GROUP 3

INFILTRATION
• REPORTERS:
BONDALO, JEFFERSON
BUHISAN, KATRINA
CATULONG, HARVEN
SALVO, ANTONEL
SANCHEZ, MARVEN
SEROJE, JOHN
 INFILTRATION
TOPICS:
• DEFINITION OF INFILTRATION
• FACTORS AFFECTING INFILTRATION, AND INFILTRATION
MEASUREMENTS
• HORTON MODEL AND PHILIP’S EQUATION
• GREEN-AMPT MODEL
• PONDING TIME
• FITTING INFILTRATION MODELS TO INFILTRATION DATA USING
EXCEL
 INFILTRATION
• THE PROCESS BY WHICH WATER ON
THE GROUND SURFACE ENTERS THE
SOIL.
• ONCE WATER INFILTRATES THE SOIL, IT
MOVES THROUGH THE PORE SPACES
AND CAN EITHER BE STORED AS SOIL
MOISTURE OR PERCOLATE FURTHER TO
REPLENISH GROUNDWATER AQUIFERS.
TWO IMPORTANT ASPECTS RELATING INFILTRATION:
• INFILTRATION CAPACITY IS THE MAXIMUM RATE AT WHICH
GROUND CAN ABSORB WATER
• FIELD CAPACITY IS THE AMOUNT OF WATER GROUND CAN HOLD.

INFILTRATION RATE IS THE VELOCITY AT WHICH WATER CAN SEEP

INTO THE SOIL. IT IS COMMONLY MEASURED BY THE DEPTH (IN MM)
OF THE WATER LAYER THAT THE SOIL CAN ABSORB IN AN HOUR.
- INFILTRATION TAKES PLACE IN THE VADOSE ZONE, ALSO TERMED THE
UNSATURATED ZONE.
VADOSE ZONE (UNSATURATED ZONE) - IT IS THE PART OF
EARTH BETWEEN THE LAND SURFACE AND THE TOP OF THE
PHREATIC ZONE, THE POSITION AT WHICH THE
GROUNDWATER (THE WATER IN THE SOIL’S PORES) IS AT
ATMOSPHERIC PRESSURE (“VADOSE” IS FROM THE LATIN
WORD FOR “SHALLOW”).

SATURATED ZONE - THE UNDERGROUND AREA WHERE

ALL THE AVAILABLE SPACE IN THE SOIL AND ROCKS IS
FILLED WITH WATER. IT'S ALSO KNOWN AS THE PHREATIC
ZONE OR ZONE OF SATURATION.
 HOW INFILTRATION WORKS

-THE PROCESS OF INFILTRATION CAN

CONTINUE ONLY IF THERE IS ROOM
AVAILABLE FOR ADDITIONAL WATER AT THE
SOIL SURFACE.
- THE AVAILABLE VOLUME FOR
ADDITIONAL WATER IN THE SOIL
DEPENDS ON THE POROSITY OF THE SOIL
AND THE RATE AT WHICH PREVIOUSLY
INFILTRATED WATER CAN MOVE AWAY
FROM THE SURFACE THROUGH THE SOIL.
HOW INFILTRATION WORKS:
1. RAINFALL (WATER REACHES THE SURFACE)
- WHEN PRECIPITATION OCCURS (RAIN, SNOW, OR
HAIL), WATER LANDS ON THE EARTH'S SURFACE.
THIS WATER CAN EITHER SOAK INTO THE GROUND
(INFILTRATION) OR REMAIN ON THE SURFACE,
LEADING TO RUNOFF.
2. INFILTRATION (WATER ENTERS THE SOIL)
- INFILTRATION BEGINS WHEN WATER PENETRATES
THE GROUND SURFACE THROUGH TINY PORES AND
CRACKS. INITIALLY, DRY SOIL ABSORBS WATER
QUICKLY, BUT AS THE SOIL SATURATES, THE RATE
SLOWS DOWN.
 HOW INFILTRATION WORKS
3. PERCOLATION (WATER MOVES DEEPER)
- AFTER INFILTRATION, WATER CONTINUES TO
MOVE DOWNWARD THROUGH THE SOIL UNDER
THE FORCE OF GRAVITY. THIS PROCESS IS KNOWN
AS PERCOLATION.

