Rafael Bras, a professor of civil and environmental engineering who pioneered the field of hydrologic science, is MIT's James R. Killian Jr. Faculty Achievement Award winner for 2008-2009. If he doesn’t have the whole world in his hands, Rafael Bras certainly grasps more pieces of the gigantic puzzle than most of us. Often credited with launching the science of hydrology — the study of water’s crucial role in Earth systems -- Bras has developed passions for pretty much the rest of the Earth sciences as well. In this fond, valedictory lecture to MIT (he’s recently taken the post of Dean of Engineering at UC Irvine), Bras describes some of the research problems that have long fascinated him.  Bras enjoys wrapping his mind around big things, such as the size of the world’s oceans, whose numbers are in the billions of cubic kilometers. What interests Bras even more are the ways huge amounts of water cycle from the atmosphere as rain, into the soil, as runoff to the sea, and back again. He says “a lot of what we depend on is the result of differences between large numbers. It is those differences between very large numbers that makes it so uncertain, variable and so sensitive to our intervention or changes.” The physics behind the various water cycles involves vast and continuous transfers of energy: rain changes soil moisture, which changes the amount of radiation the earth reflects, which affects evaporation, which changes the convection potential energy, which impacts cloudiness, which leads again to rain. It’s a “very nonlinear, very interacting cycle,” says Bras, which is “elegant and quite pretty.” Bras helped lay out the models for these cycles. His studies describe how nature seems to prefer extremes like flood and drought, and how in river basins all over the world, nature favors fractal organization and minimal energy expenditure. Other observation and modeling projects may have consequences for the future of the planet: A nine-year study...