with Damon Grace, environmental engineer, COtL
Local weather conditions heavily impact the safety and efficacy of herbicide spray applications. New weather station networks, or mesonets, in South Australia, New South Wales and Queensland are providing growers with real-time, local inversion data to help them avoid spray drift conditions while maximising safe spray windows.
Damon Grace, general manager of the COtL Mesonet in South Australia, says the missing piece in making safe spray decisions was the ability to identify hazardous temperature inversions.
“All herbicide labels now stipulate that the products must not be applied when a hazardous temperature inversion is in place,” Damon says. “Until very recently, it has been very difficult for growers and applicators to reliably detect inversion conditions, and impossible to determine if the inversion is hazardous.”
“Spray drift is always unintentional. Many factors influence the risk of spray drift, and it is difficult to make decisions with the necessary degree of certainty,” he says. “Specialised mesonets can provide the data for informed decisions surrounding spray drift and other weather-related herbicide efficacy parameters, such as optimal temperature for applying specific herbicides.”
Spray efficacy is an essential component of the WeedSmart Big 6 for reducing herbicide resistance risk. If spray drifts off-target, it can potentially damage or contaminate other sites, such as sensitive crops or the environment. It also results in potential low dosing of the target weeds, which increases selection pressure for herbicide resistance.
Ambient temperature is also critical for maximising efficacy. As a general rule of thumb, Group 1 [A fops], paraquat (Group 22 [L]) and glyphosate (Group 9 [M]) are more effective at lower temperatures, while Group 1 [A dims], atrazine (Group 5 [C]) and glufosinate (Group 10 [N]) are more effective at higher temperatures.
What conditions increase the risk of spray drift?
In brief: Surface temperature inversions, wind speed, temperature and humidity all combine to create conditions that can increase the risk of spray drift. All inversions must be assumed to be hazardous unless otherwise indicated by a recognised system. It is illegal to spray during a hazardous inversion.
The details: Many factors affect the potential for long-distance spray drift, including nozzle selection, boom height and ground speed. Hazardous inversion is the primary weather condition that contributes to long-distance spray drift.
Inversions occur when warm air sits above a layer of cool air. Inversions generally happen in the evening and overnight as the earth cools, but they can happen at other times. It is important to note that laminar wind flows often occur during an inversion and are sometimes quite strong (> 5 km/ha).
A hazardous inversion occurs when the wind turbulence in the vertical direction is very weak. In a hazardous inversion, spray droplets remain suspended in the air and drift over long distances, resulting in off-target deposition at high concentrations.
Computer models can assist in predicting how the various meteorological conditions affecting spray drift might interact with the landscape.
What is a mesonet?
In brief: A network of local weather stations that collect reliable, real-time ag-weather data and make it available to users via an online app.
The details: While Bureau of Meteorology (BOM) sites are sparse in the regions, mesonet towers are 60 km apart, or less. The spacing is dependent upon the land topography and based on advice from meteorologists, using meteorological models. The location of the stations aims to provide favourable coverage of the area. They are typically closer together in complex (hilly) terrain or near the coastline, and wider apart on plains.
Each weather station includes a variety of sensors that are the same or better than those used at official BOM weather stations.
Each tower is 10 m tall, with temperature sensors at 10 m and 1.2 m above ground level to measure the vertical temperature difference (VTD), an indicator of inversion conditions. The solar-powered stations use state-of-the-art electronic sensors including sonic wind anemometers that measure wind speed even under very still conditions.
The data collected is available on the online platform and updates every 10 minutes. Users can monitor the conditions at the station most relevant to their location (e.g. closest station in the same valley).
Where are Australia’s mesonets?
In brief: In South Australia, COtL mesonets are in the Mid North, Riverland & Mallee regions, and the Limestone Coast mesonet is under construction. WAND (Weather And Networked Data) is another large mesonet in Queensland and New South Wales.
The details: The WAND network of around 100 towers covers the broadacre grains and cotton growing regions from Central Queensland to the Victorian border. WAND provides applicators with real time data indicating the presence or absence of hazardous inversions, including all other ag-weather data.
The establishment of WAND represents six years of collaborative research by the Grains RDC and Cotton RDC, led by meteorologists Dr Graeme Tepper, Micro Meteorological Research and Educational Services and Dr Warwick Grace, Grace Research Network.
WAND is a $5.5 million investment between the RDCs and Goanna Ag, the commercial partner responsible for installation and management of the WAND mesonet.
The COtL Mesonet incorporates over 70 weather stations across the Mid North and Riverland & Mallee. In response to calls-to-action by agronomists in the Clare Valley, the South Australian government funded the development and installation of a pilot mesonet in the state’s Mid North. The Mid North Mesonet opened in 2019, and COtL began to manage the ongoing operations.
Once complete, the COtL Mesonet will cover 50 per cent of the arable land in South Australia. The South Australian Government has invested $4.5 million to establish the physical infrastructure for the COtL Mesonet.
GRDC Factsheet – Hazardous surface temperature inversion
GRDC Update Paper – Spray drift hazard warning system
WAND mesonet (NSW and Qld)
CoTL mesonets (SA)