Tǔrǎng de yǔshí shì shuǐtǔ liúshī de zhǔyào yuányīn zhī yī.
Rain erosion of soil is one of the main causes of soil loss.
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Technical term used in geology, geography, and conservation contexts. In everyday speech you'd just say 雨水冲刷 (yǔshuǐ chōngshuā, rainwater washing away).