual riverflow at a given monitoring site, relative to the mean, Qrel; x Annual precipitation sum recorded at a precipitation station close to the center of C.A., again relative to the mean annual sum, Prel; x Difference, ǻTav, between the average annual temperature and the mean temperature at that station To arrive at relationships, data were grouped into categories according to deviations of average annual temperatures from the mean values for that station, at intervals of 0.5°C Average values were then calculated for each category of temperature deviation, and of the annual discharge and precipitation relative to their mean values, respectively These data were then used to construct graphs of the relationships between the derived values, displaying also the linear and 3rd degree polynomial fit to the composite data shown and the associated coefficient of determination R2 Even though each of the studied C.A exhibits specific features, there is no major difference between them: all show the expected trend of an average decline in riverflow with increasing temperature and vice-versa It is, therefore, fully justifiable to synthesize all relevant data into a single data series This enlarges the data series by 58 members, of each of the analyzed series, to 58x18 = 1044, and decreases the effect of random, non-standard years, especially in classes which otherwise have few data points A synthesis of all data (part 3.2; Figure 2) yielded average values and the derived trends can be considered highly representative for assessing the average temperature impact on river discharges and precipitation in Serbia 2.3 The reliability of the Regional climate and hydrology model’s projections Apart from climate change (CC), the hydrologic regime of a river is affected by changes in land use (LU) within the C.A and changes in the extent and method of human use (HU) of water [8], [9], [10] As a result, some of Serbia’s rivers record a considerable decrease in discharge The discharges of only a small number of rivers have increased, largely due to water transfers from other river basins, which began in the 1970’s and 1980’s All three components are very important and the degree of significance varies very much from C.A to C.A Based on precipitation and temperature trend distributions, the greatest negative trend changes were noted in eastern Serbia But, for stations at some other parts of Serbia, especially where changes in human use of water or in land use are significant, climate change impact is often negligible The same is valid for the future Most of the recent projects are based on different climate models and different scenarios that provide basic inputs for hydrological models A large number of studies based on regional climate-hydrological models address only the first type of change (CC) Neither overestimate or underestimate the importance of particular projects, we have reviewed projects, and all available data on mean annual level from these projects were analyzed [11] Five different groups of experts have analyzed 10 different rivers at 15 different hydrological stations in Serbia and 54 different cases (different scenarios or climate – hydrology model) In just few of them all three impacts on water resources (CC, LU and HU) were analyzed Detailed results are not reported here due to lack of space 147 148 Dejan Dimkić / Procedia Engineering 162 (2016) 145 – 152 Results 3.1 Observed climate and hydrological changes in Serbia The annual average temperature trend in Serbia was found to be about 0.6°C/100 years, while the average precipitation trend was slightly negative In the same period hydrological trend was about -30%/100 years The spatial annual distribution is shown in Figure [4], [5], [12] Serbia, especially its eastern part, experiences a downward riverflow trend However, contrary to climate parameters, it is difficult to spatially generalize because several factors affect these trends [13], [14] : The size of the river, The transfer of water, if any, between C.A.s upstream form a given hydrological station, The volume of water used by man in a given C.A., The presence or absence of river reservoir(s) in the C.A., Land use changes (forest have been generally increased in Serbia last 60 years), and Climate change (including differences associated with geographic locations within Serbia) Fig Recorded annual trends in Serbia (1949-2006), based on (a) T-26, (b) P-34 and (c) Q-18 stations Small rivers are much more stochastic in nature and sensitive to water withdrawal for human consumption, while large international rivers (the Danube, the Sava and the Tisa) often not adequately reflect what happens within one country or region, in this case Serbia As such, medium-size rivers are the most reliable representatives of the overall trend in a particular area (including climate change and human activity), provided that Factor – water transfer between C.A.s – is not significant Factor is dominant at many hydrological stations and such stations need to be excluded from analyses, in order to derive relevant results Factor is significant, and the degree of significance ranges from negligible (small volumes of water withdrawn from large rivers) to dominant (large volumes withdrawn from small rivers), within the framework of the recorded trend A favorable circumstance from a trend analysis perspective is