HYDROLOGY
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HYDROLOGIC LOSSES
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Collectively, that part of the rainfall that does not show up as
a runoff.
Also called abstractions.
Infiltration, Evaporation, Transpiration, Interception.
Occurs when water flows over the surface of the land.
It happens when the ground is saturated, impermeable, or the
rainfall rate exceeds the soil's infiltration capacity.
Water moves across the land, entering streams, rivers, or
lakes.
THROUGHFLOW
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Refers to the lateral movement of water within the soil.
Occurs when infiltrated water moves through the unsaturated
zone of the soil and flows downslope toward a water body.
Water travels underground but near the surface, often feeding
streams and rivers gradually.
INTERCEPTION
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Is that portion of the rainfall that is intercepted by trees, plants,
obstacles, and vegetation before it can reach the ground.
Rainfall that drops through the vegetation is called throughfall.
It is subsequently lost to the atmosphere through evaporation
and transpiration.
INFILTRATION
INFILTRATION RATE
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Plays a crucial role in hydrology and should be a key
consideration in water resource management.
Replenishes soil moisture, recharges aquifers and ultimately
supports stream flows during dry periods.
The process by which the water enters the soil from the ground
surface, usually fast at the beginning and slow at later stages.
Takes place at uppermost stratum of soil and movement is
vertical.
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Refers to the infiltration per unit time; expressed in volume per
unit area per unit time, or depth per unit time.
Example: An infiltration rate of 5L per square meter per hour
(It means that for every square meter of land, 5 liters of water
is absorbed by the soil every hour).
Example: An infiltration rate of 2cm per hour (Every hour, water
is moving 2 cm deeper into the soil).
CUMULATIVE INFILTRATION
PERCOLATION
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When water enters the ground surface and travels through the
soil to finally join the ground water reserves.
Happens at a deeper depth when water move vertically to join
water table.
In between unsaturated zone and saturated zone of the soil.
Usually, slow compared to infiltration.
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Refers to the summation of the depth of water absorbed by a
soil in a specified elapsed time in reference to the time of
initial water application.
For example, if water was applied at the start of a rainstorm,
and after 1 hour, the soil has absorbed 3 cm of water, and
after 2 hours, it has absorbed a total of 6cm, the cumulative
infiltration at the 2-hour mark would be 6cm.
SEEPAGE
IMPORTANCE OF INFILTRATION
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the process of seeping by which a liquid leek through a porous
substance.
Thus, water seepage occurs through a porous soil.
Movement of water is not restricted to vertical direction only
as in above two cases.
Slowest among the three.
When water enters the ground surface at the upstream side of
a retaining structure like a dam and comes out at the
downstream side.
SURFACE RUNOFF
Water conservation
Reduced soil erosion by runoff
Water reservoir design
SOIL HORIZONS/LAYERS
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Humus (O Horizon) - organic layer of soil made mostly of
litter and decomposed organic matter
A Horizon - where seeds germinate and plant roots grow
B Horizon - contains clay and mineral deposits
C Horizon - plant roots do not penetrate in this layer, very
little organic material is found
Bedrock - unweathered rocks
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Loam – best soil type
Clay – worst soil type
SOIL STRUCTURE
FACTORS AFFECTING INFILTRATION
INFILTRATION RATES
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1. WATER (fluid)
Intensity of rainfall - if rain falls too heavily, it may exceed the
infiltration rate, leading to runoff.
↑ Flow rate
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Viscosity - refers to a fluid's resistance to flow, essentially
measuring how thick or thin it is.
↑ Viscosity
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↓ Infiltration
Turbidity - a measure of how cloudy or murky water is due to
the presence of suspended particles. Water carrying fine
sediments can clog soil pores, reducing infiltration rates.
↑ Turbidity
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↓ Infiltration
↓ Infiltration
2. SOIL (medium)
Soil texture, soil structure;
The larger the pore size, the greater is the infiltration rate;
Surface sealing or puddling of soil - formation of a thin
compact layer on the surface as a result of severe breakdown
of soil structure caused by the beating action of raindrops and
flow of water over the surface
Porosity: High porosity promotes infiltration as more spaces
are available for water to fill.
Porosity - porosity, moisture holding capacity, permeability
Structure - the more stable the soil aggregates are, the higher
is the soil capability to store and transmit water.
SOIL CLASSIFICATION
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USDA Soil Classification (United State Department of
Agriculture) defines twelve major soil texture classifications
based on factors like their physical properties, climate and
biological activity.
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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.
Reduces surface sealing.
Tillage practices loosen up the upper soil layer thereby
increases the rate of surface entry and increases porosity of
the plowed layer.
4. ANTECEDENT MOISTURE CONDITION
Refers to the soil's moisture level before rainfall or irrigation.
High antecedent moisture condition causes reduction in pore
space and rate of water movement.
o Dry Soil: Dry soil can initially absorb water faster
because of available pore space.
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Wet Soil: Water infiltration slows when the soil is
already saturated because there's little or no pore
space left for water to enter.
5. TOPOGRAPHY
Influences the characteristics of surface runoff and interflow.
Slope speeds up overland flow and, hence, the depth and time
of distribution of direct runoff and infiltration.
6. USE OF SOIL ADDITIVES
Use of organic matter (compost)- improves soil structure and
porosity, enhancing infiltration.
Chemicals (Polymers and soil stabilizers) that improve the soil
structure hence increase infiltration.
Use of mulches - a layer of material spread on top of soil to
protect it and improve the soil's health. Reduces evaporation
and prevents soil compaction, indirectly aiding infiltration.