Harvesting and supplying flood water to irrigators, on demand

The Rangitata South Irrigation Scheme will help improve water management in mid Canterbury and dramatically increase productivity, benefiting the entire country. 

Taking and storing flood water to help meet future demand, when water is otherwise scarce, will add a huge amount of supply capacity to the region. When Canterbury’s Rangitata River is running high, the new Rangitata South Irrigation Scheme (RSIS) can divert up to 20 cubic metres of ‘excess water’ into its seven massive storage ponds, every second.

These ponds, located near Arundel, supply water when the demand for irrigation is high but the river level is too low to permit water to be taken from it.

NIWA has designed, supplied and installed a complex web-enabled automatic control system, comprised of nearly 60 control stations, and software to manage the water distribution across RSIS’s 14,000 hectare-wide irrigation scheme. The system automatically schedules irrigation water and delivers it on demand, through 48 controlled turnout nodes, to approximately 35 users. 

‘Control central’ — the intelligent communications hub

The scheme’s ‘60’ active control nodes communicate through an intelligent central hub; a computer/server, running Neon Server software including NIWA’s own Irrigation Sentinel scheme-management application, configured specifically for RSIS. Communication works in both directions, to and from the hub.

The hub is located in Rangitata Water Ltd’s office, alongside the ponds. 

What happens if communications between hub and nodes is lost?

Control nodes can operate in ‘standalone’ mode, proactively controlling water-level or flow-rate at a ‘local level’ using a control gate. So if communication was lost (e.g. if the Internet Service went down) control stations would simply maintain the ‘standalone status quo’ until communications were restored and updates issued to the nodes.

Managing the storage – controlling the water into, through and out of the ponds

If the river flow is above Environment Canterbury’s consent threshold, then the scheme is allowed to take its allocation of water. Then, at the scheme intake, three control gates admit river water to the ponds. Around the intake, an extreme-flood bypass and fish spawning race accommodate the natural environment.

As water enters the seven-pond system, each of the active water-level control nodes (one per pond) maintains a set-point level difference between any pond and the one below it. When the level of a downstream pond falls below this set-point, the gate opens to admit more water from the pond above, to maintain the targeted level difference. All the ponds are filled concurrently as a 'managed cascade'.

We continuously monitor the water level of each buffer pond with a PumpPro compressed-air-bubbler water-level instrument. This measures the weight of the water above the sensing point and converts it to depth. The sensing point is referenced to a fixed datum to convert depth to water level.

The scheme takes its water from the last/lowest pond, pond 7, and from here it is distributed to the downstream users, on demand.

Managing the distribution – controlling flow-rates

Each node regulates the flow-rate to a set-point target, sent by the Neon Server, by continuously adjusting the gate position to keep the flow-rate ‘on target’. Each node sends flow measurements to the hub, where checks are continuously performed to verify that the flow-rate is maintained within the expected range.

To meet the scheduled demand, the hub sends set-point target updates to the nodes upstream of the scheduled delivery point. An allowance for water ‘travel time’ is already built in. 

Robust communications provide operational security

There are two modes of wireless communications. Both use Internet Protocol:

• Communications for pond management - a local private Wi-Fi network enables each of the 10 active nodes to communicate with the hub.

• Communications for downstream distribution management - the 48 downstream nodes and hub communicate over commercial cellular networks, using NIWA’s secure Internet Access Point Node (APN).  

Supplying the right amount of water to the right farms at the right time

NIWA’s automatic monitoring and gate control systems enable ‘smart distribution’ of irrigation water, through a complex network of races, to around 35 downstream farms spread across 14,000 hectares of surrounding land.

NIWA’s Irrigation Sentinel computer application provides access to the operation of the entire scheme. It consists of several systems working together to monitor, control, maintain and distribute water throughout the irrigation scheme. Sentinel ‘supervises’ 24/7, unattended, and relieves RSIS staff of many routine water-scheduling and supervisory tasks.

Irrigation Sentinel enables the scheduling and supply of irrigation water to order. It generates the schedules that ensure the right amount of water is delivered automatically to the right farms at the right time, while ensuring that the total water taken from the river stays within RSIS’s consent limit.

Demand-based scheduling allows for the time it takes the water to reach each user in a schedule. It may take several hours for water to reach those furthest from the intake. The scheduling system allows for this and admits water to the scheme early, to allow for the travel time.

Viewing scheme operation and making changes anytime, anywhere

Even when the RSIS is operating on ‘autopilot’, authorized users can view real-time and historical data on the server via the Internet.

Scheme operators can make changes, such as editing flow schedules, entering customer orders (with flow-rates, start and stop times), make allocation changes and view scheme operation via an Internet-connected communications device such as a Smartphone, tablet or desktop computer.

What happens if something unexpected happens?

If any data move outside the expected range the system sends text message alarms to nominated communications devices, such as Smart phones. Nominated users, such as scheme race-men, can easily configure these alarm ranges, add appropriate messages, and query the current status of any of the nodes in the network.

In the unlikely event of total loss of the automatic system, the scheme can be switched to ‘manual control’.

Other partners

RSIS primary contractor, Rooney Earthmoving Ltd (Timaru), contracted NIWA to carry out the automatic control and demand-scheduling of water.  Andar Holdings Ltd manufactured the farm turn-out gates.