US - Kenya Doctoral Dissertation Enhancement Project: The Impact of Macropores on the Spatial and Temporal Patterns of Soil Moisture in Dryland Ecosystems of Central Kenya

普林斯顿大学PI Kelly Caylor将指导他的博士生Trenton Franz进行本次博士论文强化项目。肯尼亚中部旱地生态系统的畜牧业活动正受到土地利用政策变化和气候变化的威胁。在1960年代末？20世纪80年代，当地牧民被安置在集体牧场，这从根本上改变了他们的游牧生活方式。人们的定居，加上医疗保健的改善和人口的增长，导致了对景观的过度放牧。从原生的树草稀树草原到退化的贫瘠景观的变化对人口、野生动物种群、水和能量平衡产生了严重的影响。关于水平衡，对系统的一个影响是降雨分布的变化。该地区的降雨记录表明，该地区的降水正在向强度更大的罕见事件转变，而年总量保持不变。土地利用和气候变化对系统的扰动可以用生态水文模型来解决。旱地生态系统过程是由大气、土壤和植被之间复杂的相互作用所控制的，这些相互作用通过水的质量平衡紧密地耦合在一起。完全解析的地表和地下过程的物理模型需要数值程序和参数化，这对于解决生态水文兴趣的空间（山坡）和时间（许多植物世代）尺度是很重要的。为了降低模型的复杂性，进行了水平衡研究，从不同景观斑块类型（裸土、草地和树木）中经验推导通量项。研究结果揭示了水平衡的一个强大的地下组成部分，白蚁可能在创造通量方面发挥了重要作用。为了解决山坡水平衡的地下成分，提出了三种创新的地球物理方法研究。两项研究将使用直流电阻率来跟踪注入土壤介质的水的脉冲。第一项研究将在实验室进行，在一种均匀的沙子中测试方法和仪器。第二项研究将把水注入白蚁巢中，追踪水在下游的再分配情况。电阻率测量将用于创建白蚁巢水分再分配的总体图像，并希望使用解析解来近似该过程。解析解对于生态水文景观模型的实现是可取的，因为它们将大大降低复杂性。第三项研究将探讨多白蚁巢对山坡水平衡的时空影响。电磁感应（EMI）将用于获得土壤顶部1米的总体电导率（5-10公顷）的调查数据。这些实验将于2009年在肯尼亚中部的Mpala研究中心进行。该研究中心主任Margaret Kinnaird博士将担任该项目的主持合作者。拟议活动的智力价值将是对肯尼亚中部旱地生态系统的山坡水文进行实地和模拟研究。创新研究将具体解决白蚁对山坡水平衡的时空影响。研究结果将用于证明在更合适的空间（山坡）和时间（许多植物世代）尺度上对生态水文系统的数值模型进行合理的简化。拟议活动产生的更广泛影响将为Mpala研究中心等团体提供分析工具，以解决社区和政府对土地利用和气候变化的担忧。旱地生态系统覆盖了地球的20%，对扰动极其敏感。该模型可以运行气候变化和土地利用政策的多种情景，为当地社区提供关于当前放牧做法和未来政策决策的更准确、更可靠的估计。

PI Kelly Caylor of Princeton University will supervise his doctoral student Trenton Franz in this Doctoral Dissertaion Enhancement Project. Pastoralist activities in the dryland ecosystem of central Kenya are being threatened by changes in landuse policy and shifts in climate. In the late 1960?s, native pastoralists were placed on Group Ranches, which fundamentally changed their nomadic lifestyle. The sedentarization of the people coupled with improved healthcare and population growth has lead to overgrazing of the landscape. The change from native tree-grass savanna to degraded barren landscapes has severe implications to human populations, wildlife populations, water and energy balances. With regard to water balance, one impact to the system is the change in the distribution of rainfall. Rainfall records in the region indicate a shift to more intense infrequent events, while annual totals remain unchanged. Perturbations to the system from landuse and climate change can be addressed with ecohydrologic models. Dryland ecosystem processes are governed by complex interactions between the atmosphere, soil, and vegetation that are tightly coupled through the mass balance of water. Fully-resolved physical models of surface and subsurface processes require numerical routines and parameterization that are not trivial to solve for the spatial (hillslope) and temporal (many plant generations) scales of ecohydrologic interest. In order to reduce model complexity, a water balance study was installed to empirically derive flux terms from different landscape patch types (bare soil, grass, and tree). The results of the study revealed a strong subsurface component of the water balance and that termites may play a substantial role in creating the flux. In order to resolve the subsurface component of the hillslope water balance, three studies are proposed using innovative geophysical methods. Two studies will use direct current resistivity to track a pulse of water injected into a soil medium. The first study will be performed in the laboratory in a uniform sand to test the methodology and instrumentation. The second study will inject water into a termite nest to track the redistribution of water downstream over time. The resistivity measurements will be used to create a general picture of water redistribution from termite nests with the hope of using analytical solutions to approximate the process. The analytical solutions are desirable for implementation in an ecohydrologic landscape model as they will greatly reduce complexity. The third study will investigate the temporal and spatial impacts that multiple termite nests have on the hillslope water balance. Electromagnetic induction (EMI) will be used to obtain survey data (5-10 ha) of the bulk electrical conductivity of the top 1 meter of soil. The experiments will be carried out in 2009 at the Mpala Research Center of Central Kenya. The director of the research center, Dr. Margaret Kinnaird, will serve as the host collaborator on the project. The intellectual merit of the proposed activity will be field and modeling studies on the hillslope hydrology of the central Kenya dryland ecosystem. The innovative studies will specifically address the spatial and temporal impacts that termites have on the hillslope water balance. The results of the studies will be used to justify reasonable simplifications to a numerical model of the system at more appropriate spatial (hillslope) and temporal (many plant generations) scales of ecohydrologic interest. The broader impacts resulting from the proposed activity will be to provide groups like the Mpala Research Center with analytical tools to address concerns of the community and government about landuse and climate change. Dryland ecosystems cover 20% of the earth and are extremely sensitive to perturbations. Multiple scenarios on climate change and landuse policy can be run with the model that will provide more accurate and robust estimates on current grazing practices and future policy decisions to local communities.