Block 2 Lecture 2: how water structures the British Landscape

Why is there an east west divide in rainfall

partly due to topography- Durham is in the rain shadow of the Pennines

Prevailing wind direction, all the cyclonic systems come across the North Atlantic

ET depends on

evaporation - exposure and ventillation / albedo / seasonality

Transpiration - stomatal cover / seasonal growth / growth stage / canopy health

interception - vegetation height / canopy depth / leaf are and shape / crop spacing

potential ET

evapotranspiration under the condition of unlimited water supply

Actual ET

amount of ET that actually takes place (limited by water availibility)

The effect of rainfall

the amount of precipitation (P) - the amount of Et = Effective rainfall (Re)

Infiltration-excess overland flow

if intensity of rainfall is lager than the infiltration rate then overland flow develops as water can’t get into the soil

Rainfall intensity letter

i

rate of overland flow production letter

o

infiltration rate letter

f

Infiltration-excess overland flow equation

o = i - f

Typical rainfall intensity for thunderstorms

50-100 mm h-1

Typical rainfall intensity for light rain

< 0.5 mm h-1

Typical infiltration rate for forest loam

100 -200 mm h-1

Typical infiltration rate for pasture loam

10-70 mm h-1

Saturation Overland flow

In some cases, even if there is very low rainfall and input of rainfall never exceeds the infiltration rate, overland flow can still happen if ground is getting saturated

Darcy’s Law

Allows us to calculate the amount of water flowing through a substance. if saturated, the only thing in the soil that can change is its flow rate or flux. Says that flux is a function of the pressure head (or gradient in potential energy) and a parameter Ks that saturated hydraulic conductivity

Q (discharge, m3 s-1) = Ks (a measure of how easy it is for water to move through soil (saturated hydraulic conductivity)) A (cross-sectional area) H (hydraulic gradient → difference in water levels / column length m m-1)

Ks is the gradient of the line on the graph, therefore telling us about the saturated hydraulic conductivity of the material that was in the column

Richards equation

if unsaturated, then the flux may change but so may the ‘porosity’, so Darcy’s law is modified to allow for suction

Variable source areas

locations where we get overland flow. Runoff locations vary in time and in space

can use a topographic index to predict which areas are likely to contribute runoff

Effect of vegetation

infiltration rates - look at paired catchment studies

Sediment transport on hillslopes

creep / mass movement / interrill erosion / rill erosion / splash

Characteristic forms

if we can estimate what the transport rate is at a point on the slope we can estimate what the changes in slope form should be through time

what type of surface do diffuse processes cause

convex (most profiles are convex near the top of the slope)

what type of surface do advective processes cause

concave (most profiles are concave near bade

Equifinality

same hill slope might get produced by different processes