CAREER: Hydroclimatic Response to Natural and Anthropogenic Land Cover Change over South America: A Focus on the La Plata River Basin

本研究旨在了解土地覆盖变化对拉普拉塔河流域(LPRB)水文气候学的影响。LPRB是南美洲仅次于亚马逊的第二大水文流域，面积约为美国大陆的三分之一。该区域的天气和气候以南美季风系统(SAMS)为主，在该系统中，夏季(12月至2月)的降雨由赤道信风进入大陆、穿越亚马逊并由南美低空急流输送到LPRB的水汽提供。在这一过程中，水分会以降雨的形式下降，并通过蒸散作用多次返回到大气中，这一过程被称为水分循环。PI和其他机构之前的工作表明，LPRB上的年平均降水量有60%到70%来自南美大陆的蒸散，其中约24%来自LPRB上的当地蒸散。由于地表是降水的重要水分来源，土壤水分异常可能会持续存在，因为它们会导致降雨量进一步减少。此外，陆面条件可以通过影响低层大气的热力学结构来影响降水，从而调节大气环流和稳定性。陆地-大气耦合在LPRB中的关键作用表明，土地覆盖的变化可能对当地的水气候产生重大影响。国际森林倡议指出，该区域存在严重的森林砍伐现象，2000年至2012年期间的森林砍伐率可能是世界上最高的，因此有必要了解森林砍伐如何影响该区域的季风系统。更具体地说，PI试图了解水分循环和热力/动力陆地-大气反馈对南美降水模式的相对影响。一种假设是，与SAMS相关的最强烈的陆地-大气相互作用发生在春末夏初季节，其空间格局突出了连接热带和亚热带的过渡区。在决定LPRB降水年际变率方面，热力学/动力效应比降水循环的变化更为重要。但PI还假设，预计未来南美洲的森林砍伐将改变水分循环和热力学/动力效应的相对重要性，这种转变将伴随着LPRB降雨量的显著减少。这项研究还将考察森林砍伐在多大程度上改变了降水的频率和强度。此外，该项目将考虑这些反馈在预计未来地表条件变化的情况下可能会如何变化。研究议程围绕三个工具建立，即被称为广义平衡反馈评估(GEFA)的统计方法，以及天气和研究预报(WRF)模型的两个增强版本。统计方法是一种用于量化反馈强度的多变量滞后回归技术，可用于在空间变化的基础上确定水分循环的强度。WRF模式的一个版本(WRF-WVT，以前由PI开发)已得到扩充，包括用于跟踪水蒸气输送的示踪剂，这一配置可用于确定水蒸气的来源区域，从而确定输送和局部蒸发对降水的贡献。该模式还被修改为使用生态系统功能类型(EFT)，它可以调整为代表林地和农田，并且该模式能够模拟中尺度对流系统，该系统占LPRB的大部分降雨量。WRF的另一个版本是WRF-HYPORO，这是一个代表水文循环的地下部分的社区模型，国际水文计划声称这一模型“在南美洲特别重要，因为地下水动力学可以完全改变大气通量”。这项研究伴随着一项教育努力，将开发三门新课程，为学生提供全面的水文气候学基础知识，其中一门课程侧重于水文气象观测，并将为选定的学生提供在夏季为期两周的时间内进行实地考察的机会。该实地考察计划是与斯克里普斯海洋研究所的西部极端天气和水研究中心(CW3E)合作开展的。在2015-2018年期间，学生将参加Calwater 2的实地活动，观察大气河流，测量降雨量、溪流和土壤湿度。第二门课程向学生介绍使用WRF-HEDIO模式的数值模拟，在这门课上，被选中的学生在夏季前往国家大气研究中心参与模式开发。开发了第三门课程，向具有大气科学背景的学生教授基础水文学。这门课程取代了传统的水文学课程，后者从大气科学的角度教授这门课程。该课程旨在作为典型水文学课程的替代课程，在水文学课程中，重点放在与自来水厂的建设和运营有关的问题上。本课程试图开发一种描述表层和次表层过程的统一语言，类似于用于大气过程的概念框架，并帮助学生使用包含大气、表层和次表层成分的模型。

This research is an effort to understand the influence of land cover change on the hydroclimatology of the La Plata River Basin (LPRB). The LPRB is the second largest hydrological basin in South America after the Amazon, at about a third the size of the continental US. The weather and climate of the region is dominated by the South American monsoon system (SAMS), in which summer (December to February) rains are fed by moisture which enters the continent in the equatorial trade winds, crosses the Amazon, and is transported to the LPRB by the South American low level jet. Over the course of this route the moisture can fall as rain and be returned to the atmosphere through evapotranspiration multiple times, a process referred to as moisture recycling. Previous work by the PI and others suggests that 60 to 70 percent of the mean annual precipitation over the LPRB comes from evapotranspiration from the South American landmass, with about 24 percent coming from local evapotranspiration over the LPRB. Since the land surface is an important source of moisture for precipitation, soil moisture anomalies can become persistent as they lead to further reductions in precipitation. In addition, land surface conditions can affect precipitation through their influence on the thermodynamic structure of the lower atmosphere, which can modulate atmospheric circulation and stability. The key role of land-atmosphere coupling in the LPRB suggests that changes in land cover could have significant impacts on the local hydroclimate. The PI notes that there has been intensive deforestation in the region, with perhaps the highest deforestation rate in the world for the period 2000 to 2012, thus it is of interest to understand how deforestation is affecting the monsoon system in the region. More specifically, the PI seeks to understand the relative impacts of moisture recycling and thermodynamic/dynamic land-atmosphere feedbacks on South American precipitation patterns. One hypothesis to be pursued is that the strongest land-atmosphere