Lecture 8: Water Balance in a Drainage Balance (Interception Storage,…
Lecture 8: Water Balance in a Drainage Balance
Hydrology is the study of H2O associated w/ the earth's land mass, including surface & sub-surface H2O
precipitation: conversion & transfer of moisture in atm to land
. forms when H2O(g) cools to
dew pt. (temp. to which air would have to cool to be saturated.
state of saturation exists when air is holding max. amt. H2O(g) possible) & condenses into tiny droplets/ice particles that form clouds. after aggregating & reaching a critical size, leave clouds as ppt
depending on size of drainage basin, ppt totals affect input & output. ppt varies over space, time & scale.
H2O delivery to a river is in 2 stages: 1. ppt as it falls & 2. what happens to it when it arrives at surface
evaporation: return of H2O from bare soil/open bodies of H2O to atm
transpiration: transfer of H2O to atm through stomata of veg.
must have soil moisture to be taken up by plants via roots (H2O infiltrated into soil has not moved on through basin system)
evapotranspiration: diffusion of H2O from veg. & H2O surfaces to atm.
does not occur in unvegetated deserts or snow/ice fields
Channel Flow & Storage
rivers are a medium for H2O to drain from system into ocean (flow) but also act as stores.
river discharge is a measure of vol. of H2O moving in a river.
also describes output.
receive input indirectly through flows & directly from ppt
are dry most of the time, occupied only after rainstorm
are seasonally occupied by flowing H2O, as in basins when H2O table rises to surface in one or more parts of the year
are always occupied by some flowing H2O
Infiltration & Soil Moisture Storage
infiltration: vertical downward flow of H2O from surface storage to soil moisture storage.
entry of rain H2O facilitated by
pores (small spaces btw mineral & organic particles)
alternatively, H2O forms capillary H2O, held by thin films adhering to indiv. soil particles by surface tension.
water held in soil by molecular attraction is soil moisture storage.
adhesive & cohesive forces to retain H2O in soil.
these vary in size & interconnect to greater/lesser extent depending on size & shape of soil particles. provide capillaries through which rain H2O can pass vertically downwards or laterally on slopes under gravity.
infiltration capacity: ability of soil to allow entry of H2O
aka max. infiltration rate (mm/hr)
Interception Storage, Throughfall & Stemflow
interception is part of rainfall that is intercepted by earth's surface (includes everything that becomes wet after rainfall event)
can amt. up to 15-50% of ppt
interception loss (output): H2O retained then lost as evapo. after rainfall
interception storage refers to H2O caught & stored/retained on surface
throughfall 1. raindrops that penetrate gaps in canopy (fall to ground) 2. rain that falls on surfaces & when cap. of leaves & surface storage is reached, drip to the ground
interception flow is movement of H2O from interception storage to surface storage e.g. throughflow, stemflow
stemflow: raindrops that run down branches & tree trunks
storage cap. varies
season to season
for deciduous (shed leaves) species or
place to place
, depending on leaf size, size of canopy
refers to any parts of the system where H2O lies above ground on earth's surface.
occurs in natural & artificial areas e.g. roof tops, canals. most apparent in urban areas.
include biological H2O storage, which is H2O temporarily absorbed by plants, subsequently released as transpiration
Hortonian Overland Flow
H2O will sink into ground when rainfall intensity is less than infiltration capacity (
is high (thunderstorms) or
is low (baked clay soil), H2O cannot penetrate soil sufficiently rapidly. excess H2O accumulates, occupies small irregularities (depression storage) but they will overflow to form a continuous sheet of H2O flowing downslope
layer of H2O thickens downslope as water is added at any pt. (cumulative increment of surface H2O). velocity increases due to gradient AND energy losses due to friction btw H2O & soil becoming proportionately less as depth of flow increases.
recognised in 1. semi-arid environments e.g. BSh climate, where
is high and
is low due to little veg (aids infiltration, impedes surface slow) or baked soil surface during dry season (high temp., low rainfall) so soil capillaries close up
paved urban areas w/ impermeable granite/concrete surfaces
areas devoid of veg. (bc deforestation or degradation) or little
agricultural fields w/ topsoil compacted/removed to expose less permeable soil
Saturation Overland Flow
combination of return flow & direct ppt falling onto saturated areas
build-up of H2O @ slope bottom when there exists permeable top soil layers & less permeable lower soil horizons, during rainfall
active throughflow so downslope migration of H2O, causing saturated soil at slope base. with time, saturation zone extends upwards
on lower parts of slope, surface flow will be increased bc soil H2O which has migrated from upslope will seep out again, by throughflow (return flow)
when rain continues to fall on saturated soil, cannot be absorbed
appear in areas w/ veg. (encourage infiltration) & areas w/ thin top soil over impermeable surface. gentle & concave shaped slope.
lateral downslope flow of H2O from one surface storage to another
Throughflow & Springs
permeability: ease w/ which H2O flows through a rock/soil/material by its pores & lines of weaknesses like joints & bedding planes
permeability of soil > bedrock e.g. granite (largely impermeable but weathers to give sandy soil). sandy soil w/ large pore spaces are permeable.
throughflow is a lateral flow of H2O from soil moisture storage to channel storage
H2O transported by throughflow emerges as small springs (on lower parts of slope or in banks of stream channels), contributing to surface run-off
springs occur where the land surface & H2O table intersect.
once a spring starts to flow, dip in H2O table that creates pressure grad. in aquifer (underground layer of H2O-bearing permeable rock), encouraging H2O to move to the spring
Percolation, Groundwater Storage & Groundwater Flow
infiltrated H2O (not removed by plants) will continue to migrate vertically downwards under gravity to pass thru permeable underlying rock.
percolation is vertically downward flow of H2O from soil moisture storage to groundwater storage
zone of saturation: when at depth within rock, pores, joints, fractures & bedding planes become filled by ground H2O
H2O table: uppermost surface of saturation zone, dividing saturated rock below from unsaturated rock above.
zone of aeration: spaces within rock are aerated (unsaturated) but have H2O in transit to saturation zone
groundH2O can escape to surface as
baseflow (lateral flow of H2O from groundwater storage to channel storage
), where zone of saturation intersects land surface. makes up over 24% of earth's fresh H2O.
global annual H2O balance is P=E+R+/-S where P is annual precipitation, E is evapotranspiration, R is surface run-off and S is gains/losses due to changes in storage
Catchment Hydrology is the study of hydrology in drainage basins.
A drainage basin is a catchment area within which H2O collects & drains into a main river channel, entering eventually into an ocean, sea or lake.
The drainage basin H2O balance refers to the hydrological systems that are made up of many diff. hydrological processes: input, pathways, storages, output.
Pathways/Flows refer to paths taken by H2O as it travels from one storage to another
Storages are parts of the system that hold/retain H2O for periods of time