I want to calculate moisture stress index for a deciduous forest using the formula from the article 'Detecting vegetation leaf water content using reflectance in the optical domain' (Ceccato et al. 2001) (SWIR/NIR). However, my forest browns/loses its leaves in autumn. This leads to the NIR decreasing, which leads to lower MSI values (meaning less water stress). However, this does not seem an accurate reflection of reality, as my area actually records far more rainfall in autumn than in other seasons. Also, the true colour images show that in autumn the grass fields are more yellow/bare in summer, and thus summer should be more dry and have higher MSI values.
How to deal with this issue? Is the MSI from the linked article simply not suited for deciduous forest, and are there any suggestions for moisture stress index that are accurate in all seasons?
I've added a picture showing the seasonal differences (values for nir and swir are the original sentinel values, so not divided by 10000), and a graph showing the monthly rainfall in mm (clearly summer is drier)