A friend of mine, Chris Reilly, summarizes everything you need to know about measuring flows into your surface water diversion: “If you can’t measure it, you can’t manage it!” Except for riparian rights and some very small water rights, diverted flows have to be measured. Why?
Legally, to ensure your neighbors, the Board, and/or a Superior Court Judge that you are diverting no more than your water right. Practically, how do you know if you are getting as much water as you should? As surface flows decrease through the summer, every bit less means some pasture, hay, orchards, row crops, or something else does not get irrigated.
If you have never measured flow into a ditch before, well, here goes, I am going to leak the secrets right here, I’m going to violate the Unspeakable Code Of The Water Measuring Brotherhood, the ve
ry deepest, most powerful wisdom of how to measure your flow will appear on this very page. After this, who knows if you will ever hear from me again, once this classified information is made public? Well, not really, but few people have heard of the Bible Of Water Measurement, the USBR Water Measurement Manual (WMM)
Let’s look at 3 common measurement devices detailed in the manual: weirs, orifices, and flumes. Properly installed and maintained, these devices can measure flow within plus or minus 5% of the actual amount. The photos below are from the WMM, which has lots of diagrams that make it easy to see the details of how each device works. First, the weir:
You have seen these before, they’re just a level plate or board of a specific width, with a relatively still pool behind them. That’s it! By measuring the height of the pool above the edge of the plate or board, you can use tables or equations from the WMM to determine what the flow is.
Above is shown an orifice. Not much to see, is there? In this case, it is just a hole, lower than the upstream flow. That is physically all an orifice is. Knowing the size of the hole, and how high the water is over the center of the hole, and how high the water is down the ditch, a table or equation can be used to figure out the flow. The gentleman above is using a square gate with a certain width. The area changes with how high the bottom of the gate is, not hard to figure out.
The photo above shows a Parshall Flume. These are great for measuring high flows without needing a lot of “head” or the drop in the water from upstream to downstream. By knowing the depth at a certain point, a table or equation can give the flow amount.
We’ll go into how to use tables for specific measurement devices in later posts. It’s enough for now to know that if you have a decent measurement device, then you CAN manage your flow, as well as proving that you are taking no more than your legal water right.