Does the water content in milk change between seasons?
Sure in a minor sort of way. Let's say that a milk contains 12% solids. Half this (Jersey Cows) is fat because the it's spring and the cows have been eating grass. So, the fat content is 6%. Now it's fall and the fat content drops to 5% so the total solids drop to 11%. The grand total still has to be 100% so the water content increases to make up for the drop in the fat content. Got it?
It can also change daily depending on how hot it is because a cow will drink more water and lay around more (eat less) on hot days so generally if a cow is on pasture you will have more water and less solids during the summer compared to winter.