Barbara Stanwyck's hair is not getting wet when she is running around in the rain with the shotgun; but when she returns inside the house, it is suddenly wet---and even wetter in the next scene before she returns outside, where it is dry again.
The year of the film is 1971. Susan has been away for seven years. That means that she left in 1964. The styles depicted in the beginning of the movie, which takes place in 1964 supposedly, are early seventies. Also, Barbara Perkins hair is very wavy. The actress who plays young Susan has very straight hair. And, the young girl's hairstyle is early seventies not sixties. Plus, Barbara Stanwyck's character seems to have not aged at all in seven years.
At one point in the film, it is established that Susan Wilcox took a short drive around town in the station wagon. It would be implausible for Susan to have any driving skills, much less a driver's license, since she spent the past seven years in an institution, where she was admitted at age 13. She was also catatonic during many of those years.
In the outdoor pool scene with Miriam and Susan having breakfast.