interactions occur during the late spring and early summer season in association with the SAMS, with a spatial pattern that highlights the transition regions linking the tropics and subtropics. Another is that the thermodynamic/dynamic effects are more important than changes in precipitation recycling in determining the interannual variability of LPRB precipitation. But the PI also hypothesizes that projected future deforestation in South America would change the relative importance of moisture recycling and thermodynamic/dynamic effects, and the shift would be accompanied by significant reductions in LPRB precipitation. The research will also examine the extent to which deforestation changes the frequency and intensity of precipitation. In addition, the project will consider how these feedbacks may change under projected future changes in land surface conditions.The research agenda is built around three tools, a statistical approach known as the generalized equilibrium feedback assessment (GEFA) and two enhanced versions of the Weather and Research Forecast (WRF) model. The statistical method is a multivariate lagged regression technique developed to quantify feedback strength, which can be used to determine the strength of moisture recycling on a spatially varying basis. One version of the WRF model (WRF-WVT, previously developed by the PI) has been augmented to include tracers used to track water vapor transport, and this configuration can be used to determine the source regions of water vapor and hence the contributions of transport and local evaporation to precipitation. The model has also been modified to use ecosystem functional types (EFTs) which can be adjusted to represent forested and agricultural land, and the model is able to simulate the mesoscale convective systems which account for much of the rainfall in the LPRB. The other version of WRF is WRF-hydro, a community model which represents the subsurface component of the hydrological cycle, which the PI claims is " particularly important over South America, as ground- water dynamics can completely change the atmospheric fluxes".The research is accompanied by an education effort in which three new courses would be developed to give students a thorough grounding in hydroclimatology One course focuses on hydrometeorological observations, and will give selected students an opportunity to perform fieldwork during a 2-week period in the summer. The fieldwork program is developed in partnership with the Center for Western Weather and Water Extremes (CW3E) at the Scripps Istitute of Oceanography. For the period 2015-2018 students will participate in the Calwater2 field campaign to observe atmospheric rivers, taking measurements of precipitation, stream flow, and soil moisture. A second course introduces students to numerical modeling using the WRF-hydro model, and in this class selected students travel to the National Center for Atmospheric Research during the summer to participate in model development. A third course is developed to teach basic hydrology to students with an atmospheric science background. This course replaces traditional hydrology courses, in which the subject is taught from an atmospheric science perspective. The course is intended as an alternative to typical hydrology courses in which the focus is on issues relating to the construction and operation of waterworks. The course attempts to develop a unified language for the description of surface and subsurface processes, which is analogous to the conceptual framework used for atmospheric processes and helps students to work with models which incorporate atmosphere, surface, and subsurface components.