| 英文摘要: | 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. |