INFILTRATION

Infiltration

The flow of water into the ground through the earth's surface

Infiltration Process

Water from rain, snowmelt, or other surface sources enters the soil and moves downward through soil pores and cracks in rocks, driven primarily by gravity and assisted by capillary action

Infiltration Rate Behavior

Initially, dry soil absorbs water rapidly, filling available pore spaces, but as the soil becomes saturated, the infiltration rate slows

Infiltration Role in Hydrologic Cycle

It contributes to groundwater recharge, maintaining aquifers that supply drinking water, irrigation, and base flow to streams and rivers

Infiltration and Runoff Relationship

Water that infiltrates may remain in the soil for plant use, be absorbed by roots and later transpired, or continue downward to replenish deeper groundwater

Infiltration Effect on Surface Runoff

Reduces surface runoff, mitigating flooding and soil erosion

Infiltration Ecosystem Role

Supports ecosystem health by providing water for plants and soil organisms, maintaining soil moisture, and filtering pollutants before water reaches aquifers

Infiltration Management Importance

Essential in agriculture, urban planning, and water resource conservation, helping to sustain water supplies and reduce environmental hazards such as flooding and erosion

Seepage (S)

A lateral subsurface movement of water within the soil profile

Seepage Flow

The slow movement of fluid through small openings or cracks in the surface of unsaturated soil

Seepage Mechanism

The fluid fills the pores in the unsaturated bottom layer and moves into the deeper layers as a result of the effect of gravity

Percolation (P)

A vertical subsurface movement of water

Vadose Water

Water percolating in aerated soil

Ground Water

Water that reaches the saturated part of soil

Water Table

The boundary between vadose water and groundwater

Percolating Water Function

Replenishes aquifers, huge underground reservoirs filled with water

Factors Affecting Seepage and Percolation

Soil Type, Water Depth, Land Slope, Hard Pan Depth

Soil Type Effect on Percolation

Fine textured soil results in deep standing water; coarse textured soil results in shallow standing water

Water Depth Effect on Percolation

Percolation loss in deep flooded land is greater than in shallow flooded land

Land Slope Effect on Surface Losses

Surface losses are greater in sloping land than in flat land

Hard Pan Effect on Percolation

Percolation loss is affected by the depth of the hard pan layer below the surface

Percolation States

Percolation is high (deep water), Percolation is low (shallow water), Percolation is zero (no standing water)

Surface Water Hydrology

Deals with the transfer of water along the earth's surface

Runoff

The portion of the precipitation that makes its way towards rivers or oceans, etc as surface or subsurface flow

Runoff Definition 2

Portion which is not absorbed by the deep strata

Runoff Definition 3

When water flows over land areas and mixes with soil, minerals, and other contents

Surface Runoff Condition

Generally occurs when the rainfall intensity exceeds the rate of infiltration, or if the soil is at its water holding capacity

Runoff Paths (Number)

Three runoff paths that water follows to reach a stream channel

Runoff Path 1

Throughflow

Runoff Path 2

Overland flow

Runoff Path 3

Groundwater flow

Throughflow

Lateral subsurface flow path water takes to reach a stream channel

Overland Flow

Surface flow path water takes to reach a stream channel

Groundwater Flow

Subsurface saturated zone flow path water takes to reach a stream channel

Groundwater

Stored in subsurface void spaces below the water table

Aquifer

The geologic material that stores, transports, and yields groundwater to wells

Three Types of Aquifers

Unconfined Aquifer, Confined Aquifer, Perched Aquifer

Unconfined Aquifer

No confining layers between the zone of saturation and the land surface; if a fully screened well is drilled into an unconfined aquifer, the water level will rise to the water table, which is the top of the zone of saturation

Confined Aquifer

Overlain by a confining layer or aquitard, which is geologic material with little or no permeability/hydraulic conductivity; this layer does not allow water to pass through or the rate of movement is extremely slow

Aquitard

Geologic material with little or no permeability/hydraulic conductivity

Perched Aquifer

A saturated zone within the zone of aeration that overlies a confining layer; sits above the main water table

Interception

The process of interrupting the movement of water in the chain of transportation events leading to streams

Interception Sources

Can take place by vegetal cover or depression storage in puddles and in land formations such as rills and furrows

Stemflow

Interception through the stems

Throughfall

Interception because of the leaves

Data Selection Criteria I

Relevance

Data Selection Criteria II

Adequacy

Data Selection Criteria III

Accuracy

Plotting Positions

Method in flood frequency analysis involving return period and rank of events

Return Period Formula

Tr = (n + 1) / m

Return Period Variable – m

Rank of the event in order of magnitude

Return Period Variable – n

Number of years of record

Return Period Variable – Tr

Return period

Probability and Return Period Relationship

p = 1/Tr

Return Period Example

If a place has a 2% (0.02) probability of a flood striking in any given year, then that community would expect such a flood, on average, every 50 years

Return Period Calculation Example

0.02 = 1/x, therefore x = 50

Expected Occurrences Formula

Ek = (n + 1) / m

Expected Occurrences Example

Given n = 40 years, m = 10 years: Ek = (40+1)/10 = 4.1, therefore Ek = 4 times

Theoretical Distribution of Floods Formula

X = X̄ + Ks

Flood Distribution Variable – X

Flood of specific probability

Flood Distribution Variable – X̄

Mean of the flood series

Flood Distribution Variable – s

Standard deviation of the series

Flood Distribution Variable – K

Frequency factor

Log-Pearson Type III Distribution

A theoretical method (Step IV) used for flood frequency analysis using logarithmic transformation

Extreme-Value Type 1 Distribution

P = 1 – e^(–e^(–y))

Extreme-Value Type 1 – p

Probability

Extreme-Value Type 1 – e

Base of napierian logarithms

Extreme-Value Type 1 – y

Reduced variate

Fisher and Tippett Finding

The distribution of the maximum or minimum values selected from n samples approached a limiting form as the size of the samples increased

Extreme-Value Type 1 Distribution – X Formula

X = X̄ + Ks, where y is the reduced variate used to determine K

Design Frequency Selection Principle

The design of any structure with essentially unlimited life would be based on average probabilities

Binomial Probability Formula

Jk = [N! / (k!(N-k)!)] × p^k × (1-p)^(N-k)

Binomial Variable – N!

Year period

Binomial Variable – k!

Times of occurrence

Binomial Variable – p

1/Tr