Saturday, March 7, 2009

Sneaky Water Bandits or “The Great Rain Robbery” by Gardener Dave

How Much of This Will Get To Your Roots?

There are some subtle, perhaps even “insidious” natural forces at work “stealing” our water. When we hear reports of several inches or even feet of snow in our watershed areas, in the foothills, or (rarely) even on the plains near Denver, we really can’t assume that is the amount that will finally be available to us. Let’s look at these natural forces, and let’s ignore the water absorbed as it moves downslope to reservoirs. That part is very beneficial to trees, shrubs and other vegetation – I don’t consider that to be “stolen”. But, as an old English proverb says: “There’s many a slip ‘twixt the cup and the lip”.

1) Sublimation – This process turns snow directly into a gaseous state, bypassing the beneficial liquid water state entirely. I have no actual estimate of how much of our snow moisture is lost by this process, but it can be significant, and we certainly need all the moisture we can get. Have you ever noticed that sometimes snow can shrink a lot even before we get the warm Chinooks and sunshine? Some shrinking is from compaction, and some is because of sublimation.

Sublimation occurs at below-freezing temperatures. It is the same phenomenon used in the commercial freeze-drying process. It also explains why Grandma could hang her clothes out on the line in below-freezing temperatures and later retrieve them all nice and dry. Our very dry and many times fast-moving winter air also accelerates the sublimation process. In case you’re interested, this is also how “frost-free” freezers keep their walls and shelves free of frost and ice, by using a fan within the freezer compartment to circulate dry, cold air. Even ice cubes in open trays will sublime and get smaller if left unused for a long time.

To further illustrate the sublimation process by way of something that we may be more familiar with, frozen carbon dioxide (dry ice) sublimes at atmospheric pressure and room temperature. That’s how our stage acts, plays and movies create all that spooky “fog”. The opposite of sublimation is “deposition”. Frost is an example of this. In frost deposition, water goes from a gaseous vapor directly to solid ice crystals.

2) Evaporation – We lose a lot of water by evaporation, especially from the surfaces of reservoirs, lakes and even streams. Again, our dry air and winds speed up this process. We are all familiar with this process, so I won’t belabor it here, except to give you an estimate of how much water can be lost. For arid regions, I have seen estimates of over 25% of the water, stored to be used for human purposes, can be lost to evaporation.

3) Virga – You may have said “Gee, look – it’s really raining heavily from that cloud”, only to find out that it never actually reaches the ground at all. And – you guessed it – our dry air also contributes to this effect.
Now, in my mind, virga qualifies as one of our Sneaky Water Bandits because if we see rain falling from the sky – it sure would be nice if it reached the ground and soaked into our lawn and gardens… oh well, maybe some other time!

The word virga is derived from Latin, meaning twig or branch (go figure that out), and a popular “backronym” (ever hear that word?) in meteorology is "Variable Intensity Rain Gradient Aloft”. (See, we learn something every day if we pay attention). Interestingly – one effect of virga can be pockets of cold air that descend rapidly, creating dry air called microbursts which can be extremely hazardous to aviation.
4) Run-off – When we see water running downhill we can all spot this one, right? But perhaps less-known is the fact that all the water that falls on our property doesn’t actually belong to us. According to Colorado Water Law, we can’t just use rain-barrels on our downspouts or a big catch basin to retain every possible drop. I may be selfish if I consider this run-off water “stolen” (from me), but if rain falls on my property – I’d like to keep it!

However – Colorado Water Law requires that precipitation fall to the ground, run off and into the river of the watershed where it fell. An individual may not capture and use water to which he/she does not have a right. Also… the reuse of household water (gray water) is regulated by Colorado State Board of Health Guidelines. Colorado Water Law allows each customer just one use of the water before it goes down the drain, through a wastewater treatment plant and back into the river for others to use. By law, Denver Water customers are not permitted to take their bath or laundry water (“gray water”) and dump it on their outdoor plants or garden. After this water is used once by Denver Water customers, it must return to the South Platte River where it will be used seven or eight more times before it gets to the state line (Nebraska).

Gardener Dave