简要分析黄土高原水土流失严重的原因。
植被：                                                                ；
降水：                                                                ；
土壤：                                                                ；
人为：                                                                。

简要分析黄土高原水土流失严重的原因。

植被：                                                                ；

降水：                                                                ；

土壤：                                                                ；

人为：                                                                。