Weeks 3&4 Flashcards
What is the vapour pressure deficit?
Vapour pressure deficit is the difference between vapour pressure and saturated vapour pressure, Pvap,sat - Pvap, (moisture in the air vs. how much moisture it can hold when saturated)
What is the rate of evaporation (E) proportional to?
The rate of evaporation is proportional to the vapour pressure gradient.
What is the equation for vapour pressure gradient?
delta Pvap / delta Z = (Pvap,surf - Pvap,a) / delta Z
Describe relative humidity and dew point in terms of the saturated vapour pressure curve?
Consider a point A on the graph. The relative humidity is the ratio of that vapour pressure (A) to the corresponding saturated vapour pressure for that same temperature (point B). The dew point is the temperature corresponding to that vapour pressure on the saturated curve (point C).
How does wind affect evaporation?
When warm dry air (wind) is blown across the surface, it removes the accumulated water vapour, thus increasing the vapour pressure deficit.
What is the evaporation equation and what are the variables?
A.E = I - O +- dS/dt
A: surface area (m2)
E: evaporation rate (m/s)
I, O, dS/dt: as per water balance equation (m3/s)
What is the average pan factor value (Kp)?
Kp = 0.7
How are vapour pressure deficit and evaporation related?
Evaporation ‘fills’ the vapour pressure deficit, and occurs when pressure at a surface is greater than pressure in the air (e_surf > e_air).
What is the equation for aerodynamic evaporation calculation and what is its other name?
Lumped mass transfer approach: dmv/dt = B.pw(Pvap,sat - Pvap).A dmv/dt: mass flux [kg/s] B: mass transfer coefficient [m/Pa.s] pw: water density [kg/m3] Pvap,sat - Pvap: vapour pressure deficit [Pa] A: surface area [m2]
Evaporation rate = dmv/dt / pw / A
What are Eo and ETo?
Eo is evaporation and ETo is evapotranspiration from an open water body.
What is the equation for calculating evapotranspiration from pan evaporation data?
ETo = Kp.Ep
What is a ‘lysimiter’ and what is it used for?
A lysimeter is a tank installed into the ground, and is used to measure evapotranspiration.
What is ‘Eddy covariance’?
Eddy covariance is a technique that uses turbulence and statistics to calculate vertical flux of gas i.e. actual evapotranspiration
What is AET and PET?
Actual evapotranspiration and potential evapotranspiration
What is the difference between PET and AET?
Potential evapotranspiration is the amount of water that is possible to be evaporated. The actual evapotranspiration is generally a lot less than potential, but they are equal when there is ample water.
What is the name for a method of calculating evapotranspiration using modelling?
Penman-Monteith
What are the three different processes that can happen once water has infiltrated the soil surface?
Water can become soil moisture (for use by vegetation), become interflow (laterally moves towards surface water body), or percolate towards the groundwater system
What is ‘Hortonian overland flow’?
Hortonian overland flow is when excess rain moves downslope to a water body as overland flow. The rainfall intensity is higher than the soil’s infiltration capacity. (Soil does not have to be saturated)
Describe the 3 cases of infiltration.
Case 1: the rainfall intensity is less than soil infiltration capacity - infiltration occurs with no runoff
Case 2: the rainfall intensity is greater than soil infiltration capacity - infiltration occurs with runoff
Case 3: the soil is saturated from below and has no infiltration capacity (saturation excess) - runoff with no infiltration
What is porosity, and how is it calculated?
Porosity is a measure of the pore space volume within a soil.
Porosity (n) = volume of voids / total volume
What is hydraulic conductivity?
Hydraulic conductivity (k) is the rate of water movement through saturated soil.
What are macropores?
Macropores are large passages in soil that acts conduits for water flow in saturated conditions (enhance infiltration)
Name the 4 basic soil profiles from top to bottom.
Organic matter, surface soil, subsoil, substratum
What is soil moisture content, and how is it calculated?
Soil moisture content is the volume of water contained in a given volume of soil [m3/m3]
Soil moisture content (theta) = volume of water / total volume
What is the effect on porosity of well-sorted (compacted) vs. poorly-sorted soil?
