# Shared Flashcard Set

## Details

Hydrology - Soils
Soil aspect of hydrology
27
Geography
Graduate
03/18/2011

## Cards Return to Set Details

Term
 Material Properties of Soil
Definition
 Particle density, bulk density and porosity.
Term
 Soils
Definition
 Soils are mixtures of solids, air and water.
Term
 Particle Density
Definition
 Average density of mineral grains in the soil.  Pm = Mm / Vm
Term
 The value of tension for a given  water content  also depends  on
Definition
 the history of wetting and  drying (hysteresis).
Term
 Bulk Density
Definition
 Bulk density:  Pb = Mm/Vs = Mm/(Va+ Vw + Vm)  Where Vs is the total volume of the soil sample, and Va, Vw and Vm are the volumes are the air, water and mineral components of the soil. Typical values are 1000-2000 kg m-3
Term
 Porosity
Definition
 Porosity (proportion of pore space in a volume of soil):   Φ = (Va + Vw)/Vs = (1- ρb / ρm).        Porosity is typically around 0.5.     In many soils porosity decreases with depth due to compaction.
Term
 Volumetric water content
Definition
 ratio of water volume to total volume   θ =  Vw/Vs    Equivalently θ = (Mswet – Msdry )/( ρw.Vs )
Term
 Degree of Saturation ( wetness )
Definition
 the proportion of pores that contain water, is: S = Vw/(Va + Vw) = θ/ Φ   If the soil is saturated, then  S = 1
Term
 As the soil dries
Definition
 the capillary  tension increases,  the  water is  held  more tightly to grains (the  menisci have a smaller radius of  curvature
Term
 Field Capacity
Definition
 The maximum amount of water that a soil can hold after gravitational drainage.
Term
 Wilting Point
Definition
 is the water content at which plants can no longer extract water from the soil.
Term
 Fine grained soils
Definition
 high porosity high field capacity high wilting point
Term
 Course grained soils ( sand )
Definition
 low field capacity low surface area capacity leading to low wilting point
Term
 Intermediate soils ( silty )
Definition
 intermediate wilting points and field capacity
Term
 Darcy’s Law states that the flow rate in the horizontal depends on
Definition
 1) the ability of the soil to “conduct” water; (2) the magnitude and direction  of the slope, which is the  gravitational  potential energy gradient per unit weight of flowing water (soil water wants to  flow downhill, towards lower gravitational potential energy), and (3)  the magnitude and  direction  of the horizontal gradient in soil water pressure (if soil water pressure decreases  in the x direction, the water wants to flow in that direction)
Term
 Darcy’s Law states that the flow rate in the vertical depends on
Definition
 For a given hydraulic conductivity, the vertical flow rate depends on the sum of  the gravitational potential energy  gradient per unit weight of water (dz/dz =1) and the vertical gradient in soil water pressure
Term
 The hydraulic conductivity Kh (θ)  depends on:
Definition
 a)    The type of soil,  tending to be larger for coarse-grained soils (water path is less sinuous); b)   The volumetric soil water content θ;  it increases with increasing  soil moisture to is maximum value at saturation( the saturation hydraulic conductivity  K*h
Term
 The pressure head ψ is being more negative means
Definition
 more tension in the soil
Term
 The   pressure head  ψ(θ):
Definition
 a)   Is negative for unsaturated soils, and increasingly negative the drier the soil.  It is positive for saturated soils. b)    It depends on soil type.  For a given degree of saturation, it is more negative in fine-grained soils than in coarser-grained soils.
Term
 Hydraulic diffusivity
Definition
 Relationships between matric suction,  volumetric water content and hydraulic conductivity can also be expressed in terms of the hydraulic diffusivity, Dh(θ):
Term
 Soil profiles: Pedalogic horizons
Definition
 Surface Horizon   Subsoil   substratum
Term
 Infiltration rate
Definition
 Rate (m s -1) at which water enters the soil from   the surface
Term
 Water input rate
Definition
 Rate at which water is added to the surface
Term
 Infiltration capacity
Definition
 :Maximum rate at which infiltration can  occur (also called infiltrability)
Term
 Factors affecting the infiltration rate
Definition
 Water input rate from rainfall, snowfall or irrigation •The saturated hydraulic conductivity of the soil profile •The degree to which pores are already filled with water when the infiltration  process begins •Variations in hydraulic conductivity through the soil profile  •The inclination and roughness of the soil surface •Chemical characteristics of the soil surface •Physical and chemical properties of water
Term
 Richards Law
Definition
 The time rate of change in volumetric soil moisture for  a given  thin layer of soil  depends on the vertical rate of change of the hydraulic conductivity (itself a function of θ) and the vertical rate of change  of the product of (a) the hydraulic conductivity, and (b) the vertical rate of change of the pressure head ψ, (the matric suction gradient) the pressure head also being a function of θ. In this expression, z is taken to increase downwards.
Supporting users have an ad free experience!