HYDROMETEOROLOGY

Part 1II
Elements of Hydrology
: • Infiltration
• Groundwater
• Streamflow and Streamflow
Hydrographs

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INFILTRATION

• Passage of water
into the soil surface
• Supplies water to
plants
• Replenishes
groundwater
INFILTRATION

May be considered as a
3-stage sequence of :
• Surface entry
• Filling up of soil profile
storage potential
• Movement of water
within the soil profile
Without
INFILTRATION

• Wells would go dry

• Streams would
stop to flow soon
after a rain
• There would be
frequent flooding
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Definition of Terms
• INSTANTANEOUS INFILTRATION RATE (f) – rate of water
entry to the soil at a specific time (mm/s)
• CUMULATIVE INFILTRATION DEPTH (i) – total depth of
infiltrated water from start of wetting (mm)
• INFILTRABILITY or INFILTRATION CAPACITY – maximum
rate at which rain or irrigation water can be absorbed
by a soil under a given condition (mm/hr)

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FORCES ACTING ON SOIL WATER

•adhesive force -causes the dissimilar particles and/or

surfaces to cling to one another
•cohesive force-causes the similar or identical particles/
surfaces to cling to one another
•gravitational force-due to the effect of gravity
•osmotic forces-caused by salt or ion concentration
differences or gradients
SOIL MOISTURE WATER BASED ON
AVAILABILITY TO PLANTS

•Unavailable water -held too tightly by capillary

forces and is generally not accessible to plant roots
•Gravitational water -drains quickly from the root
zone under normal drainage conditions
•Available water -difference between gravitational
and unavailable water
SOIL MOISTURE WATER BASED ON PREDOMINANT
FORCES ACTING ON THE SOIL

•Hygroscopic water -water on the soil grains that is not

capable of significant movement by the action of gravity or
capillary forces
•Capillary water -water that exists in the pore spaces of the
soil and is retained against the force of gravity in a soil that
permits unobstructed drainage
•Gravitational water -water that will readily move out of the
soil if favorable drainage is provided
HYGROSCOPIC POINT
–amount of water the soil profile will hold against the
soil moisture tension of 10,000 bar
PERMANENT WILTING POINT
–amount of water the soil profile will hold against the
soil moisture tension of 15 bar
-an arbitrarily defined soil moisture level, it represents
the point where a plant permanently wilts
FIELD CAPACITY
–the point where the gravitational forces equal the
cohesive forces capacity
–a measure of the soil’s water holding capacity
-amount of water the soil profile will hold against the
soil moisture tension of 1/3 bar
SATURATION POINT
–amount of water the soil profile will hold when all its
pore spaces are filled up
–water at saturation is subject to free drainage
FACTORS AFFECTING INFILTRATION
1. The Fluid (water)
•rate of application/rainfall -↑flow rate, ↑infiltration
•viscosity -↑viscosity, ↓infiltration
•turbidity -↑turbidity, ↓infiltration
•↑depth of standing water over the soil surface, ↑infiltration
2.The Medium (soil)
•porosity –↑porosity, ↑ moisture holding capacity, ↑permeability
•structure –more stable the soil aggregates are, the higher is the soil capability to store and transmit water
3.Vegetation and cultural practices
•Vegetal cover protects the soil against raindrop energy and improves soil structure through production of
organic matter and root penetration
•Tillage practices loosens up the upper soil layer thereby ↑rate of surface entry and ↑porosity of the
plowed layer
•Compaction ↓porosity, ↓pore diameters and infiltration
4.Topography
•influences the characteristics of surface runoff and interflow
•slope speeds up overland flow and, hence, the depth and time distribution of direct runoff and infiltration
MEASUREMENT OF INFILTRATION
1. Infiltrometers
a.Flooding type (Double ring infiltrometers)
•two open-ended cylinders embedded into the soil
•a constant head is maintained inside the inner ring where measurement is made
•the same depth is maintained at the outer ring to minimize errors due to lateral flow at the inner ring
•advantage: simplicity and ease of measurement
•disadvantage: small area being sampled; maintenance of constant head which may not reflect actual field
conditions
b.Rainfall simulator type
•water is applied at a predetermined rate comparable with natural rainfall and surface runoff is measured
•the amount of water that infiltrated into the plot is computed as the difference between the water
applied and the surface runoff
•advantage: simulates natural rainfall conditions
•disadvantage: small sample area for an expensive experimental set-up; complexity of construction and
field installation
MEASUREMENT OF INFILTRATION
2. Basin method
•same as flooding method but a large area is ponded
•rate of drop in the water depth is measured with time
•not practical; it is very difficult to measure the initial infiltration rates
3.Watershed Hydrograph Method
•characterize infiltration through the subtraction of runoff rates from rainfall rates in
watersheds areas
•involves hydrograph analyses to separate surface runoff from total streamflow as
well as making corrections for evapotranspiration and detention storages
•adv: resulting infiltration data is representative of the entire catchment area
•disadv: large amount of data to be collected and analyzed
 INFILTRATION EQUATIONS
•may be expressed in either
instantaneous rate (f ) or cumulative
depth (i) form
•infiltration rate curve shows a high
initial rate that diminishes towards a
constant value at very large time
•the curve is a decay type function
which can be expressed into an
equation with two or three parameters
depending on the magnitude of the
constant infiltration rate (fc)
INFILTRATION EQUATIONS

