Sustaining Irrigation Agriculture for the Long-Term: Lessons on Maintaining Soil Quality from Ancient Agricultural Fields in the Phoenix Basin and on the North Coast of Peru

Irrigation agriculture has been heralded as the solution to feeding the world’s growing population. To this end, irrigation agriculture is both extensifying and intensifying in arid regions across the world in an effort to create highly productive agricultural systems. Over one third of modern irrigated fields, however, show signs of serious soil degradation, including salinization and waterlogging, which threaten the productivity of these fields and the world’s food supply. Surprisingly, little ecological data on agricultural soils have been collected to understand and address these problems. How, then, can expanding and intensifying modern irrigation systems remain agriculturally productive for the long-term? Archaeological case studies can provide critical insight into how irrigated agricultural systems may be sustainable for hundreds, if not thousands, of years. Irrigation systems in Mesopotamia, for example, have been cited consistently as a cautionary tale of the relationship between mismanaged irrigation systems and the collapse of civilizations, but little data expressly link how and why irrigation failed in the past. This dissertation presents much needed ecological data from two different regions of the world – the Phoenix Basin in southern Arizona and the Pampa de Chaparrí on the north coast of Peru – to explore how agricultural soils were affected by long-term irrigation in a variety of social and economic contexts, including the longevity and intensification of irrigation agriculture. Data from soils in prehispanic and historic agricultural fields indicate that despite long-lived and intensive irrigation farming, farmers in both regions created strategies to sustain large populations with irrigation agriculture for hundreds of years. In the Phoenix Basin, Hohokam and O’odham farmers relied on sedimentation from irrigation water to add necessary fine sediments and nutrients to otherwise poor desert soils. Similarly, on the Pampa, farmers relied on sedimentation in localized contexts, but also constructed fields with ridges and furrows to draw detrimental salts away from planting surfaces in the furrows on onto the ridges. These case studies are then compared to failing modern and ancient irrigated systems across the world to understand how the centralization of management may affect the long-term sustainability of irrigation agriculture.灌溉农业被誉为解决全球日益增长人口粮食供给问题的方案。为实现这一目标，全球各地干旱地区的灌溉农业正同时朝着扩张化与集约化方向发展，以期构建高产农业系统。然而，超过三分之一的现代灌溉农田已显现出严重的土壤退化迹象，包括盐渍化（salinization）与涝渍（waterlogging），这些问题正威胁着这些农田的生产能力乃至全球粮食供应。令人意外的是，目前针对农业土壤的生态学数据仍十分匮乏，难以支撑这些问题的研究与解决。那么，不断扩张并集约化的现代灌溉系统，该如何长期维持农业生产能力？ 考古案例研究可为灌溉农业系统如何实现数百年乃至数千年的可持续发展提供关键洞见。例如，美索不达米亚的灌溉系统常被引为警示案例，用以阐释管理不善的灌溉系统与文明衰亡之间的关联，但鲜有数据能够明确说明历史上灌溉系统失效的具体路径与根源。本论文采集了全球两个不同区域的生态学数据——亚利桑那州南部的菲尼克斯盆地（Phoenix Basin）与秘鲁北海岸的查帕里潘帕斯（Pampa de Chaparrí）——以此探究在多样的社会经济背景下，长期灌溉对农业土壤的影响，其中涵盖灌溉农业的寿命与集约化发展进程。 来自前西班牙殖民时期（prehispanic）与历史时期的农田土壤数据显示，尽管两地均实施了长期且集约化的灌溉耕作，当地农民仍通过制定适配策略，依靠灌溉农业维持了数百年来的大规模人口生存。在菲尼克斯盆地，霍霍坎文明（Hohokam）与奥哈姆族群（O’odham）的农民借助灌溉水携带的泥沙沉积（sedimentation），为原本贫瘠的荒漠土壤补充了必要的细粒沉积物与养分。同样在查帕里潘帕斯地区，农民一方面在局部区域依托泥沙沉积改良土壤，另一方面通过修建垄与沟（ridges and furrows）的农田结构，将种植区域沟内的有害盐分引流至垄上。随后，本研究将这些案例与全球范围内失效的现代及古代灌溉系统进行对比分析，以解析管理集权化（centralization of management）如何影响灌溉农业的长期可持续性。