The depletion of transboundary aquifers is similar to that of domestic aquifers

This is an interesting article on the management of transboundary aquifers because it provides the first global assessment of agricultural groundwater use and depletion.

The main conclusion is that it finds no evidence that transboundary aquifers are, on average, more overexploited than those that are not. The article highlights that the specific geographical and hydrogeological conditions of each aquifer are decisive, and that the governance of transboundary groundwater should be organised according to the geographical distribution of water uses and potential interactions between countries, rather than treating shared aquifers as homogeneous management units.

The importance of the geographical location of water uses and of hydrogeological factors is well known and is not a new finding. This applies to all aquifers, whether transboundary or not. In my view, the most novel aspect of the study is that transboundary aquifers do not, on the whole, show a higher degree of depletion than national aquifers.

This conclusion raises interesting questions. Could it be due to more effective management than expected in some shared systems? Or does it reflect the fact that management problems are equally common in both national and transboundary aquifers? The study does not allow us to answer these questions directly, but it highlights the need to explore them further.

We must bear in mind that groundwater has historically been the great neglected aspect of water resource management. If we are to adapt to climate change, this situation must change, particularly in regions facing water scarcity or an increasing risk of drought, such as Spain and Portugal

Is the study of good quality? Are the conclusions supported by sound data?

“Yes, the study is of good quality and its conclusions are supported by sound data. This includes official statistics or previous work by other researchers published in high-impact, peer-reviewed journals. However, it should be noted that agricultural water use is an estimate, based on crop requirements and the area under cultivation, as in the vast majority of countries there are no on-site meters that allow the actual water use to be determined. This is the case, for example, in Spain”.

How does this study fit in with the existing evidence?

“The study confirms the assessment of groundwater resources and their consumption that has been carried out over recent decades in various countries around the world, including Spain. It classifies transboundary aquifer systems between countries into four categories and provides a future projection of their global evolution, highlighting the significant increase in consumption forecast for those shared aquifers used for agriculture.”

Are there any significant limitations that need to be taken into account?

“The main limitation is that the exact volume of groundwater contained in an aquifer is difficult to calculate, as these are underground structures. Another limitation, as already mentioned, is the calculation of actual agricultural water use—both surface and groundwater—due to the absence, in the vast majority of countries worldwide, of effective measurement systems at plot level.

Finally, the study focuses on national-level conflicts between countries, but there are notable conflicts at a regional level within countries, as is the case in Spain, for example, between irrigators in Castilla-La Mancha and the south-east of the Iberian Peninsula.”

How relevant are these findings for Spain and Portugal? Are there any aquifers shared between the two countries for which this study is particularly relevant?

“On the Iberian Peninsula, there are aquifer units shared between Spain and Portugal, although the vast majority of these aquifers do not present problems of overexploitation or pollution. Consequently, there are no conflicts over groundwater between the two countries. The Albufeira Convention regulates the sustainable use and management of shared water resources – both surface and groundwater – between the two countries. And it has never given rise to any significant tensions between Spain and Portugal.

How would you assess the management and cooperation between Spain and Portugal regarding shared groundwater resources?

"Cooperation on water management between Spain and Portugal is exemplary and has never caused tensions between the two countries. The only complaints come from certain groups of irrigators involved in the Tajo-Segura water transfer, who take issue with the fact that the transfer of a significant volume of water to Portugal is regulated by the Albufeira Convention, as they believe that existing water demands in the south-east of the Iberian Peninsula must be met first.

Therefore, when it comes to the management of shared water resources, the model established for Spain and Portugal is appropriate and works well. The problem of shared groundwater management in Spain is regional in scale and arises between regions with scarce resources that are demanding more water and others with abundant resources that consider the guarantee of their own water needs to be paramount.”