In late November I'll return to one of my favourite places: Ruaha National Park in Tanzania.
I'll be there to work with WWF colleagues and other partners in a water resources programme but I hope to have a little spare time to enjoy the scenery. Compared to the celebrated Serengeti it's not as well known, but Ruaha has stunning wildlife and a quiet beauty.
The lifeline of the park is the Great Ruaha River, which provides the main source of water for the wildlife, especially during the four month dry season, from July until November.
It's a seasonal river, with natural variation between wet and dry season flows. But in recent years, this variation has increased. In fact, over the last decade or so, the river has stopped flowing altogether during the dry season. Wildlife is dying as a result.
This is not a natural turn of events; any variation in rainfall over the period has been insufficient to cause the river to dry up.
The issue is that more and more water has been diverted from the river upstream of the National Park, often by very poor farmers whose livelihoods depend on irrigated rice and vegetable crops.
This is a problem not only for the wildlife. Downstream of the National Park are the Mtera and Kidatu dams which generate a significant chunk of Tanzania's electricity. The dam operator, TANESCO, is concerned. When the river level drops, it becomes difficult to generate power and the risk grows that the lights will go out in Dar es Salaam.
So, for a few hundred kilometres, the Great Ruaha River is the hydrological thread tying together water, food and energy security. The river is the nexus.
This is not a unique situation. Increasing numbers of rivers, and lakes too, are drying up because we have taken too much water from them, for too long. The Yellow River, the Indus, the Aral Sea, Lake Chad... the list goes on.
Nor is this only a challenge for low income countries. In the US, the Rio Grande often runs dry. If we broaden our scope to groundwater, we might consider the diminishing Ogallala Aquifer, which underpins much of America's farming belt.
All of this is now well-documented. Fred Pearce wrote eloquently about the global water crisis in his book, When the Rivers Run Dry. The recent UN report on the The Economics of Ecosystems and Biodiversity, drawing on a plethora of academic papers, made many of the same fundamental points, albeit in technical language.
But as I've read through the blurb for the Bonn conference on the water-food-energy nexus, I've seen relatively few mentions of rivers or lakes or aquifers. In the thematic brief for conference sent to me as a briefing for this blog, the word "water" appeared 37 times. The words "river", "lake" and "aquifer" were conspicuous by their total absence.
So here is the problem. There is a fourth part of the water-food-energy nexus: ecosystems. Unless we can maintain sufficient water in our rivers, lakes and aquifers, all the taps and pipes and dams in the world will be of little use. Sustainable, equitable and productive management of these natural systems should be at the heart of the nexus.
Indeed, I would contend that the litmus test of "water security" is the continued flow of water through rivers, lakes and aquifers to downstream users.
This will be a daunting challenge in a world of 7 billion people (and growing), many of whom are on the road to a better, but thirstier, lifestyle. It will require trade-offs between different water users and strong involvement of farmers, local communities, researchers, NGOs and the private sector. This will, in turn, necessitate increased political will and better institutions to manage decision-making processes.
But more than anything else, the water-food-energy nexus demands that we get real about where our water comes from in the first place.
I'm an optimist. I believe that in most places, most of this can be done. And there is evidence. In 2008 and 2009, the Great Ruaha River flowed even through the dry season for the first time in fifteen years. The flow was small, but it was progress nonetheless.
Last year, 2010, was more difficult and the river ran dry again. So we face a long-term challenge, rather than a short-term fix. As long as Tanzania, and the world, needs water, food and energy, there'll be work to be done.
Dave Tickner is Head of Freshwater Programmes at WWF-UK.