Reliability Issue in Projections of the Impacts of Climate Change and Other Changes on Water Resources - page 2

What are All the Uncertainties and What Can be Expected in Serbia?

There are many uncertainties associated with hydrologic projections (IPPC 2000, SEPA 2014, IPPC 2007). In the case of regional climate-hydrological models, the uncertainties of assessments of the climate change impact on hydrology can, conditionally, be divided into climate (including climate change) prediction uncertainties and uncertainties of hydrologic regime predictions based on assumed future climate conditions.

The causes of unreliable climate change (climate) predictions should primarily be sought in:

  • the natural complexity and variability of the climate itself, and
  • insufficient knowledge of climate system response to climate change.

The author believes that these two (particularly the second) are likely the most significant uncertainties inherent in predictions of the climate change impact on hydrology, regardless of the approach followed.

The causes of unreliable hydrologic predictions based on assumed climate conditions include:

  • the inability to fully examine the correlation between the climate and hydrology of a given catchment (region),
  • the difference in correlations between present and significantly altered climate conditions, and
  • demographic and other changes in the study area.

In addition to the above uncertainties, apart from those stemming from measurement inaccuracies, the following uncertainties can be associated with regional climate-hydrological models:

  • credibility of the selected climate scenario (assumed GHG increase, assumed economic and social developments, and the like),
  • credibility of the selected hydrological model,
  • non-representative input quantities selected for the given region (e.g. if the base time series reflects 30-year data, significant extremes are often lost, for instance the droughts in England (Fowler et al. 2007) in 1928 and 95-96 where the time series includes data from 1961-90).

Taking into consideration all the previously-mentioned projects, an analysis of the results led to two conclusions:

  1. In all the projects and nearly all case studies, smaller changes are expected in the near future rather than the distant future, which seems logical.
  2. The results vary considerably, even for the same river, especially with regard to the distant future.

Let us address the second finding. 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 a correctly selected hydrological model), which is rather cumbersome and highly questionable, or expert judgment of the study (project) analyst, which is certainly debatable.

Instead, let us look into what can be expected, on average, for Serbia's national rivers in the near future, with a high degree of reliability. Considering research based on recorded data (Institute Jaroslav Černi 2012, Dimkić et al. 2013), Figure 2 shows the relative values of the direct correlation between the average annual temperature and precipitation/river discharge in 18 study areas. An important characteristic of this approach is that it takes into account all three changes: CC, LU and HU.

It is apparent that the coefficient of determination of both plots is very high, which indicates high reliability. This means that, on average, a mean annual temperature variation of +1°C has an inversely proportional effect on annual precipitation of about 7%, and on average annual river discharges of some 20%. The implication is that in the near future, if there are years with mean annual temperatures which are 2°C higher than the average of the past 60 years, 50% less water in Serbia's rivers can be expected (most probable value). As already indicated, these correlations refer to annual averages and the distribution of average annual discharges as a function of average annual temperatures which are similar to Gaussian, which corroborates the correlations even further. If considerably more water is withdrawn in the future for irrigation, or if adequate adaptation measures are implemented, the above correlation could change.

Of course, the results differ between catchments (more significantly in the eastern than in the western part of the country) and the average dependency can only be taken as an indication. 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 achieve an average for central Serbia that is similar to the values of the above correlations.

 

Fig02
Figure 2: Relative values of average annual discharge and precipitation as a function of average annual temperature deviation.