In 2001 and 2002 the Rio Grande, draining one of the largest watersheds in North America, failed to produce enough flow to reach the Gulf of Mexico. | Photograph by Jack Dykinga.
The theme of the 2012 World Water Day is “Water and Food Security”. The tag line is that “the world is hungry because it is thirsty”.
The provision of safe and secure food to supply the needs of a growing population is inextricably linked to management of our fresh water resources. Our ability to manage the supply of water is, in turn, linked to our ability to measure it. If we know how much of the water supply is renewable on a sustainable basis then we can manage the allocation of our water resources to stay within this envelope of sustainability.
All You Eat – Issues which concern you this year on World Water Day
I am going to present the argument that sustainability of water supply can only be quantitatively managed with respect to non-zero flow.
The corollary of this argument is that in-stream flow needs (IFN) have to be integral to any water management plan. Zero is a quantity that can be added or subtracted the same as any other number. The problem is that when we observe zero flow in a stream channel that observation is irrespective of whether the water table is 1mm or 100m below the channel bed. Non-zero flow is an integral of all water sources providing a cumulative measure of the net effect of all anthropogenic water allocations.
I was recently in El Paso, Texas. Coming from Canada, it was alarming for me to see the trickle of water upstream of the American dam being entirely diverted into the American Canal. This is entirely OK under the terms of the various agreements negotiated between the US and Mexico. These treaties allocate all of the water between the two countries with none left over for IFN. It was even more astounding to learn that the water that I saw was entirely sourced from the sewage treatment plant and that the hydrograph peaks twice a day – in the morning when everyone flushes their toilets and in the evening when everyone prepares their dinner and washes the dishes.
In 2001 and 2002 the Rio Grande, draining one of the largest watersheds in North America, failed to produce enough flow to reach the Gulf of Mexico. Whereas it is considered ‘normal’ for long reaches to go dry every year, it is unusual for there to be absolutely no water making it to the Gulf. The size of the prize being divided by treaties between the two countries may be getting smaller as a result of climate change or, perhaps, by over-allocation.
My curiosity is piqued by Rio Grande basin area statistics. The watershed is 870,000 km2 in total area, of which only 472,000 km2 is hydraulically connected. Almost half of the possible contributing area is isolated in endorheic sub-basins. This means that a very large percentage of the water in the watershed must be moving through the basin underground. I would assume that, under natural conditions, the groundwater contribution from these endorheic basins to the lower Rio Grande must be very substantial because of the large contributing area and high hypsometric relief (3658 m of elevation difference along the length of the river), creating a strong pressure potential gradient.
The Rio Grande is managed with respect to surface water quantities. It seems that the quantity of water abstracted directly from aquifers may not be properly accounted for.
So how could inclusion of IFN in these treaties have resulted in more sustainable water sharing agreements?
The in-stream flow is the sum of upstream sources and groundwater discharge. If the water table in a groundwater discharge zone drops below the channel bed then the reach becomes a groundwater recharge zone. Maintaining the in-stream flow to some minimum requirement would then require a compensating increase in release of upstream flow. An IFN requirement imposes a penalty on over-exploitation of the groundwater resource by automatic reduction in surface water resource availability. Such a penalty does not exist for zero flow water management. In contrast, when surface water availability becomes limited in a management plan with 100% anthropogenic allocation then there is an inherent, unsustainable, economic reward for increased groundwater exploitation.
It is not my intention to second guess the many and various benefits of the treaties for allocation of Rio Grande water. I believe these agreements were made with trust in the best science and engineering practice available at the time. Neither do I do not think either country would agree to the exact same terms and conditions if they had the opportunity to do it all over again with benefit of hindsight. The challenge now is in how to best manage the resource in the context of many and various legally binding agreements. The opportunity is to learn from the past and to ensure that, as we inevitably increase the extent of water exploitation in other watersheds, we limit our impact to sustainable levels.