I like this idea. There are a lot of interesting implementation possibilities in addition to the ones already mentioned.
Water magic working best next to rivers, oceans, swamps and whenever there is fog and rain. Death magic better near swamps and graveyards and during the times of the year when life is difficult, freezing winter and bitter heat. Life magic having much increased force in the Springtime and somewhat better in farmland, rivers and light rain.
Troops who grew up in the far north or in the mountains can perform better in winter...but worse in the summers or in warm climes while tropical troops are just the opposite.
I also like the implication that significant military campaigns would have to be well planned. An empire might not be conquored in a single campaign covering one year. A city may fall but winter approaches so the invader would have to hunker down until spring and the defender gathers forces during the interim. Lots of interesting roleplaying possibilities.