that much more water is used in Serbia for drinking water supply (where relatively accurate data are available), than irrigation (where there are only rough estimates) The existence of a river reservoir upstream of a given hydrological station is generally a negligible or has a small contribution to the overall hydrological trend Exceptions are small rivers, or large river reservoirs relative to the surface area of the C.A Land use changes diver from negligible to very significant 149 Dejan Dimkić / Procedia Engineering 162 (2016) 145 – 152 Climate change is often the most important contributor to the overall hydrological trend Climate change has to date had the greatest impact and resulted in the most distinct recorded downward precipitation and riverflow trends in Eastern Serbia Conversely, only minor change has been noted in Southwestern Serbia, where many rivers exhibit near-zero trends as a result of an upward precipitation trend, but also an upward evapotranspiration trend due to a slightly higher temperature increase in that region An approximate geographical distribution of the downward average annual riverflow trend in Central Serbia is shown in Figure 1, compiled based on the trends recorded at 18 selected river monitoring stations, where Factors and were assessed as having an acceptable degree of impact, and where Factor was either absent or negligible It should be kept in mind that the above hydrological results are given in terms of averages, while the discharge trends for specific C.A.s can be significantly different, both up and down, due to differences in human activities (factors 3, and especially 2) We can see that the direction of the discharge changes in Serbia (decline trend from west to east) is generally in accordance with the forecasts based on IPCC scenario A1B [15], like in many parts of the world [1], [16], [17] For the same period (1949-2006), for Temperature and Precipitation, trend results were checked by Mann-Kendal test Results are grouped depending of the part of the country (Table 1), and it could be seen that they are generally in line with graphs on Figure Table Results of Mann-Kendal test: Trend description for Temperature and Precipitation for different regions in Serbia Serbia’s region ĺ North Serbia West part of Central Serbia Central part of Central Serbia East part of Central Serbia Temperature clearly increasing clearly increasing negligible between unclear and slightly positive Precipitation between unclear and slightly positive clearly increasing between unclear and slightly negative clearly decreasing Temperature deviation category (°C) ǻT av < -1.0°C -1.0 < ǻT av < -0.5 -0.5 < ǻT av < 0.0 All data for 18 C.A 0.0 < ǻT av < 0.5 0.5 < ǻT av < 1.0 1.0°C < ǻTav Relative discharge (average) 1.27 1.11 1.04 1.00 0.96 0.90 0.72 Relative precipiTemperature tation (average) difference (average) 1.09 -1.22 1.05 -0.72 1.00 -0.24 1.00 0.00 1.00 0.22 0.99 0.70 0.88 1.36 1,60 1,60 y = -0,0508x + 0,0141x - 0,1313x + 0,9981 R2 = 0,9986 1,50 1,40 1,40 1,30 Prel Qrel 1,20 1,10 1,00 0,90 0,80 0,70 0,40 -2,0 y = -0,0348x - 0,0021x - 0,0243x + 1,0055 R2 = 0,9703 1,50 1,30 1,20 0,60 0,50 Number of data points (years) 74 148 327 1044 278 123 94 1,10 1,00 0,90 0,80 0,70 y = -0,1997x + 1,0033 R2 = 0,9809 -1,5 -1,0 0,60 0,50 -0,5 0,0 0,5 ǻT av ( °C ) 1,0 1,5 2,0 0,40 -2,0 y = -0,0728x + 1,0015 R2 = 0,9032 -1,5 -1,0 -0,5 0,0 0,5 1,0 1,5 ǻT av ( °C ) Fig Average annual riverflow and precipitation, relative to the average, as a function of temperature deviation (all 18 C.A.s) 2,0 150 Dejan Dimkić / Procedia Engineering 162 (2016) 145 – 152 3.2 Correlation between air temperature and river discharges and precipitation Results of correlation between air temperature and river discharges and precipitation is shown on Figure It should be noted that the coefficient of determination is very high in both graphs, leading to the conclusion that a deviation of the average annual temperature by +1°C has an inversely proportional effect on the average annual precipitation levels of about 7%, and on the average annual riverflow of about 20% The results differ from C.A to C.A., but in most cases this variation is not great If these linear and 3rd degree polynomial trends are extrapolated to +2°C, values derived for relative riverflow and relative precipitation are shown in Table (mean annual level) Table Relative riverflow and relative precipitation as a function of Temperature increasing ǻTav ( °C ) Relative riverflow (Qrel) Relative precipitation (Prel) ĺ 0.5 1.0 1.5 2.0 0.60 Linear trend 0.90 0.80 0.70 3rd degree polynomial trend 0.93 0.83 0.66 0.39 Linear trend 0.97 0.93 0.89 0.86 3rd degree polynomial trend 0.99 0.94 0.85 0.67 This methodology could be basis for the most probable average river discharge assessment (decline) for the near future (30 years) in Serbia, in dependence of the average yearly temperature increasing The same methodology could be applied for many countries and regions, especially for those where the negative annual trend for precipitation is recorded 3.3 The reliability of the Regional climate and hydrology model’s projections An analysis of the results obtained from regional models led to the conclusion that the results vary considerably, even for the same river, especially with regard to the distant future If hydrologic predictions result in a broad range