World Water Day is on March 22
The theme for World Water Day this year is “Water and Food Security”. The www.unwater.org/worldwaterday website provides a lot of interesting information about the importance of water for feeding a growing population that I would like to talk about.
First, I would like to invert the World Water Day theme and talk about “Food and Water Security”. This animation of the water cycle provided on the UN world water day website demonstrates watershed function very clearly as an integrated system. This, however, is not how we manage our water resources.
The Water Cycle: Surface Water and Ground Water | UN World Water Day 2012
There is a truism that “we manage what we measure” and we have proven to be remarkably adept at measuring surface water runoff volume.
This is not to say that it is not difficult to measure surface water, nor to say that we have been able to produce data fully adequate for water management purposes, but we can measure quantities that can be managed by addition, subtraction or division (unfortunately, we can’t multiply our water resources). We subtract and add by storage and release from reservoirs. We subtract by abstractions and diversions. We divide by appropriation and apportionment amongst license holders. The same is not true for groundwater. Groundwater management is largely disconnected from quantitative guidance. More importantly, it is disconnected from the quantitative management of surface water.
There are many things that we can measure that would allow us to manage watersheds as integrated systems. As illustrated in the water cycle animation, water from the atmosphere either runs off immediately as surface water or percolates down through the soil to become groundwater. The mass of this new water raises the pressure potential of the water table contributing to the pressure gradient driving ‘old’ water into the stream. We can distinguish ‘new’ from ‘old’ water by measuring isotopic composition. The ratio of the conservative isotopes oxygen-18 (18O) to deuterium (2H) in the water provides a unique signature that is useful for differentiating water by source. There are also a variety of other parameters than can be monitored to provide an understanding of water source. Groundwater tends to have a thermal signature near the mean annual temperature, whereas storm water tends to have a temperature near that of the atmosphere. Groundwater is enriched in solutes raising its conductivity relative to atmospheric water. The chemical composition of ions in the water can be measured and this chemical signature can be linked to contact time of the water with rock and to the chemistry of the rock in the source aquifer. These variables are relatively easy to monitor and can be used to provide a useful measure of groundwater flux in the receiving stream.
Food production requires masses of water
This water can be intercepted directly from the atmosphere; stored in reservoirs; abstracted from streams; or pumped from aquifers. This water is not lost completely, it may short-circuit back to the atmosphere by transpiration or evaporation; it may be exported as part of the food product; or it may be returned to the stream as agricultural runoff. Agricultural runoff is characteristically loaded with nutrients, pathogens, and pesticides.
Our food security is dependent on water and our water supplies are vulnerable to the activity of food production. We will not be able to manage our water security effectively until we learn to measure the right things, in the right places, at the right times to be able to understand our watersheds as integrated systems.