Tutor profile: Carlos O.
Subject: Environmental Science
What is the difference between the center of mass of rainfall and the center of mass of runoff?
The center of mass of rainfall is the point at which half of the total rainfall has fallen over a period of time. That means that you need to have data regarding how long the rain has fallen over the period of time that you wish to investigate, and you also need to measure how much rain fell per unit of time. Once you have that information, you can calculate when 50% of the rain had already fallen. This is the center of mass of rainfall. The center of mass of runoff is very similar. It is the point at which half of the total runoff has already happened. Again, you need to have information over the period of time you wish to investigate about how much runoff happened per unit of time and then you can calculate when, along that time period, 50% of the total runoff had already happened.
Subject: Earth Science
Why is it that permeability and porosity aren't always directly related?
Porosity is a description of the physical condition of a material: how much of this material is just empty space, how many holes does it have? Primary porosity is a reflection of the amount of pores within the material itself. Secondary porosity is a reflection of the amount of other spaces caused by external factors, like tunneling animals, or tree roots. Permeability, on the other hand, is a description of how well a fluid can travel through the pores of the material. Permeability will depend on the kind of fluid we study. If it is water, for example, then permeability will reflect the system's hydraulic viscosity: how well water can move through the pores in the system. Materials with low permeability typically promote erosion and have higher runoff rates. In short, permeability depends on porosity and it integrates other factors too, like viscosity of the fluid. As a couple of examples: sandstone is somewhat porous and has moderate permeability, which encourages flow of fluid throughout its entire structure. However, granite, for example, has extremely low permeability and extremely low porosity, which encourages higher runoff and surface erosion.
Subject: Environmental Engineering
What is a simple explanation of Darcy's Law?
Darcy’s law gives information on the flow (Qout ) of a fluid through porous material depending on the following factors: hydraulic conductivity, the cross-sectional area of the soil column, and the hydraulic gradient. Hydraulic conductivity, K, is a measure of how well water can travel across the material. The cross-sectional area of the column, Ac, indicates how wide the tunnel through which the fluid is moving is (the bigger the cross-sectional area, the greater the flow). The hydraulic gradient is a measure of the difference in height , Δh , over the difference in length, Δl, of the soil column. The bigger the difference in height, for example, the more flow you will get because of gravity. The bigger the difference in length, the longer it will take for the fluid to traverse the material. Qout = KAcs(Δh/Δl)
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