of possible discharges of the same river (extremes of +20% and -40%), clearly the reliability (probability) of the predictions needs to be examined This requires either the selection of certain criteria for assessing the probability of the prediction, which is theoretically possible (e.g probability of adopted climate prediction multiplied by the probability of correctly selected hydrological model), which is rather cumbersome and highly questionable, or expert judgment of the study (project) analyst, which is certainly debatable Instead, we can use the relation explained in the part 2.2 and results shown in the part 3.2 of this paper An important characteristic of this approach is that it takes into account all three changes: CC, LU and HU In order to be applied to individual catchments, it might be useful to produce the same models for a number of catchments and try to arrive at an average for the analyzed region (in this case central Serbia) that is similar to the values of the correlations given on Figure Conclusions The recorded average river discharge trends are about -30%/100 years, and depend on a large number of factors Climate change is only one of the factors that impact water resources The impact of climate change has been noted at all gauging stations, but its significance varies In eastern Serbia, based on precipitation trend distributions, it is generally dominant In some parts of the country it is often not of primary concern, and elsewhere it is minor given the magnitude of other impacts [4], [9] If the average annual temperature were to increase by 2ºC, based on the correlations established to date between average annual river discharges and average annual temperatures, one could expect, on average, approximately half the amount of water in rivers whose C.A.s largely lie within Serbia Even though uncertainty (the consequently unreliability) is inherent in any projection, it is quite certain that it grows with the period of time for which the projection is made Considering all the above, the author believe that all projections that pertain to the distant future (50 or more years) should be deemed purely “academic” and the focus of professional circles and society as a whole should remain on the near future (about the next 30 years), as the average decrease in water resource availability is not only probable but to an extent measurable It is recommended that one Dejan Dimkić / Procedia Engineering 162 (2016) 145 – 152 of the future projects that assess the impact of climate change on water resources in the near future (max increase in average annual temperatures by to 2°C) should include a larger number of national river catchments and seek out climate scenarios and hydrological models that produce average dependencies between river discharges and temperatures at mean annual levels, according to the correlation shown in Figure Who can perhaps benefit from the outcomes of this research? Apart from Serbia, it is believed that the presented results will be of interest to the entire region of Southeast Europe, particularly eastern and southern part of Balkan peninsula Further, the results indicate that an in-depth study of all observed data (above all temperature, precipitation and hydrological data) should be undertaken before a regional model is produced Ultimately, the proposed methodology for the assessment of average temperature impact on average river discharge and precipitation could certainly be applied in many parts of the world, especially in regions where a decreasing precipitation trend is recorded Some general, but in the case of paper’s subject very important remarks are emphasized in closing: ¾ Observed data are extremely important [12], as is continued systematic monitoring in the future ¾ Regional integration is extremely important [3], SEE or the Danube River Basin in the case of Serbia, as is the use of the same approach to produce various maps ¾ Exchange of knowledge, experience and ideas between countries and regions that share the same problem is important (e.g regions that record downward precipitation trends, or sub-arid regions, etc.) ¾ It is important to apply various methods to assess past and predict future climate and hydrological developments If predictions for the near future under RCMs for numerous C.A.s and through correlation and extrapolation of observed data not differ to a large extent, the reliability of such predictions is quite high Acknowledgements This paper is an outcome of the scientific project “Assessment of Climate Change Impact on Water Resources in Serbia” (TR37005), funded by the Ministry of Education and Science of the Republic of Serbia References [1] N W Arnell, Effects of IPCC SRES emissions scenarios on river runoff: A global perspective, Hydrol Earth Syst Sci (2003) 619–641 [2] Y Fujihara, K Tanaka, T Watanabe, T Nagano, T Kojiri, Assessing the impacts of climate change on the water resources of the Seyhan River Basin in Turkey: Use of dynamically downscaled data for hydrologic simulations, J Hydrol 353 (2008) 33–48 [3] Climate Change and Impacts on Water Supply (CC-WaterS) - 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(decline) for the near future (30 years) in Serbia, in dependence of the average yearly temperature increasing The same methodology could be applied for many countries and regions, especially for