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Variability of Hydroclimate in the North American Southwest: Implications for Streamflow, the Spring Dry Season and Ecosystems

机译：北美西南部水文气候的变化性：对水流，春季干旱季节和生态系统的影响

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摘要

The Southwest United States (SWUS) is facing an ongoing drought which has led to water short- ages, in addition to forest mortality due to wildfire and bark beetle outbreaks associated with increased temperatures. This region has a population of 9.6 million people and is one of the fastest growing parts of the United States, and pressure on its resources can be expected to increase in the future. The SWUS is also projected to become more arid in the coming century under greenhouse gas induced climate change, which will impact its environmental, economic and social vitality. This thesis explores the climate dynamics which control water availability, streamflow, and vegeta- tion green-up in the SWUS, in order to constrain our understanding of the mechanisms controlling the ecohydrology of the region, and to inform projections for the 21st century.;Chapters 1 and 2 investigate the climate drivers responsible for producing the observed vari- ability in streamflow for the Gila River, a tributary of the Colorado, and the upper Rio Grande. The Gila is the southernmost snowfed river in the SWUS, and has a spring streamflow peak that responds to melting of the snowpack at its headwaters in New Mexico. The Gila is also sufficiently south so that it has a secondary streamflow peak in the summer which is fed by rains from the North American Monsoon (NAM). On interannual timescales, the Gila's spring peak is primarily influenced by natural variability associated with Pacific sea surface temperature (SST), while the summer peak apparently does not respond to interannual variability. The upper Rio Grande is fur- ther north and east in the SWUS, and only has one streamflow peak occurring in spring-summer which is influenced by both tropical Pacific SST and Atlantic SST. Spring streamflow has also declined in each river post-1998, and this is due to a shift in the tropical Pacific leading to negative precipitation anomalies and drying in the SWUS.;Chapter 2 assess a region of the SWUS that receives both winter storm track precipitation and NAM, and therefore has two periods of vegetation green-up annually with an intervening spring dry season. The first peak in vegetation occurs during the spring, and is influenced by the magnitude of winter precipitation and snowmelt, which gradually adds water to the soils. The second peak in vegetation follows the spring dry season when soil moisture recovers with the arrival of the NAM. A climatic shift in the tropical Pacific occurred in 1997/98 and produced a shift to an earlier and more severe spring dry season, and reduced vegetation green-up. An earlier extended dry period in the mid-century (1948 to 1966) also was influenced by a cool phase of the tropical Pacific, which led to a reduction in precipitation of a similar magnitude as the recent drought. However, the recent drought is more severe - and temperatures also have been greater during the recent period. Using a decomposition of the impact of precipitation and potential evapotranspiration (PET) on soil moisture, we found that PET contributed 39% to the negative soil drying anomalies in the recent post-1998 drought, compared to 8% during the earlier extended dry period. This indicates an increased role of temperature during the recent drying.;In