- THE WATER TRAVELS THROUGH THE DIFFERENT

SOIL LAYERS UNTIL IT REACHES AN IMPERMEABLE
LAYER (LIKE BEDROCK) WHERE IT CANNOT MOVE
FURTHER.
HOW INFILTRATION WORKS

5. GROUNDWATER RECHARGE (FILLING

AQUIFERS)
- WHEN WATER MOVES DEEP ENOUGH, IT
REACHES THE SATURATED ZONE, WHERE ALL THE
PORES AND CRACKS ARE FILLED WITH WATER.
THIS AREA IS CALLED GROUNDWATER. THE UPPER
SURFACE OF THIS SATURATED ZONE IS THE
WATER TABLE.
 HOW INFILTRATION WORKS

6. SURFACE RUNOFF (EXCESS WATER FLOWS AWAY)

- WHEN THE SOIL IS SATURATED AND CAN NO
LONGER ABSORB WATER, OR RAINFALL IS TOO
INTENSE AND THE SOIL IS UNABLE TO ABSORB
WATER QUICKLY ENOUGH, THE EXCESS WATER
FLOWS OVER THE LAND’S SURFACE AS RUNOFF. THIS
WATER EVENTUALLY REACHES RIVERS, LAKES, OR
OTHER BODIES OF WATER.
FACTORS AFFECTING INFILTRATION
1.) SOIL PROPERTIES
2.) VEGETATION COVER
3.) LAND USE AND SURFACE
CONDITIONS
4.) PRECIPITATION
CHARACTERISTICS
5.) TOPOGRAPHY
6.) TEMPERATURE
FACTORS AFFECTING INFILTRATION
1.) SOIL PROPERTIES
A. SOIL TEXTURE - (SUCH AS LOAM, SANDY LOAM OR CLAY) REFERS TO THE PROPORTION OF
SAND, SILT AND CLAY SIZED PARTICLES THAT MAKE UP THE MINERAL FRACTION OF THE SOIL
-COARSE-TEXTURED SOILS (SANDY) HAVE HIGHER INFILTRATION RATES COMPARED TO
FINE-TEXTURED SOILS (CLAY) DUE TO LARGER PORE SPACES.

B. SOIL STRUCTURE – REFERS TO HOW SOIL PARTICLES ARE ARRANGED AND ORGANIZED;
DESCRIBES THE ARRANGEMENT OF THE SOLID PARTS OF THE SOIL AND OF THE PORE SPACE
LOCATED BETWEEN THEM
-WELL-AGGREGATED SOILS ALLOW BETTER WATER MOVEMENT THAN COMPACTED SOILS.

C. SOIL MOISTURE CONTENT - ALSO REFERRED TO AS WATER CONTENT, IS AN INDICATOR OF THE

AMOUNT OF WATER PRESENT IN SOIL
- DRY SOILS TYPICALLY ABSORB WATER MORE QUICKLY INITIALLY, WHILE SATURATED
SOILS REDUCE INFILTRATION.
FACTORS AFFECTING INFILTRATION
2.) VEGETATION COVER - IS THE
PERCENTAGE OF LAND COVERED BY
PLANTS, SUCH AS TREES, SHRUBS, AND
GRASSES.
-VEGETATION INCREASES INFILTRATION
BY REDUCING THE IMPACT OF
RAINDROPS, MINIMIZING SOIL CRUSTING,
AND ENHANCING SOIL ORGANIC
MATTER. ROOTS ALSO CREATE PATHWAYS
FOR WATER TO ENTER THE SOIL. Example: A forested area typically
has higher infiltration than a parking
lot because plant roots and organic
matter improve soil permeability.
FACTORS AFFECTING INFILTRATION
3.) LAND USE AND SURFACE CONDITIONS
- URBANIZATION, PAVING, AND OTHER LAND-USE
CHANGES REDUCE INFILTRATION BY LIMITING EXPOSED
SOIL. AGRICULTURAL PRACTICES SUCH AS TILLAGE CAN
EITHER INCREASE OR DECREASE INFILTRATION
DEPENDING ON SOIL MANAGEMENT.

