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Lecture: Soil Pores: Narrator: Listen to partof a lecture in an Earth science class. Professor: Okay. We're going to talk about the second half of the chapter now, the part on soil hydrology. Now I want you to skim that section so you'll be familiar with the material but it won't be on the test because well it's a little outdated. It doesn't reflect the most recent research. You see up to about a year or so ago we believed that rainwater entered soil, mixing and displacing water that are already there, pushing down deeper in the soil. It becomes groundwater, heads downhill and eventually enters local stream and river systems. Over time it evaporates into the atmosphere. Now most of the time this is still the case as we know. But recent findings have shown that how water behaves depends on the soil and the climate, that it isn't always so clear-cut. New technology is available that provides us with a more detailed stable isotope analysis, which led to track water travels through soil as part of the hydrological cycle, the water cycle and ... Yes, Jannet? Jannet: Before you go on, could you please explains stable isotope analysis? I mean, how it works? Professor: Ah, right, right. Okay. Stable isotope analysis. New stable isotope analysis is a research method that tracks water using isotopes. Isotopes are well, each element – hydrogen, oxygen, and so on – has different isotopes. They are like different versions of the same element. And just as we can identify people from their fingerprints, we can trace isotopes using stable isotope analysis. So researchers collect water from different locations during a rainfall, look at the isotopes of the elements and use that information to track the water through the soil and into nearby streams. So a fairly recent study looked at water in the region in the Cascats, mountains in the Northwest United States. That area has a seasonally dry climate, with dry summers followed by a rainy season. The researchers tested the water. They analyzed the isotopes of the water in the soil and the trees and in nearby streams and they found significant differences. The samples had two, two distinct isotopic signatures which means the samples came from different rainfalls. It turned outthe water in the soil near the trees did not contribute to stream flow. Furthermore, they discovered the water from the first rainfall of the rainy season remains in the soil far longer than expected. You see soil consists of particles that are packed together but not so tightly, that there is air or space between them. We call these spaces pores and different soil textures mean different pore sizes. In the area studied the soil around the roots of trees is characterized by small pores. When rain water to startthe rainy season saturates the ground there, it fills the small pores around the roots and it's held there. So you might see lessthan 5% of the first rainfall actually moving down to the soil, enter to the streams. Instead it'stightly bound in that soil beneath the trees and what we discovered is that throughout the rest of the rainy season these trees just draw from that water that has filled those small pores. You see small pores are slower to drain. They have what'scalled the longest water residence time. So in subsequent rainfalls, after the moisture in the soil around the tree roots reaches a maximum, new rain water flows down into empty larger pores, away from the tree roots, but this water never really mixes with the water in the smaller pores and it doesn't stay in the soil as long. It moves down through the soil into the streams. Now obviously different soils have different pore sizes and not every location has the same dry rainy seasonal pattern. So the study is just a preliminary look at one specific region, one specific circumstance. We need to repeat the research elsewhere, but with the advances in stable isotope analysis we can now do it. And here we reach what is in my view the most valuable aspect of these research findings, because you see this new 72 understanding of water movement has been changing the way we understand other things as well. For example, it's making us look at how pollutants move through or are trapped in soil and that has far-reaching implications outside the hydrological cycle.