This is updated from a previous post, which was an example for a stream with adjudicated water rights. However, it also works for any stream where there are water rights with legally defined diversion quantities, if all the diverters have headgates in good condition and/or measurement devices such as weirs, flumes, and pipe meters.
Is John Stealing Water?? John Casey has a cattle ranch near Adin, where he grows pasture and hay to raise about 70 Angus steers. His ranch is 240 acres with lower irrigated land and forest on the higher part. He has an a licensed water right of 2.00 cubic feet per second (cfs) from Preacher Creek, to irrigate 80 acres, from April 1 to November 1.
John’s downstream neighbors claim he steals water. He says he can show that he takes only 2 cfs, or less when the flow drops down in the summer. Can he prove it?
As we can see, he has a square headgate at the head of his ditch. It is 2.0′ wide, and can open up to 1.5′ high. Right now, John says he is diverting 1.05 cfs. His evidence is that his gate is open 0.15′, the water is 0.57′ deep on the upstream side, and the water is 0.20′ deep on the downstream side. Is that enough to check what he says?
The box in which the gate sits has smooth walls, and the gate closes flush with the bottom when John is not diverting. The water continues in a straight path from upstream to downstream. That means the weir has “suppressed” sides.
This is in contrast with, for example, a hole cut in the middle of a 2″ x 12″ weir board. The water on the sides has to make the turn to go straight through, so the hole in the board is an example of a “contracted” orifice.
Let’s look at the tables for orifices in the back of the Water Measurement Manual. Table A9-3 is for submerged, suppressed weirs.
We can’t see the downstream side of the weir, but the water is above the bottom of the edge of the gate, so it is submerged rather than free-flowing.
This table has flows calculated for a minimum area of 2.0 square feet (sq. ft.). However, the area of the opening at John’s headgate is 2.0′ wide x 0.15′ high, or 0.30 sq. ft. Fortunately, the equation, Q=0.70A(2g Δh)^0.5, is listed right at the top of the table. We can calculate the flow using that. Q is the flow in cfs, A is the area of the orifice hole, g = the acceleration due to gravity, or 32.2 ft/second^2 (feet per second squared), and Δh is the difference between the upstream and downstream water depth.
So the flow Q = 0.70 x (2.0′ x 0.30′) x (2 x 32.2 x 0.37′)^0.5 = 1.03 cfs. So far so good – John is taking 52%, or just over half of his right when 100 percent of flows are available. But, how much flow is actually available right now?
Let’s use the “sum of the boxes” method. Instead of measuring the amount of water in Preacher Creek at the top, before any diversions, and then estimating how much flow is being lost to evaporation, transpiration, and infiltration, and then estimating how much flow is subsurface above John Casey’s ranch and “pops up” out of the ground below, we’ll look at what each diversion amount is, plus the amount still in the creek after the last diversion. This is very useful because none of the instream losses have to be estimated – we just add the diversions and flow still in the creek, and that amount IS the available supply.
Water Board Permits and Licenses are usually not interrelated – they specify water rights without considering the other water rights on the stream. This is different from adjudicated streams, whether done by the Water Board or the Department of Water Resources. Some Superior Court judges in past decades were pretty smart and actually ordered that available flows be calculated by the sum of the boxes:
The paragraph above, from the Susan River Decree, defines available water supply as what is being diverted, plus the flow passing the last diversion.
There are 4 diversions on Preacher Creek, and here are the amounts being diverted:
- Diversion 1 (John Casey) 1.03 cfs of a 1.60 cfs water right, 52% of his total right
- Diversion 2 (Amy Hoss) 1.67 cfs of a 3.80 cfs water right, 44% of her total right
- Diversion 3 (Mark and Cindy Sample) 0.55 cfs of a 0.88 cfs water right, 62% of their total right
- Diversion 4 (Quint and Marcie Minks) 1.32 cfs of a 2.50 cfs water right, 53% of his total right
- Flow still in the creek past the Minks Diverison – Quint estimates about 0.7 cfs
The total diversion-plus-bypass flow is about 5.3 cfs. The total rights on the creek are 9.48 cfs. Therefore, the total available flow = 5.3 / 9.48 = 56%.
So, John is right, he is not stealing water! He is taking 52% of his water right, when he could be taking 56% according to the “sum of the boxes” method. Not only that, but Amy could take more, the Samples should reduce their diversion, and the Minks’s could take a tad more. Well, that’s theoretical – Quint and Marcie Minks probably cannot seal up their dam completely, so there may be a little bit less flow actually available for diversion.
eirs© with two flashboard slots. The upstream flashboard slot holds the nominal 2-inch wide weir flashboards. The downstream slot provides the air gap for the nappe, as specified in the 
ith weir sticks, as specified in the 
What size diversion is this, really? An irrigation diversion of 1,000 AF over 6 months, with flows starting at 100% of the water right, declining to 50% of the right by the end of the season (month 6) would calculate out to a 
To answer the 
cfs. This category does not have to comply as quickly – the deadline is July 1, 2017, or nearly a year from now. The measurement frequency is daily, which is possible to be done by a diligent person, if not not recorded automatically. For a sense of how much water this is, depending on where you are, what the soils are like, and how efficient the diversion is, the acreage of hay or pasture irrigated ranges from about 8 to 370 acres.
as the summer proceeds, so what size of water right are we really looking at? Let’s say flows decline evenly from 100% at the beginning, to 50% of available flows at the end of 6 months. The right that would divert 1,000 AF per year under these conditions is 
e is a 4′ weir, capable of measuring up to 4 cfs very accurately, at plus or minus 5% (sometimes better) accuracy with new lumber. A headgate like the one to the left is easily capable of passing 4 cfs and, if the gate is used as a measurement orifice, the accuracy can be 5%, certainly within plus or minus 10% if care is used with an older gate.
and make sure it’s reasonable. The data that is recorded is “stage”, or water surface elevation. Using the correct weir, orifice, or flume equation, or table from the Water Measurement Manual, the stages have to be converted to flows. For hourly flows, that means 8,760 data points per year, which will require a spreadsheet like Excel to make the conversions. A 
and then a different zero-flow datum used to convert stages to flows starting when the change was made.
the least expensive, long-lasting, accurate devices. This weir was prefabricated and shipped from Briggs Manufacturing. Installation takes a few hours, and with new 2″ lumber, accuracy is plus or minus 5 %. That is better than the Water Board’s requirement of 10 % accuracy.
together correctly, doors would not swing easily, windows might get stuck open, or worse.
actually going past. Inside the device, the boards have to be sealed. They might be sealed with plastic and gravel, as shown below, or just well-fitted. If the soil is sandy, it is easy to dump shovelfuls of sand along the backs of the boards until the sand fills the cracks. As long as the boards are not changed out, they will stay sealed for the season.
together anymore. Besides fit, an old board can’t be leveled; one end will be level but the other end will not because of the warp or twist.
Standing in front of the Wigno Weir, getting ready to “
WHY? I did not wait the 5 minutes it would take for the upstream head to stabilize. It was cold and about to get dark and the videographer was patient but getting cold. 🙂