Example: A paved road has almost

zero infiltration, while a plowed
field may have moderate infiltration
depending on soil compaction.
FACTORS AFFECTING INFILTRATION
4.) PRECIPITATION CHARACTERISTICS
A. INTENSITY - THE RATE AT WHICH PRECIPITATION FALLS IS CRUCIAL; A HIGH-INTENSITY
RAIN WILL LIKELY RESULT IN MORE RUNOFF DUE TO EXCEEDING THE SOIL'S INFILTRATION
CAPACITY.
B. DURATION - THE LENGTH OF A PRECIPITATION EVENT ALSO IMPACTS INFILTRATION; A
LONG RAIN CAN SATURATE THE SOIL, REDUCING ITS ABILITY TO ABSORB FURTHER WATER.
C. AMOUNT - THE TOTAL VOLUME OF PRECIPITATION PLAYS A ROLE, AS MORE
PRECIPITATION GENERALLY LEADS TO MORE INFILTRATION UNTIL THE SOIL REACHES
SATURATION.
 FACTORS AFFECTING INFILTRATION
5.) TOPOGRAPHY - AFFECTS INFILTRATION BY
INFLUENCING THE RATE OF RUNOFF, WHICH IN
TURN IMPACTS HOW MUCH WATER CAN
INFILTRATE THE SOIL.
- STEEP SLOPES ENCOURAGE RUNOFF AND
REDUCE INFILTRATION, WHILE FLAT TERRAINS
ALLOW MORE WATER TO INFILTRATE.

Example: Water infiltrates more easily

on a flat meadow than on a steep
hillside where it runs off quickly.
 INFILTRATION MEASUREMENTS
- REFER TO THE PROCESS OF QUANTIFYING THE RATE
AT WHICH WATER ENTERS THE SOIL FROM THE
SURFACE
- THIS IS TYPICALLY EXPRESSED IN MILLIMETERS PER
HOUR (MM/HR) OR INCHES PER HOUR (IN/HR).
- THESE MEASUREMENTS ARE CRUCIAL IN FIELDS LIKE
CIVIL ENGINEERING, HYDROLOGY, AND AGRICULTURE
TO ASSESS SOIL PERMEABILITY, MANAGE WATER
RESOURCES, AND DESIGN DRAINAGE SYSTEMS.
- ARE ESSENTIAL FOR DESIGNING STORMWATER
MANAGEMENT SYSTEMS, PREVENTING SOIL EROSION,
AND IMPROVING AGRICULTURAL IRRIGATION
EFFICIENCY.
COMMONLY USED INFILTRATION MEASUREMENTS
1. INFILTROMETERS
- USED TO MEASURE THE RATE OF WATER INFILTRATION INTO SOIL OR OTHER
POROUS MEDIA UNDER CONTROLLED CONDITIONS. EACH TYPE HAS SPECIFIC
APPLICATIONS DEPENDING ON THE ACCURACY REQUIRED AND FIELD
CONDITIONS.
MOST COMMONLY USED INFILTROMETERS:
A. DOUBLE-RING INFILTROMETER
B. SINGLE-RING INFILTROMETER
C. TENSION (OR DISC) INFILTROMETER
D. RAINFALL SIMULATOR
E. AUTOMATED INFILTROMETERS
 MOST COMMONLY USED INFILTROMETERS:
A. DOUBLE-RING INFILTROMETER
DESCRIPTION: CONSISTS OF TWO CONCENTRIC RINGS
(USUALLY METAL) DRIVEN INTO THE SOIL. THE OUTER RING IS
INTENDED TO REDUCE LATERAL FLOW FROM THE INNER RING,
ALLOWING THE DATA FROM THE INNER RING TO BE ANALYZED
AS IF THE FLOW WERE ONE-DIMENSIONAL.
- WATER IS ADDED TO BOTH RINGS TO MEASURE HOW
QUICKLY IT INFILTRATES THE SOIL WHILE MINIMIZING LATERAL
FLOW.
USES: IDEAL FOR MEASURING SATURATED INFILTRATION RATES
IN VARIOUS SOIL TYPES. WIDELY USED IN AGRICULTURAL AND
ENVIRONMENTAL STUDIES.
ADVANTAGES: REDUCES EDGE EFFECTS AND PROVIDES MORE
ACCURATE INFILTRATION RATES.
LIMITATIONS: TIME-CONSUMING AND REQUIRES A LARGE
AMOUNT OF WATER.
THERE ARE TWO OPERATIONAL TECHNIQUES USED WITH THE DOUBLE-RING
INFILTROMETER FOR DIRECTLY MEASURING SOIL INFILTRATION RATES:
➢ CONSTANT HEAD METHOD – IN THIS METHOD, THE WATER LEVEL IN THE INNER
RING IS MAINTAINED AT A FIXED LEVEL AND THE VOLUME OF WATER USED TO
MAINTAIN THIS LEVEL IS MEASURED
➢ FALLING HEAD METHOD – IN THIS METHOD, THE TIME THAT THE WATER LEVEL
TAKES TO DECREASE IN THE INNER RING IS MEASURED.

