- predict the likely proportional change (%) in the annual runoff available for the downstream country; and
- anticipate the standardized Reconnaissance Drought Index (RDIst) and SDI values due to a wide range of possible future climate change.
2. Materials and Methods
2.1. Meteorological Drought Severity (MDS)
2.2. Streamflow Drought Index
|Extremely wet||(SDI and RDIst) ≥2.00|
|Very wet||1.5 ≤ (SDI and RDIst) < 2.00|
|Moderately wet||1.00 ≤ (SDI and RDIst) < 1.50|
|Near normal||−1.00 < (SDI and RDIst) < 1.00|
|Moderately dry||−1.50 < (SDI and RDIst) ≤ −1.00|
|Severely dry||−2.00 < (SDI and RDIst) ≤ −1.50|
|Extremely dry||(SDI and RDIst) ≤ 2.00|
2.3. Rainfall-Runoff Model
2.4. Methodological Approach
- The Medbasin-M rainfall-runoff model was calibrated using the monthly P and PET data for the period of twelve hydrologic years (1962−1973) for which unimpaired streamflow data were observed and used for model calibration. The climatic conditions for the calibration period were mostly near to normal, with some years characterised by moderately wet or moderately dry conditions (Table 1). This allowed the unbiased calibration of the model. About 75% and 92% of the annual precipitations during the calibrated period were between the mean −SD and the mean +SD, and between mean − 2SD and mean + 2SD, respectively. The minimum, mean and the maximum annual precipitations were 520 mm, 729 mm and 1,187 mm, respectively. The SD was approximately 201 mm. The annual precipitation observed in 1969 was interpreted as an outlier. No other outliers were observed over the calibration time interval. The minimum, mean and maximum PET values were 1,215 mm, 1,287 mm and 1,364 mm, respectively. The SD was about 58 mm. No outliers were observed.
- The simulation results were validated using monthly climatic data (P and PET) and the unregulated observed runoff for a period of nine water years (1974−1982). Some statistical goodness-of-fit tools were employed for calibrating the model and validating the simulation results. These measures are r, RMSE, MAE and IoA ([34,35] for the latter). The following formulas (Equations (8) to (11)) were applied:
- The RDIst values were calculated over a time window of 30 years between 1962 and 1991. This was performed to specify which period represents nearly the normal climatic condition (on average) and to assess the constants a and b of the linear Equations (12) and (13):
- The Medbasin-M model was used to compute the reference mean annual runoff for the normal climatic condition.
- The Fifth Assessment Report of the IPCC  suggests that the region where the examined basin is located is likely to face a decline in precipitation amount and an increase in mean air temperature as part of climate change. The IPCC  highlighted that the annual average river runoff availability is projected to decrease by between 10% and 30% over some dry regions at mid-latitudes by 2050. Some impacted regions presently have water-stressed areas. Furthermore, an increase in drought spells is also projected for mid-latitudes. The IPCC reports also point out with high confidence (defined by IPCC) that climate change has the potential to exacerbate water resource stresses in most regions of Asia. The regional projections of temperature and precipitation in Asia based on a so-identified A2-forced emission scenario using the Atmosphere-Ocean General Circulation Model (AOGCM) simulations show that the rate of decrease in precipitation could reach −40% in winter (between December and February) and −50% in summer (between June and August). The increase in temperature would be in the order of +10% in winter and +6% in summer. It should be noted that these predictions should be considered as valid until the end of the 21st century. The synthetic scenarios for assessing the runoff sensitivity to climate change were formulated through an incremental shift of the historical P and PET values by a 2% step for a P reduction range from 0% to −40% and a PET increase from 0% to +30%. Correspondingly, 336 scenarios were developed, representing the mutual impact of deviations in P and PET values that lie within the aforementioned assortment of scenarios. These scenarios include all possible basin-wide climate change projections, as well as a wide array of drought severity conditions. The Medbasin-M model was repetitively used to simulate the runoff for the 336 scenarios.
- The anticipated proportional change in the annual runoff (%), corresponding to each scenario, was determined relative to the reference mean annual runoff.
- A nomograph was used for estimating the projected proportional change (%) in the climate-impacted runoff. The predictable proportional changes in the runoff represent the anticipated runoff reductions (%) relative to the long-term unimpaired mean annual runoff under the normal climatic condition.
2.5. Study Area and Data Availability
3. Results and Discussion
3.1. Model Calibration and Simulation Results after Validation
3.2. Anticipated Runoff for Various Climatic Scenarios
4. Conclusions and Recommendations
Conflicts of Interest
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