Climate Change Impact
Part 6: Example – Tejo-Tajo-Tagus
The Tagus (Tejo or Tajo) is one of the most important rivers in Europe. It rises in Spain and flows through Portugal to the Atlantic Ocean. The river basin is highly developed with several large dams and abstractions for irrigation and urban water supply. Data on reservoir storage and abstractions were used to estimate the natural flow of the river. A hydrological model of the river (HYSIM) was calibrated to observed flows. The calibrated model was then run with projected climate changes. The conclusions were that flows in the river would reduce but that the changes associated with human activity were of a similar order of magnitude.
The Tagus River (in English, Tajo in Spanish and Tejo in Portuguese) rises in the hearth of Spain and flows into the Atlantic close to Lisbon. The total basin area is 80,000 km2 and the length of the river is 1060 km.
The general approach to hydrology and water resources of the Tejo/Tajo basin was to calibrate hydrological model (HYSIM) which uses precipitation and potential evapotranspiration as input data and flow data for model calibration. Once the model had been calibrated the observed precipitation and climate data could be replaced with projected values related to climate change scenarios and the impact of climate change on river flows assessed.
|Figure 1 shows the Tejo/Tajo basin and the principal flow stations.|
The Tejo/Tajo is a highly developed basin and this development has been ongoing for several decades. The total reservoir storage is 9 billion m3 in Spain and 11 billion m3 in Portugal. For comparison, the average natural flow at Alcantara is 6.5 billion m3/year.
The simulation started with Spain and then moved on to Portugal. Some of the data sources had data for both countries and others were only for one country.
In Spain, comprehensive climate and flow data were available from a variety of web sites run either at a national or European level. These included precipitation and the climate parameters necessary to calculate potential evapotranspiration. Data were also available for Madrid water supply, one of the principal users. Annual data were available at province level for irrigation.
The following shows the cumulative effect of artificial influences on flow.
|Figure 2 River Tajo at Alcantara - Spain - artificial influences on flow|
The red line shows the influence of reservoirs. Basically, during winter, the reservoirs fill and during summer water is released – principally for hydropower. The Blue line shows the effect of irrigation with abstractions in the summer. The green line represent others uses of water, principally for urban populations. These data, combined with the observed discharge of the Tajo at Alcantara, were used to calculate the natural flow in the river.
The following chart shows the simulated and naturalised daily flow for the River Tajo at Alcantara.
|Figure 3 Simulated and naturalised flow - River Tajo at Alcantara - Spain|
Simulation of the naturalised flow was not straightforward. Normally peak flows have a major influence on the calibration but in this case the effects of storage masked some of the variation. However, the general pattern of low and high flows is well represented.
For the Tejo river basin in Portugal data on flow, climate, storage and abstractions was also available at the locations in figure 1. The station furthest downstream with flow data for the Tejo is at Almoural. To calculate the naturalised flow at this point it was necessary not only to include the influence of reservoirs and abstractions in Portugal but the cumulative effect including those in Spain as well. The following chart shows the simulated and naturalised flow.
|Figure 4 Simulated and naturalised flow - River Tejo at Almoural - Portugal|
Considering the many factors that went into simulating flow, the accuracy was good. The correlation coefficients were 0.91 for daily flows and also 0. 91 for monthly flows. The simulated and observed means were 353 m3/s in both cases. The simulated and observed standard deviations of daily flows were 717 and 716 m3/s and for monthly flows were 550 and 563 m3/s.
To examine the effect of climate change six scenarios were considered: A1B, A1B max, A1B min, A2 and B1. The A1B is considered the most probable projection.
|Figure 5 Flow duration curve - observed and project flow - River Tajo at Alcantara|
A flow duration shows the percentage of the time that a flow was higher than a given value. On this curve, the flows are plotted using a logarithmic axis. Some inferences which can be drawn from this graph are:
- Downstream of Alcantara flows are zero for more than 15% of the time.
- The difference between the natural and observed flow is of a similar order of magnitude to the effect of climate change.
- Average flows will be about 30% lower by the mid-century.
The next chart shows the average monthly flow for the River Tejo at Almourol. This is the lowest point of the Tejo with flow measurement.
|Figure 5 Average monthly observed, naturalised and projected flow - River Tejo at Almourol|
This chart confirms that climate change will, not surprisingly, have a similar impact on flows at Almourol as at Alcantara. As with Alcantara, the effect of abstraction and reservoirs is of a similar magnitude to the effects of climate change. It also shows that under the A1B scenario, average flow will be reduced.