IN BOTH CONSTANT HEAD AND FALLING HEAD TESTS, THE WATER LEVEL IN THE
OUTER RING IS MAINTAINED AT A CONSTANT LEVEL TO PREVENT LEAKAGE
BETWEEN RINGS AND TO FORCE VERTICAL INFILTRATION FROM THE INNER RING.
 MOST COMMONLY USED INFILTROMETERS:

B. SINGLE-RING INFILTROMETER
DESCRIPTION: A SINGLE METAL RING IS INSERTED
INTO THE SOIL, AND WATER IS POURED IN TO
MEASURE THE INFILTRATION RATE.
USES: SIMPLER AND FASTER METHOD FOR BASIC
INFILTRATION ASSESSMENTS.
ADVANTAGES: EASY TO USE AND REQUIRES LESS
WATER THAN THE DOUBLE-RING METHOD.
LIMITATIONS: MORE PRONE TO LATERAL WATER
MOVEMENT, LEADING TO LESS ACCURATE
MEASUREMENTS.
 MOST COMMONLY USED INFILTROMETERS:

C. TENSION (OR DISC) INFILTROMETER

DESCRIPTION: USES A POROUS DISC TO APPLY WATER
AT A CONTROLLED NEGATIVE PRESSURE (TENSION) TO
MEASURE UNSATURATED INFILTRATION.
USES: MEASURES INFILTRATION UNDER UNSATURATED
CONDITIONS, MIMICKING NATURAL SOIL MOISTURE.
ADVANTAGES: PROVIDES ACCURATE MEASUREMENTS
OF UNSATURATED FLOW AND CAPILLARY ACTION.
LIMITATIONS: REQUIRES SPECIALIZED EQUIPMENT AND
IS MORE COMPLEX TO OPERATE.
 MOST COMMONLY USED INFILTROMETERS:

D. RAINFALL SIMULATOR
DESCRIPTION: SIMULATES/REPLICATE
NATURAL RAINFALL OVER A D..EFINED AREA
TO STUDY INFILTRATION AND SURFACE
RUNOFF.
USES: IDEAL FOR LARGE-SCALE FIELD STUDIES
OF INFILTRATION, EROSION, AND RUNOFF.
ADVANTAGES: SIMULATES REAL-WORLD
CONDITIONS AND CAPTURES COMPLEX
INTERACTIONS.
LIMITATIONS: EXPENSIVE AND REQUIRES
EXTENSIVE SETUP AND WATER SUPPLY.
 MOST COMMONLY USED INFILTROMETERS:

E. AUTOMATED INFILTROMETERS
DESCRIPTION: USE SENSORS AND DIGITAL CONTROLS
TO MEASURE INFILTRATION AUTOMATICALLY OVER
TIME.
USES: USEFUL FOR CONTINUOUS MONITORING AND
LARGE-SCALE RESEARCH PROJECTS.
ADVANTAGES: PROVIDES PRECISE, CONSISTENT, AND
TIME-SAVING MEASUREMENTS.
LIMITATIONS: HIGH COST AND POTENTIAL TECHNICAL
DIFFICULTIES.
COMMONLY USED INFILTRATION MEASUREMENTS
2.) PERCOLATION TESTS
- COMMON IN CIVIL ENGINEERING APPLICATIONS,
THESE TESTS DETERMINE THE INFILTRATION RATE FOR
DRAINAGE SYSTEM DESIGN.
- (COLLOQUIALLY CALLED A PERC TEST) IS A TEST TO
DETERMINE THE WATER ABSORPTION RATE OF SOIL
(THAT IS, ITS CAPACITY FOR PERCOLATION) IN
PREPARATION FOR THE BUILDING OF A SEPTIC DRAIN
FIELD (LEACH FIELD) OR INFILTRATION BASIN
HORTON MODEL AND
PHILIP’S EQUATION
 UNDERSTANDING THE HORTON MODEL
▪ THE HORTON MODEL WAS INTRODUCED IN 1933 BY
ROBERT E. HORTON, WHICH DESCRIBES HOW THE
INFILTRATION CAPACITY OF SOIL RAPIDLY DECREASES
DURING THE EARLY STAGES OF A RAINFALL EVENT,
EVENTUALLY REACHING A CONSTANT MINIMUM
VALUE OVER TIME
▪ IT'S WIDELY USED IN HYDROLOGY STUDIES AND
WATERSHED MANAGEMENT.
▪ IT IS AN EMPIRICAL (BASED ON OBSERVED DATA)
FORMULA THAT DESCRIBES THE PROCESS OF
INFILTRATION IN SOIL.

