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Professor Rodriguez: The kingdom of Xincua experienced significant population loss in the 10th century. The best explanation for this is a drier climate that lasted about 100 years. That period included some less dry decades but also at least four extremely dry decades, as attested by tree-ring and sediment data. During these decades, growing crops became harder, especially since the semiarid Xincua territory needed extensive irrigation even in less dry periods. The resulting population loss and unrest-arising from out-migration, increasing mortality due to poorer nutrition, and a lower birth rate-likely made maintenance of irrigation systems more difficult. Some small dams, for example, deteriorated significantly during the dry period. : Multi Source Reasoning (MSR)