Well-sorted, compacted soil has a lower porosity than poorly-sorted, due to less void volume. This results in more runoff as there is less infiltration.
What is a ‘wetting fluid’ and ‘non-wetting fluid’.
A wetting fluid in the presence of air will form a convex interface (curved downwards). A non-wetting fluid will form a concave interface (curved upwards) i.e. air becomes the wetting fluid
What is a ‘capillary fringe’?
The capillary fringe is the zone between the unsaturated and saturated zones (saturation increases with depth)
What are the following methods used for: gravimetric, tensiometer, neutron probe, TDR (time domain reflectometry), and capacitance block?
Measuring soil moisture
What are the 3 soil moisture zones during infiltration from top to bottom? Describe the moisture content in each zone.
The 3 zones are saturation zone, transmission zone and wetting zone.
The moisture content is high in the saturation zone but suddenly drops off in the transmission zone (this sudden drop off is called the transition zone). It then stays fairly constant until the wetting zone, where it drops off again (called the wetting front).
What is Horton’s equation, and what does each variable mean?
f(t) = fc + (fo - fc)e^(-kt) f(t): infiltration rate fc: final constant infiltration rate fo: initial infiltration rate k: decay constant
What is ‘ponding time’?
Ponding time (tp) is time after which soil stops absorbing rainfall and the water starts to pond
What are the 2 Phillip’s equations? What do the variables mean?
Cumulative infiltration [m]:
F(t) = S x t^1/2 + Ksat x t
Infiltration rate [m/s]:
f(t) = dF/dt = 0.5 x S x t^-1/2 + Ksat
S: soil sorptivity [m/s^1/2]
Ksat: saturated hydraulic conductivity [m/s]
What are the 4 assumptions of the Phillip’s equation?
- Soil is homogeneous
- Surface water content is constant at saturation
- Estimates only valid for short to medium events
- Infiltration is vertical and one-dimensional
What is sorptivity?
Sorptivity is a measure of the capacity of soil to absorb liquid by capillary action
What factors affect infiltration?
Vegetation cover, antecedent wetness, catchment slope, rainfall rate, soil properties & macropores
What is an aquifer?
An aquifer is a permeable unit that holds & transmits significant amounts of water
What is an aquitard?
An aquitard is a low permeability unit
What is an aquiclude?
An aquiclude is an essentially impermeable unit
What is ‘water potential’?
Water potential [phi] is the potential energy per unit mass of water [m2/s2]
phi = phi_v + phi_p + phi_g (velocity + pressure + gravity)
In which direction does water flow?
Water flows from high energy to low energy (high hydraulic head to low hydraulic head)
What creates a lake/river?
A lake is a result of the water table intersecting topography (e.g. between two mountains)
What is the difference between a perennial/effluent stream and intermittent/influent stream?
An effluent stream is one that is gaining (water is flowing into it). An influent stream is one that is losing (water is flowing out).
What is hydraulic head?
Hydraulic head is pressure head + elevation [m/kg]
Velocity is neglected as it is comparatively very small
What is Darcy’s Law?
Q = -Ks x A x dH/dl
Q: discharge [m3/s]
Ks: saturated hydraulic conductivity [m/s]
A: cross-section area (normal to flow) [m2]
dH/dl: head gradient [-]
What is the potentiometric surface?
The potentiometric surface is the line between two hydraulic head points h(x)
What is specific discharge?
Specific discharge, q = Q/A [m/s]
What is the formula for storativity?
S = b x Ss
S: storativity
b: aquifer thickness
Ss: specific storage [m-1]
What is the formula for transmissivity?
T = b x Ks
T: transmissivity
b: aquifer thickness
Ks: saturated hydraulic conductivity
What is specific yield?
Specific yield (Sy) is the volume of water released per unit horizontal area per unit drop in water table
What is capillary action?
Capillary action is the process of water moving from an area of high saturation to low saturation
What is the energy balance approach equation?
A.Rn = dmv/dt x lambda + dT/dt x Cp x m +-Q
What is the simplified version of the energy balance approach equation?
Eo = Rn / (lambda x pw)
What is the equation for intrinsic permeability?
k = K x mu / (pho x g)