• Infiltration capacity is the maximum rate at which ground

can absorb water
KEY ASSUMPTIONS OF THE HORTON
MODEL:
CONSTANT INTENSITY : ASSUMES A
CONSTANT RAINFALL INTENSITY OVER A
GIVEN AREA.
INFILTRATION CAPACITY : ASSUMES A
STEADY INFILTRATION RATE UNTIL THE
SOIL REACHES ITS MAXIMUM CAPACITY.
CONSTANT EVAPORATION : ASSUMES
A CONSTANT RATE OF EVAPORATION
FROM THE SOIL SURFACE.
ADVANTAGES OF THE HORTON INFILTRATION MODEL
1.MATHEMATICALLY SIMPLE AND EASY TO APPLY USING BASIC PARAMETERS (INITIAL RATE, FINAL RATE,
AND DECAY CONSTANT).
2. DERIVED FROM REAL-WORLD OBSERVATIONS, MAKING IT PRACTICAL FOR ESTIMATING INFILTRATION
IN DIVERSE ENVIRONMENTS.
3. SUITABLE FOR MODELING INFILTRATION OVER EXTENDED TIME PERIODS AS IT ACCOUNTS FOR THE
DECREASING INFILTRATION RATE OVER TIME.
5.CAN BE ADAPTED TO VARIOUS SOILS BY ADJUSTING THE MODEL PARAMETERS.

LIMITATIONS OF THE HORTON INFILTRATION MODEL

1. PURELY EMPIRICAL, LACKING A DIRECT CONNECTION TO THE PHYSICAL PROCESSES GOVERNING
WATER MOVEMENT (E.G., SOIL SUCTION OR MOISTURE CONTENT).
2. IT MAY NOT ACCURATELY REPRESENT INFILTRATION UNDER VARIABLE RAINFALL, HETEROGENEOUS
SOILS, OR FLUCTUATING SURFACE CONDITIONS.
3. THE DECAY CONSTANT (K) ASSUMES A FIXED RATE OF DECLINE, WHICH MAY NOT REFLECT REAL-
WORLD VARIABILITY.
4. DOES NOT ACCOUNT FOR THE TIME WHEN SURFACE WATER BEGINS TO ACCUMULATE (CRITICAL FOR
FLOOD STUDIES).
5. STRUGGLES TO MODEL VERY EARLY INFILTRATION STAGES OR WHEN THE SOIL BECOMES FULLY
SATURATED.
 THE HORTON EQUATION
▪ IT IS AN EMPIRICAL FORMULA THAT SAYS THAT INFILTRATION STARTS AT A CONSTANT
RATE, , AND IS DECREASING EXPONENTIALLY WITH TIME, T. AFTER SOME TIME WHEN
THE SOIL SATURATION LEVEL REACHES A CERTAIN VALUE, THE RATE OF INFILTRATION WILL
LEVEL OFF TO THE RATE .