1.Gardner and Windsoe Equation (1921)

•suggested an inverse exponential equation to fit derived infiltration curves
•for one dimensional downward infiltration of a given film of ponded water over the soil surface

where: i= cumulative infiltration depth at time t

f= infiltration rate
C1, C2and  = constants or parameters
INFILTRATION EQUATIONS

2.Lewis (1937) –Kostiakov(1932) Equation

•an empirical equation which assumes that the intake rate declines over time according to a power
function
•the major limitation of this expression is its reliance on the zero final intake rate

where: c and a= constants with 0 < a< 1

INFILTRATION EQUATIONS

3.Horton Equation (1940)

•continuous infiltration and wetting of
the soil will decrease infiltration due to
decrease in soil profile storage potential,
in-washing of fine soil particles into soil
voids and swelling of soil colloids and
closing of soil cracks –indicates
exhaustion phenomenon
GROUNDWATER HYDROLOGY

Occurrence of Groundwater

AQUIFER - A water bearing geologic AQUIFUGE - A geologic formation with

formation or stratum capable of no interconnected pores and hence can
transmitting water through its pores at neither absorb nor transmit water
a rate sufficient for economic extraction
by wells AQUITARD - A geologic formation of
rather impervious nature, which
AQUICLUDE - A geologic formation, transmits water at a slow rate
which can absorb water but can not compared to an aquifer (insufficient for
transmit significant amounts pumping from wells)
 GROUNDWATER HYDROLOGY

Types of Aquifer
1. Confined aquifer – or artesian
aquifer - a porous formation underneath is
sandwiched between two impervious strata (aquicludes)
and is recharged by a natural source (artesian well)

2. unconfined aquifer – or artesian

aquifer - homogeneous porous formation extending
from the ground surface up to an impervious bed
underneath, rainwater percolating down in the soil
saturates the formation and builds up the ground water
table (GWT) (water table well)
 GROUNDWATER HYDROLOGY

Types of Aquifer
1. Confined aquifer – or artesian
aquifer - a porous formation underneath is
sandwiched between two impervious strata (aquicludes)
and is recharged by a natural source (artesian well)

2. unconfined aquifer – or artesian

aquifer - homogeneous porous formation extending
from the ground surface up to an impervious bed
underneath, rainwater percolating down in the soil
saturates the formation and builds up the ground water
table (GWT) (water table well)
 GROUNDWATER HYDROLOGY

Types of Aquifer
Water table well – water table
Artesian Well – from confined
aquifer
Flowing well – below piezometric
level from confined aquifer
Perched aquifer – within the
confined aquifer
 GROUNDWATER HYDROLOGY
Darcy’s Law
Hydraulic Conductivity 𝑑ℎ
Flow rate per unit area normal to the Q= −𝑘𝐴
flow direction resulting from one
𝑑𝑙
unit of hydraulic gradient using flow Q = volume rate of flow of ground water (discharge or
yield)
K = coefficient of permeability of aquifer soil
A = cross-sectional area of the aquifer (= wb)
w = width of aquifer
b = thickness of aquifer
∆h/L, ∆h = head lost in a length of flow - hydraulic
gradient
 GROUNDWATER HYDROLOGY

Transmissivity or Transmissibility
T= 𝑘𝑏
- Ability to transmit water
throughout its entire thickness k = average hydraulic conductivity
b = thickness of the aquifer
GROUNDWATER HYDROLOGY

Storage Coefficient or Storativity Specific Yield

- Volume of water that can be - Volume of water released from
stored in or released from an storage in an unconfined aquifer per
aquifer per unit horizontal area per unit horizontal area per unit decline in
unit change in hydraulic head water table
 Q

Y Qo
where:
Qo = flow towards the well
Q = discharge from the
X well
Qo = VA
dy
Darcy’s eq’n: V = − K
dx
and: A = 2XY (surface area ⊥ to flow)

dy
then: Q = −K (2XY )
dx

But: Qo = - Q
 dy
So: Q =K (2XY ) or
dx

dy
Q = 2 KXY
dx

General equation for

Aquifer discharge
For a steady state condition in a
CONFINED AQUIFER

dy
Q = 2 KXY
dx
dh
Q = 2 KrB
dr
 dh
Q = 2 KrB for any distance, r
from the well
dr
dh
=slope of drawdown curve
dr

Integrating: At h=hw & r = rw

& h=ho & r = ro
 ro dr ho
rw r
Q = 2 KB hw dh

 ro 
Q ln   = 2 KB (ho − h w)
 rw 

2 KB (ho − h w)
Q= ro
ln( rw )
For a steady state condition in an
UNCONFINED AQUIFER

•No aquifer thickness, B

•A = 2rh; h is changing
For a steady state condition in an
UNCONFINED AQUIFER

dy
Q = 2 KXY
dx
dh
Q = 2 Krh
dr
 dh
Q = 2 Krh for any distance, r
from the well
dr
dh
=slope of drawdown curve
dr

