There is a surreal thing on my NDVI images. NDVI values on lake is ranging from -0.04 to 0.7. (The lake surrounded by forest and has freshwater). Theoretically, negative value of NDVI is correspond to water. I want to mask NDVI image to remove pixels with negative values which include waterbody. But there are still positive values on lake. So I cann't set zero mask to remove whole waterbody. So what I'm gonna ask are: (i) why NDVI value on water has so high positive value? is there any explanation for that? (ii) how can I mask waterbody in order not to remove the other pixels?
By the way, the NDVI images extracted from MOD13Q1 vegetation index product.
I have already converted reflectance value into NDVI value as dividing by scale factor (10000). I uploaded 2 images. first image is NDVI image in april where value of lake is negative (correct). In April, lake is fully covered by snow. This image doesn't have any problem. This one is NDVI image in June when no snow cover on lake. . when I check values of other lakes on image 2, they are in correct (negative) values. Only this lake has positive value. The name of lake is Khuvsgul with freshwater.
So I'd like to ask whether green plant at the bottom of the lake can effect on the NDVI value during the period no snow cover on lake or not?
I don't have any idea other than this guess.