WHERE:
- INFILTRATION RATE AT TIME T;
- INITIAL INFILTRATION RATE OR MAXIMUM INFILTRATION RATE;
- CONSTANT OR EQUILIBRIUM INFILTRATION RATE AFTER THE SOIL HAS BEEN
SATURATED OR THE MINIMUM INFILTRATION RATE;
- DECAY CONSTANT SPECIFIC TO THE SOIL.
THE OTHER METHOD OF USING HORTON'S EQUATION IS TO FIND THE
TOTAL VOLUME OF INFILTRATION/CUMULATIVE INFILTRATION, F, AFTER
TIME t.
 SAMPLE PROBLEM WITH SOLUTION USING HORTON EQUATION
The initial rate of infiltration of a watershed is estimated as 2.1 in/hr, the final capacity is 0.2 in/hr,
and the time constant, k, is 0.4 hr-1. Use horton’s equation to find: A. The infiltration capacity at t =
2 hr and t = 6 hr; and B. The total volume of infiltration over the 6-hr period.
 PHILIP’S EQUATION
- ONE OF THE WIDELY USED MATHEMATICAL MODELS FOR
DESCRIBING INFILTRATION PROPOSED BY J.R. PHILIP IN
1957.
- DESCRIBES THE PROCESS OF WATER ENTERING THE SOIL
(INFILTRATION) USING A MATHEMATICAL EQUATION BASED
ON THE CONCEPTS OF SORPTIVITY AND HYDRAULIC
CONDUCTIVITY;
- HIS MODEL EXPLAINS HOW INFILTRATION RATE CHANGES
OVER TIME DEPENDING ON SOIL PROPERTIES AND WATER
AVAILABILITY.
 PHILIP’S EQUATION
- DERIVED USING THE RICHARDS EQUATION, WHICH DESCRIBES WATER FLOW IN UNSATURATED
SOILS. BY APPLYING SIMPLIFICATIONS AND SOLVING THE NONLINEAR DIFFUSION EQUATION,
PHILIP PROPOSED A SOLUTION THAT APPROXIMATES INFILTRATION IN TWO STAGES:
EARLY-TIME BEHAVIOR - DOMINATED BY SORPTIVITY, WHERE CAPILLARY FORCES DRIVE
INFILTRATION.
LATE-TIME BEHAVIOR - APPROACHES A CONSTANT RATE GOVERNED BY GRAVITY AND
HYDRAULIC CONDUCTIVITY.

- THIS MODEL IS WIDELY USED IN HYDROLOGY AND SOIL SCIENCE TO STUDY AND PREDICT
INFILTRATION RATES IN VARIOUS SOIL CONDITIONS, WHICH IS CRUCIAL FOR UNDERSTANDING
RUNOFF, GROUNDWATER RECHARGE, AND OTHER WATER MANAGEMENT ASPECTS.
PARAMETERS OF THE EQUATION:
SORPTIVITY (S)- THE CAPACITY OF THE SOIL TO ABSORB WATER THROUGH
CAPILLARY ACTION DURING THE EARLY STAGES OF INFILTRATION. IT DEPENDS ON
SOIL TEXTURE, MOISTURE CONTENT, AND PORE SIZE DISTRIBUTION.
STEADY-STATE INFILTRATION RATE (A)- THE CONSTANT INFILTRATION RATE WHEN
THE SOIL REACHES SATURATION. IT REFLECTS THE SOIL'S PERMEABILITY AND
HYDRAULIC CONDUCTIVITY.
HYDRAULIC CONDUCTIVITY - DESCRIBES HOW EASILY WATER MOVES THROUGH THE
SOIL. IT IS A MEASURE OF THE SOIL'S ABILITY TO TRANSMIT WATER UNDER PRESSURE.
COARSE SOILS LIKE SAND TYPICALLY HAVE HIGHER HYDRAULIC CONDUCTIVITY,
WHILE FINE SOILS LIKE CLAY HAVE LOWER VALUES.
ADVANTAGES
A. PHYSICALLY-BASED PARAMETERS ALLOW FOR BETTER INTERPRETATION OF SOIL
PROPERTIES.
B. SUITABLE FOR A WIDE RANGE OF SOIL TEXTURES AND INITIAL MOISTURE
CONDITIONS.
C. PROVIDES ACCURATE ESTIMATES DURING BOTH EARLY AND LATE INFILTRATION
STAGES.