Integrating: At h=hw & r = rw

& h=ho & r = ro
 ro dr ho
rw r
Q = 2 K hw hdh

 ro  2 2
h hw
Q ln   = 2 K ( − )
o

 rw  2 2

 K (h − h )
2 2
Q= o
ro
w

ln( )
rw
Streamflow & Runoff
RUNOFF
PROCESS

STREAMFLOW ( or RUNOFF)
- water reaching the channels
- results from several factors
Typical
Watershed
– with a
mainstream
and its
tributaries
Sources of
Streamflow
1) DRO =
(Surface
Runoff + Direct
precipitation)
2) BRO or BF
= (Subsurface
flow +
Groundwater
flow)
Sources of Streamflow
1) DRO = (Surface Runoff + Direct precipitation)
2) BRO = (Subsurface flow + Groundwater flow)

DRO = DIRECT RUNOFF

BRO = BASE RUNOFF
BF = BASEFLOW

Surface runoff (Overland Flow)

Subsurface flow (Interflow)
Groundwater flow (Baseflow)
Total streamflow = DRO + BF
 E T

Water table

1st Phase – Rainless Period

RO R

2nd Phase – Initial period of rain

• Water table intersects the stream bottom
• Flow may come from DRO and BF
• Perennial streams

Water table
INFLUENT STREAM

Water table

• Water table falls below the stream bottom

• Flow may seep and join GW
• Flows during rainy period only
STREAMFLOW MEASUREMENT
 Streamflow information can be used for a
wide variety of uses:
 flood prediction, water management and
allocation
 engineering design, research
 operation of locks and dams
 recreational safety and enjoyment
STREAMFLOW MEASUREMENT OR
STREAMGAGING
 Streamflow (Q), L3/t

 Almost always a dependent variable

 Difficult to make a direct and continuous record

River Stage or Stage (S)

 height of the water surface at a location along a stream or river
STREAMGAGING INVOLVES…

1. obtaining a continuous record of stage

2. obtaining periodic measurements of
discharge
3. defining the natural but often
changing relation between the stage
and discharge
4. using the stage- discharge relation to
convert the continuously measured
stage into estimates of streamflow or
discharge.
1) Stage measurement

Using a staff gage

Using a stilling well

 Stilling Well
 Water from the river enters and leaves the stilling well through
underwater pipes allowing the water surface in the stilling well to be at
the same elevation as the water surface in the river.
 The stage is then measured inside the stilling well using a float or a
pressure, optic, or acoustic sensor.
 The measured stage value is stored in an electronic data recorder on a
regular interval, usually every 15 minutes.
2) Discharge measurement

Q = VA
V = mean velocity of flow, L
A = cross sectional area of flow, L2

a) velocity measurement
Mechanical current-meter method
- the stream channel cross section is
divided into numerous vertical
subsections
a) velocity measurement

QTOT = Qi = Q1+Q2+Q3+…+Qi

a) velocity measurement
Qi = ViAi
Current meter
• has a wheel of six metal cups that revolve
around a vertical axis.
• an electronic signal is transmitted by the meter
on each revolution allowing the revolutions to be
counted and timed.
• Because the rate at which the cups revolve is
directly related to the velocity of the water, the
timed revolutions are used to determine the
water velocity.
• meter must be attached to a wading rod for
measuring in shallow waters or to be mounted
just above a weight suspended from a cable and
reel system for measuring in fast or deep water.
3. Defining the relation between the stage
and discharge

Stage, m

Discharge, m3/s

Rating Curve – a plot of S vs Q

3. Defining the relation between the stage
and discharge
4. Using the stage and discharge relation to
calculate Q
 Analysis & interpretation of streamflow
data

Q, m3/s

t, s

Hydrograph – a plot of Q vs t
 A typical hydrograph
2

1 3

Q, m3/s

1 – Rising limb
2 – Crest segment
3 – Recession limb
t, s

Hydrograph – a plot of Q vs t
 D

i, m/s

t, s
3
4
2

Q, m3/s 5
1

t, s
 D

i, m/s

t, s TP – time to peak - time it

TP takes for flow to peak

Q, m3/s
TL – lag time = Tp – (D/2)

TL

Tb – basetime – time it takes

t, s
Tb
for water to rise and recede
FACTORS AFFECTING RUNOFF

A. Climate
B. Watershed properties
CLIMATIC FACTORS
 Rainfall pattern

b
a c

a b c

t
TPa < TPb < TPc
 Climatic factors

 Intensity QPa < QPb < QPc

i
a b c
t

t
FACTORS AFFECTING RUNOFF

Watershed properties

1) Slope & topography

2) Vegetal cover
3) Soil type
METHODS TO ESTIMATE RUNOFF
PROPERTIES

1. Hydrologic frequency analysis

2. Rational equation : Q = cIA
3. Regression and correlation
analysis
4. Hydrograph analysis