The normal distribution does not define a mathematical meaning of the term "normal". Any distribution can define a normal range relative to that distribution.
Given an expected normal distribution of soil dryness you can compare a given year's distribution.
> Any distribution can define a normal range relative to that distribution.
Any particular sample will always have a corresponding probability in a normal distribution. So what you're saying is kind of right- values always fall within the range of a normal dostribution. That's not what it means to be outside the normal distribution, though. If you have another year that only partly overlaps with the expected distribution, you'll have an area that does not overlap despite being within the same range. That area is the fraction of values outside the normal range.
Log-normal is extremely common in hydrology. Turns out an anomaly in this case is defined as outside one standard deviation[1], so in a perfectly normal year you would expect ~15.9% of soil to be dryer than normal.