LIMITATIONS
A. ASSUMES HOMOGENEOUS SOIL CONDITIONS, LIMITING ITS APPLICATION IN
HETEROGENEOUS LANDSCAPES.
B. REQUIRES ACCURATE MEASUREMENT OF SORPTIVITY AND STEADY-STATE
INFILTRATION RATE.
C. LESS ACCURATE FOR VERY LONG INFILTRATION PERIODS OR EXTREME SOIL
SATURATION SCENARIOS.
 PHILIP'S EQUATION
- PHILIP'S EQUATION EXPRESSES THE CUMULATIVE INFILTRATION AND INFILTRATION
RATE AS A FUNCTION OF TIME.
- PHILIP'S EQUATION OF THE INFILTRATION RATE IN GENERAL FORM IS GIVEN BY:
THE CORRESPONDING CUMULATIVE INFILTRATION IS OBTAINED BY
INTEGRATING THE INFILTRATION RATE:
 SOLVED EXAMPLE
A soil profile has a sorptivity (S) of 0.6 cm/hr and a steady-state infiltration rate (A)
of 0.15 cm/hr. Find the cumulative infiltration after 9 hours and the infiltration rate
at that time.
GIVEN:
GREEN-AMPT MODEL
GREEN-AMPT MODEL OF INFILTRATION
- DEVELOPED BY RESEARCHERS HEBER GREEN AND G.A. AMPT, WHICH
IS BASED ON THE ASSUMPTION OF A CONSTANT HYDRAULIC
CONDUCTIVITY WITHIN THE WETTING ZONE AND A SHARP WETTING
FRONT, MAKING IT PARTICULARLY USEFUL FOR ESTIMATING
INFILTRATION IN DEEP, HOMOGENEOUS SOILS WITH A UNIFORM INITIAL
WATER CONTENT.
PRINCIPLES OF THE GREEN-AMPT MODEL
- ASSUMES THAT INFILTRATION OCCURS AS A PISTON-LIKE MOVEMENT OF WATER
THROUGH THE SOIL. IT IS BASED ON DARCY'S LAW AND THE PRINCIPLE OF
CONSERVATION OF MASS. THE MODEL TREATS THE SOIL AS HAVING TWO DISTINCT
ZONES:
SATURATED ZONE: IMMEDIATELY BENEATH THE SURFACE WHERE THE SOIL IS FULLY
SATURATED WITH WATER.
UNSATURATED ZONE: BELOW THE SATURATED ZONE, WHERE THE SOIL REMAINS
DRY UNTIL INFILTRATION PROGRESSES.
 PRINCIPLES OF THE GREEN-AMPT MODEL
-ASSUMES A SHARP WETTING FRONT, WHICH SEPARATES
THE SATURATED AND UNSATURATED REGIONS AS WATER
INFILTRATES.
A SHARP WETTING FRONT REFERS TO THE CLEAR AND
DISTINCT BOUNDARY BETWEEN THE SATURATED AND
UNSATURATED ZONES OF SOIL DURING INFILTRATION.
IN THE GREEN-AMPT MODEL, IT IS ASSUMED THAT
WATER MOVES THROUGH THE SOIL IN A PISTON-LIKE
MANNER, CREATING A WELL-DEFINED FRONT WHERE THE
WET (SATURATED) SOIL MEETS THE DRY (UNSATURATED)
SOIL.
THIS IDEALIZED CONCEPT SIMPLIFIES THE INFILTRATION
PROCESS BY TREATING THE TRANSITION BETWEEN WET
AND DRY SOIL AS ABRUPT, RATHER THAN GRADUAL.
ASSUMPTIONS OF THE GREEN-AMPT MODEL
THE GREEN-AMPT MODEL RELIES ON THE FOLLOWING ASSUMPTIONS:
1. THE SOIL PROFILE IS INITIALLY DRY AND HOMOGENEOUS.
2. INFILTRATION OCCURS VERTICALLY AND UNIFORMLY.
3. THE WETTING FRONT IS SHARP AND DISTINCT.
4. CAPILLARY AND GRAVITY FORCES DRIVE WATER MOVEMENT.
5. CONSTANT PONDING DEPTH IS MAINTAINED DURING INFILTRATION.
ADVANTAGES:
• SIMPLE AND PHYSICALLY-BASED MODEL.
• PROVIDES ACCURATE ESTIMATES FOR HOMOGENEOUS SOILS.
• USEFUL FOR BOTH FIELD AND LABORATORY STUDIES.

