Hydrology is the name given to the scientific study of water in the environment. The collection of data from surface water systems (i.e. rivers, streams and other watercourses) is the responsibility of the Hydrometry team. This data allows scientists to understand how and why water behaves as it does – where it comes from, where it goes and the effects it has on the wider environment.

River level data is collected at a ‘gauging station': a small hut, cabin or box located on the bank of a river which contains various instruments for measuring. Many of the earliest sites were established by pioneering hydrometrist Captain WN MacClean, who established Scotland’s first gauging station near Invergarry in 1913. Although river flow gauging and level monitoring had been carried out informally by land owners and town planners concerned with the management of water supply since the mid-19th century, MacClean was specifically interested in investigating the potential for hydro-power within the River Garry catchment.

For hydrologists such as Helen James, based in SEPA’s Angus Smith Building in North Lanarkshire, this is still an essential service that the hydrometric network exists to provide.

“We receive a lot of data requests,” says Helen. “A lot of consultants come to us for various projects such as proposed new hydropower schemes. They’re interested in what kind of flow is going down a river.”

A significant part of a hydrologist’s job is to maintain the gauging stations and ensure that they are calibrated as accurately as possible and to make sure the various floats and sensors are in good working order. This requires a rolling programme of site visits to more than 400 stations across the country.

“Most sites should be visited once a month, as a matter of routine,” says Helen. “At a gauging station you normally have to check the level of the river, and make sure that the instruments are working ok.”

Modern gauging stations contain a variety of electronic sensors and data-loggers, many of which are connected to the telephone and, increasingly, 3G networks. Data is downloaded automatically once a day; sometimes, for operational reasons, more frequently. GPRS upgrades will ultimately make data available on a 15-minute basis across a large percentage of the network.

“We check all our sites on the computer system every morning to make sure that the data is looking reasonable – sometimes you can tell just by looking at the hydrograph that there is some fault with the instrumentation. But we do have back-up instruments at the sites as well, so even if the telemetry system goes down, we know the data’s still being recorded and we can then download it when we visit the site.”

Many of the gauging stations contain a stilling well, a tube of about a meter in diameter which is sunk straight down into the river bank. A pipe or conduit runs horizontally from the well to the bottom of the river bed, allowing the well to fill to the exact level of the river, free of the turbulence of the river flow. A float sits on top of the water in the well and records an accurate and constant measurement of the still surface level of the river. The float is itself connected to sensitive electronic instruments which log the measurement data and record the information for upload to SEPA’s central hydrometric database.

In order to calculate the volume of water travelling down the river, the key data hydrologists are looking for are depth and velocity. Hydrologists have a range of techniques at their disposal when attempting to measure river flow, ranging from traditional methods using rotating element current meters (which look like propellers) submerged in the river, to sensors which use sound waves to measure velocity in the water column. Methods of deployment also vary considerably, from wading across the river or using cableways and winches right up to the more modern use of remote controlled boats.
Regardless of the method used, regular gaugings are vital to maintain accurate calibration of the gauging station as shifts in the river over time (e.g. channel scour or deposition of gravel during high flows) will alter the relationship between river level and actual flow.

When carrying out a wading gauging, the hydrologist runs a measuring tape across the river, both to record the width between each bank, and also to allow them to note where each individual measurement is made relative to the banks. The hydrologist then measures flow velocity and depth at over 20 points across the channel. 

Hydrometry is evolving all the time: traditional methods are constantly being incorporated into new technologies. Today’s gauging is being carried out using a FlowTracker – a handheld sonar device mounted onto a long metal wading rod, allowing the height from the river bed to the surface of the water to be measured. An instrument panel at the top of the rod is connected to three sensors at the bottom; the device calculates river flow by measuring the velocity of particles passing through its sensors.

The one slight complication with the FlowTracker is that it works best in slightly more turbid waters. “Some rivers can be too clean for it because of its low SNR [signal-to-noise ratio] and needs more particles in the water,” explains Assistant Hydrologist Kirsty Craig.

“It’s really good with rivers in urban, industrial areas and it tends to be quite good with mid to high flows, but with really low flows it does sometimes struggle to pick up the particles.”

The data that hydrologists collect is used in a variety of important and useful ways, from drought management to flood warning; it allows local authorities to manage water resources; it enables anglers and canoeists and other users to plan their activities on Scotland’s rivers; it also provides a detailed record for scientists, planners and policy makers, and a working legacy for future generations.