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Vadose Zone
examining the region between the land surface and the water table
| Question | Answer |
|---|---|
| What is wettability? | When two immiscible fluids contact a solid surface, one will adhere more readily to the surface. Wetting phase; wets the solid surfaces. The other phase non-wetting phase. Type of surface and fluids under consideration will control the wettability |
| Hydrophilic | mineral surfaces that love to hold water |
| Interfacial Tension | interface b/w two immiscible fluids molecules have greater attraction for each other than they do for opposing fluid air-water = 0.072N/m(@25C) |
| Capillary Pressure | cohesive interfacial forces cause the fluid interface to curve and result in a pressure difference across the interface related to the radius of curvature of the interface; related to pore radius |
| When is water pressure zero? | At the water table |
| What is a tensiometer? | device used to measure pore water pressure in the vadose zone Water moves through the porous ceramic cup to reach equilibrium with the soil pore pressure. This creates a negative pressure within the tensiometer that is read with a vacuum gauge |
| what is the soil water rentention curve dependent on? | range of pore sizes in the soil degree of dryness |
| Capillary Fringe | The tension saturated zone immediately above the water table. Water is held under slight tensions, yet the porous media remains saturated |
| Vadose Zone Hydraulic Conductivity | K becomes a function of water content or pressure head in the vadose zone |
| Relative hydraulic conductivity, Kr | the ratio of the hydraulic conductivity at a given saturation to the saturated hydraulic conductivity, Ksat |
| Capillary Barriers | unique flow systems; used in landfills, mine waste covers, golf course, etc. |
| Darcy's Law and Vadose Zone | K is dependent on the variable h ( = pressure head + z) itself. This is called a non-linear dependence of K on h, and has a major influence on vadose zone flow |
| How does infiltration vary with time? | it increases |
| Infiltration Rate, f | the rate at which water enters the soil; function of both soil conditions and rainfall rate |
| Infiltration Capacity, f* | theoretical max potential infiltration rate if water is freely available at the surface; decreases w/ t during rainfall event; initially driven by gravity and capillary forces = decreases as the soil wets up and f* asymptotically approaches Ks |
| Evapotranspiration | the exit of water from the soil (exfiltration) takes water away & the soil becomes dry rate affected by depth of water table |
| recharge | net inflow across the water table into GW |
| discharge | net outflow across the water table |
| Infiltration and Recharge | Infiltration of rain is followed by periods of evapotrans. Flow direction in the vadose zone changes frequently depending on boundary condition & WT fluctuates accordingly. Recharge occurs when infiltration exceeds evapotrans over a particular period |
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