NDVI values change but there was no change in ground cover

I have a site which I have been monitoring for several months and the NDVI values have been reasonable. Suddenly, the NDVI halved for no apparent reason. There was no change in vegetation or ground cover at the site so this drop in value was not caused by local ground conditions.

The NDVI values, and dates, were:
2021-02-17: ndvi = 0.52
2021-02-22: ndvi = 0.23

I am using the FIS query to obtain the NDVI value (this is copied and pasted from the Sentinel documentation and is not my actual site. Due to confidentiality clause, I unfortunately cannot divulge the BBOX coordinates of my site):

Requesting Stats Without a Histogram
Query URL: http://services.sentinel-hub.com/ogc/fis/{INSTANCE_ID}?LAYER=5_VEGETATION_INDEX&STYLE=INDEX&CRS=EPSG%3A3857&TIME=2015-01-01%2F2015-10-01&BBOX=<BBOX_INFO>&RESOLUTION=10&MAXCC=5

Have others observed such rapid changes in NDVI when there is no apparent change in the vegetation or ground cover? Is there a known cause for such an observation?

Hi @ed6543!

There was a bbox defined in your query URL and I removed it, just in case it was your own.

As for why the NDVI would drop, there could be a number of reasons. My suggestion would be that you look at the true-colour images directly for the specified dates, in case you haven’t yet. It could be clouds, haze, cloud shadows, perhaps event wet surface, or partial data in case you’re on the border of the satellite swath, which would skew the statistic.