Exploring the influence of land-use and climate on regional hydrology and groundwater recharge.

摘要

Global energy, water, and bio-geochemical cycles are strongly linked to land-use and land-cover characteristics. Land-use and land-cover is also a component of the environment extensively impacted and continuously altered by human activity. Population growth, food, energy and other needs coupled with human ingenuity has greatly altered and will continue to change the terrestrial biosphere across the globe. With additional concerns for significant and widespread land-use/land-cover transformations due to global climate change, a need to better understand the effects of these transformations on environmental systems from local, regional, to global scale has emerged in the recent decades.;In this dissertation I have investigated the impacts of land-use and land-cover (i.e. vegetation) on groundwater recharge, which is a critical component of the hydrologic cycle. The dependence of people and many sensitive ecosystems around the world on groundwater alone warranted a closer look at how changing land-use/cover and climate are affecting the quantity and quality of groundwater. By evaluating streamflow, climate, land-cover, and other attributes in Michigan's watersheds we showed that intense agriculture reduced summer time streamflow, hence groundwater recharge in such watersheds. By quantifying baseflow discharges relative to precipitation in July-September peak growing season over multiple years we found that recharge in primarily agricultural (>70 agricultural uses by land area) watersheds was only one third of the recharge in watersheds that are mix use (50% agricultural uses).;To better grasp how land-cover; specifically vegetation, vegetation differences, and vegetation dynamics affect recharge we adopted geophysical techniques, a step beyond the traditional uses of geophysical methods as well as an unorthodox approach to terrestrial ecosystem investigations. It was hypothesized that there would be observable differences in the way vegetation interacts with the shallow subsurface and such interactions could be quantified with geophysical methods. Based on multi-year geophysical monitoring of soil moisture at a forest-grassland ecotone we found large seasonal and long-term differences in the way vegetation affects groundwater recharge as well as shallow groundwater environments. Apart from water use differences, we show that soil temperature as well as salt dynamics even at very local scales are affected by vegetation differences. For example we observed that forests in shallow groundwater regions are likely to increase groundwater salinity compared to grasslands in similar settings. These findings contribute to developing greater insights into the functioning of the natural environment and how anthropogenic forcings through land-use change may imperil or help protect the health of hydrologic systems in a range of regions.