Chapter 4 we evaluated 18 CMIP5 models based on comparisons with observations of pre- cipitation, net ecosystem exchange, leaf area index and soil moisture from land surface model output. Following our evaluation, we selected three models which best simulated the bimodal region: CanEMS2, GFDL-ESM2G and GFDL-ESM2M. These models indicate that overall this region will be drier in the 21st century; runoff is projected to decrease, particularly in the spring, soil moisture is reduced, and snow fall declines. The variability in projected precipitation, how- ever, is large, and we find that for the most part does not exceed what can be expected from model natural climate variability. The multi-model ensemble from the rest of the CMIP5 models indicate an overall decline in annual precipitation by the end of the 21st century, particularly during the spring. The three models also project an increase in net primary productivity in both the spring and summer growing seasons due to the effects of CO2 fertilization. Enhanced vegetation growth is likely to further exacerbate drying of the soils as vegetation draws down moisture, and enhances water losses via evapotranspiration. The fertilization process is, however, still uncertain and fur- ther studies are needed on the representation of CO 2 enhanced vegetation growth in the SWUS to constrain this result.;The findings of this thesis have contributed enhanced our knowledge of how climate dynamics, natural variability, and recent warming have influenced the ecohydrology of the SWUS, and also inform future climate projections. Constraining our understanding of this region is of importance given the growing populations, mounting pressures on natural resources, and anthropogenically induced climate change which is expected to affect this region in the 21st century.
机译：美国西南部（SWUS）面临着持续的干旱，这导致水短缺，此外还有因野火和与温度升高相关的树皮甲虫暴发造成的森林死亡。这个地区的人口为960万人，是美国发展最快的地区之一，其资源压力在未来可能会增加。在温室气体引起的气候变化下，SWUS还将在下个世纪变得更加干旱，这将影响其环境，经济和社会活力。本文探讨了控制SWUS中水的可利用量，水流和植被绿化的气候动力学，以限制我们对控制该地区生态水文学的机制的理解，并为21世纪的预测提供依据。第1章和第2章研究了导致观测到的科罗拉多河支流吉拉河和里奥格兰德河上游水流量变化的气候驱动因素。吉拉（Gila）是SWUS中最南端的积雪河，其春季水流高峰响应新墨西哥州上游水源的积雪融化。吉拉河也位于南部，因此在夏季会有次要的水流高峰，而北美季风（NAM）的降雨则为该水流提供了最大的流量。在年际时间尺度上，吉拉的春季高峰主要受与太平洋海表温度（SST）相关的自然变异性的影响，而夏季高峰显然对年度变异性没有响应。里奥格兰德河上游位于西南偏南，在西南夏季只有一个水流高峰，受热带太平洋海温和大西洋海温的影响。 1998年以后，每条河流的春季流量也都下降了，这是由于热带太平洋的变化导致SWUS出现负降水异常和干燥。;第2章评估了SWUS的地区，该地区既接收了冬季风暴，又接收了冬季风暴和NAM，因此每年有两个植被绿化期，中间有春季干旱季节。植被的第一个高峰发生在春季，并受冬季降水和融雪量的影响，逐渐向土壤中添加水。植被的第二个高峰出现在春季干旱季节，此时随着NAM的到来，土壤水分恢复。 1997/98年，热带太平洋发生了气候变化，导致向更早，更严重的春季干旱季节转变，并减少了植被绿化。本世纪中叶（1948年至1966年）较早的干旱时期也受到热带太平洋凉爽期的影响，这导致降水量的减少幅度与最近的干旱相似。但是，最近的干旱更加严重-最近一段时间气温也更高。通过对降水和潜在蒸散量（PET）对土壤水分的影响进行分解，我们发现，在最近的1998年后干旱中，PET对负的土壤干燥异常贡献了39％，而在更早的干旱时期则为8％。这表明温度在最近的干燥过程中作用增加。在第四章​​中，我们通过与降水，净生态系统交换，叶面积指数和土地表面模型输出的土壤湿度的观察结果进行比较，评估了18个CMIP5模型。经过我们的评估，我们选择了三个最能模拟双峰区域的模型：CanEMS2，GFDL-ESM2G和GFDL-ESM2M。这些模型表明，整个地区在21世纪将变得更加干旱。径流预计会减少，特别是在春季，土壤水分减少，降雪量减少。但是，预计降水的变异性很大，我们发现在大多数情况下都没有超出模型自然气候变异性所能预期的范围。其余CMIP5模型的多模型集合表明，到21世纪末，特别是在春季，年降水量总体下降。这三个模型还预测，由于二氧化碳施肥的影响，春季和夏季生长期的净初级生产力都将提高。随着植被吸收水分并通过蒸散增加水分流失，增强的植被生长可能会进一步加剧土壤干燥。然而，施肥过程仍然不确定，需要进一步研究SWUS中CO 2促进植被生长的表征，以限制这一结果。；本论文的发现有助于增进我们对气候动态，自然状态的认识。变异性和最近的变暖影响了SWUS的生态水文学，也为未来的气候预测提供了依据。鉴于人口不断增长，限制我们对该地区的了解非常重要，对自然资源的压力越来越大，以及人为诱发的气候变化，预计将在21世纪影响该地区。

著录项

  • 作者

    Pascolini-Campbell, Madeleine.;

  • 作者单位

    Columbia University.;

  • 授予单位 Columbia University.;
  • 学科 Atmospheric sciences.
  • 学位 Ph.D.
  • 年度 2018
  • 页码 196 p.
  • 总页数 196
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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