During the growing season, approximately October to March, primary producers and horticulturalists can access a soil Hotspot Watch from the National Institute of Water and Atmospheric Research (NIWA).
The Hotspot Report uses rain gauge data from many stations around New Zealand to create a soil/water balance model.
Meteorologist Seth Carrier says that essentially, the model starts with identifying how much water should be in the soil at a particular time of year.
Rainfall is added to that number, while evaporation from the soil due to sun, wind, warm temperatures and so on, is subtracted from the number.
“So, if we have more water in the soil than expected at a particular time and place that’s a surplus (blue colours on our maps), while less water in the soil than expected is a deficit (orange and red colours on our maps),” he says.
A hotspot is declared if soils are ‘severely drier than normal’, which occurs when Soil Moisture Deficit (SMD) is less than -110mm and the Soil Moisture Anomaly is less than -20mm.
“If an area with drier than normal soils is forecast to get even drier in the future, a farmer may consider starting irrigation to offset that trend.
“Conversely, if a dry area is forecast to get significant rain in the next week, such irrigation may no longer be necessary.
“Areas of the country that are consistently drier than normal for a long period of time may begin to experience meteorological drought, which is something we track with NIWA’s New Zealand Drought Index (NZDI). This can be accessed at: www.niwa.co.nz/climate/information-and-resources/drought-monitor
Seth says it is important to note that while NIWA does have a network of soil moisture sensors around NZ, that network is too sparse to be used for the Hotspot Watch report.
“NIWA’s soil moisture network started around 1998 with about 20 sites. It was instigated by a severe drought at that time. Currently, the network has about 91 sites, with most sites in low elevations due to its use by the primary sector.”
These sites use a variety of sensors. The most modern is called an EnviroPro, which is 80cm long and measures soil water at every 10cm interval over those 80cm.
Soil moisture maps can be found at: www.niwa.co.nz/climate/nz-drought-monitor/droughtindicatormaps)
The Hotspot Watch can be found at: www.niwa.co.nz/our-science/climate/publications/hotspot-watch
