How do you calculate hourly flows from flumes, weirs, and orifices, when what you actually measure is water depths? That’s how these devices work: the device creates an engineered curve, so that the depth at a certain distance from the entrance or exit gives an accurate flow.
Getting the flow equation is simple enough – for the flume below, flow = 11.6948 * (depth + 0.0068932) ^ 1.64505. What if you divert for seven months of the irrigation season? 7 months * 30 days * 24 hours. That comes to 5,040 measurements – too much to calculate by hand.
You’ll need to use Excel or similar software to handle all the data. Here is an example of a spreadsheet for this flume. Data is copied into the sheet from the water level logger output, then the spreadsheet automatically calculates the corrected depth, flow, and hourly volume.
Why are there columns “Year” and “Month” in this spreadsheet? Those are for pivot tables, which can quickly sum many thousands of rows of data, by year and month. A pivot table is used to get summed monthly flows and the maximum flow for each month, which are necessary to fill out your annual report to the Water Board.
A downloadable Excel file of this example spreadsheet is included at the bottom of this post.
