This is the 4797th most frequent Japanese word.
水稲
Water-grown rice.
In this sentence, "水稲" refers to its general practice as a concept or activity.
日本で水稲の栽培は重要な農業活動です。
Rice cultivation is an essential agricultural activity in Japan.
Here, "水稲" is explained as the type of rice suitable for paddy fields.
水稲は主に水田で栽培されます。
Rice is mainly grown in paddies.
In this context, "水稲" pertains to the considerations for growing this specific rice.