LIMITATIONS:
• ASSUMES A UNIFORM SOIL PROFILE AND SHARP WETTING FRONT.
• REQUIRES ACCURATE MEASUREMENT OF SOIL PROPERTIES.
• LESS EFFECTIVE FOR LAYERED OR HETEROGENEOUS SOILS.
• DOES NOT ACCOUNT FOR WATER MOVEMENT THROUGH LARGE PORES, WHICH CAN
BE SIGNIFICANT IN CERTAIN SOIL TYPES
THE INFILTRATION RATE IN THE GREEN-AMPT MODEL IS
GIVEN BY THE FOLLOWING EQUATION:

THE CUMULATIVE INFILTRATION F(T) IS CALCULATED USING:

 PARAMETERS USED IN THE GREEN-AMPT EQUATION
CAPILLARY SUCTION HEAD - THE SUCTION HEAD, REPRESENTED IN MILLIMETERS, IS THE CAPILLARY
ATTRACTION ON THE SOIL VOIDS. THE LARGER THE VALUE OF THE CAPILLARY SUCTION HEAD, THE MORE
CAPILLARY ACTION THAT IS ACHIEVED AND THE AMOUNT OF INFILTRATION THAT TAKES PLACE.

SATURATED HYDRAULIC CONDUCTIVITY - MEASURED IN MM PER HOUR, REPRESENTS THE EASE THAT
WATER CAN TRAVEL THROUGH THE SOIL WHILST IT IS SATURATED. THE SATURATED HYDRAULIC
CONDUCTIVITY IS THE EQUIVALENT OF THE LIMITING INFILTRATION RATE IN THE HORTON INFILTRATION
MODEL.

THE POROSITY VALUE REPRESENTS THE VOLUME OF DRY VOIDS PER VOLUME OF SOIL AND PROVIDES THE
MAXIMUM MOISTURE DEFICIT THAT IS AVAILABLE, THE DIFFERENCE BETWEEN THE MOISTURE CONTENT AT
SATURATION AND AT THE START OF THE SIMULATION.

THE INITIAL MOISTURE VALUE REPRESENTS THE FRACTION OF THE SOIL THAT IS INITIALLY WET. AS BOTH
INITIAL MOISTURE AND POROSITY ARE EXPRESSED AS FRACTIONS, THE SOIL CAPACITY IS DEFINED AS THE
DIFFERENCE BETWEEN THEM BOTH.
GREEN-AMPT INFILTRATION PARAMETERS FOR VARIOUS SOIL CLASSES
EXAMPLE GREEN-AMPT MODEL
PONDING TIME
PONDING TIME
- THE TIME OF PONDING ( ) REFERS TO THE
POINT WHEN THE RAINFALL INTENSITY EXCEEDS
THE INFILTRATION CAPACITY OF THE SOIL, LEADING
TO THE ACCUMULATION OF WATER (PONDING)
ON THE SURFACE.
- IT IS HOW LONG IT TAKES FOR WATER TO BEGIN
POOLING ON THE GROUND WHEN THE SOIL
CANNOT ABSORB IT FAST ENOUGH. ONCE THE
INFILTRATION RATE FALLS BELOW THE RATE OF
WATER APPLICATION, PONDING OCCURS.
TIME OF PONDING ACCORDING TO HORTON’S MODEL
TIME OF PONDING ACCORDING TO HORTON’S MODEL
TIME OF PONDING ACCORDING TO GREEN-AMPT MODEL
TIME OF PONDING ACCORDING TO GREEN-AMPT MODEL
FITTING INFILTRATION MODELS
TO INFILTRATION DATA
USING EXCEL
Using Excel to fit an infiltration model to infiltration data is crucial for civil engineering and
hydrology studies. It provides a simple and efficient platform for analyzing data, visualizing
trends, and determining model parameters.

📊 1. Data Organization and Management

•Excel allows for systematic storage of infiltration data (e.g., time vs. infiltration rate or
cumulative infiltration).
•Large datasets from field experiments can be easily organized and manipulated for further
analysis.

📈 2. Graphical Visualization
•You can create scatter plots and line graphs to visualize infiltration trends over time.
•Helps in identifying patterns in the data and assessing model fit through visual inspection.
📐 3. Model Fitting Using Built-In Functions
•Excel provides regression tools (via LINEST function) and Solver for parameter estimation.
•Supports fitting common infiltration models such as Horton's Model and Green-Ampt Model

🔍 4. Parameter Estimation
Solver Add-In enables you to minimize error (e.g., using least-squares) by adjusting model
parameters.
Can optimize infiltration model parameters to best fit experimental data.

📏 5. Error Analysis
Perform statistical analysis (e.g., R² values, Root Mean Square Error (RMSE)) to evaluate
model accuracy.
Use residual plots to check for bias and model deviations.
📚 6. Accessibility and Cost-Effectiveness
Excel is widely available and easy to use, requiring minimal specialized software.
Suitable for both academic and professional applications.