根据下图和下表资料，并结合所学知识，完成下列要求。
尼罗河三个源流流域年降水量分布
尼罗河径流量构成
（1）分析尼罗河径流量的季节变化特点及原因。
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（2）为什么尼罗河枯水期径流主要来自白尼罗河？
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（3）简述尼罗河径流季节变化对其下游河谷及三角洲古代农业生产的影响。
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