Posted by Kshama Awasthi
Green vegetation growth is a useful indicator of drought events and vegetation indices such normalised difference vegetation index (NDVI) and enhanced vegetation index (EVI) are often used for the assessment of agricultural drought. I have been evaluating the sensitivity of these indices to detect the impact of drought on forest areas in various parts of Aotearoa New Zealand. In general, a higher NDVI or EVI value represents more healthy vegetation so I expected to see a decline in the vegetation indices during drought.
Right: NDVI values for the Hunua Ranges for the month of March
There was a slight decline in NDVI in 2013 during the drought. To confirm the role of drought in the decline in NDVI, I am using soil moisture deficit (SMD) data from NIWA. SMD is calculated using incoming daily rainfall (mm), outgoing daily potential evapotranspiration (mm) and fixed available water capacity that is the amount of water in the soil reservoir that plants can use.
Above: Soil moisture deficit and vegetation health indices for summer months at Hunua Ranges.
As expected, the NDVI (r2=55%) and EVI (r2=65%) both show a strong relationships with SMD which indicates NDVI and EVI values are responsive to drought.
Above: Relationships between soil moisture deficit and NDVI and EVI vegetation indices for summertime at Hunua Ranges, 2006-2017.
EVI values decreased more in response to drought conditions as compared to NDVI, indicating that EVI is more sensitive then NDVI on the onset of drought conditions.
Since the relationship between EVI and SMD is stronger than the correlation between NDVI and SMD, EVI can be the better indicator of drought detection in forest as compared to the NDVI as EVI provides improved sensitivity in high biomass regions while minimizing soil and atmosphere influences.
This is the result for the Hunua Ranges and calculations for other sites are still ongoing.