Understanding Hydrology and the Water Cycle

Hydrology

• Deals with all phases of Earth's water

Practical Applications

• Designing and operating hydraulic structures
• Managing water supply, wastewater, irrigation, and drainage
• Hydroelectric power generation
• Flood control and navigation
• Erosion and sediment control
• Recreational use of water
• Fish and wildlife protection

Hydrologic Cycle

• Water circulates by evaporating from surfaces, moving through the atmosphere, and returning to seas through various routes

Hydrologic Cycle Process

• Solar radiation causes ocean water to evaporate
• Water vapor rises and forms clouds
• Condensation and precipitation occur as rain, snow, hail, or sleet
• Precipitation evaporates, is intercepted by surfaces, or infiltrates into the ground
• Vegetation returns water to the atmosphere through transpiration
• Precipitation reaches stream channels above or below ground, which is called runoff
• Runoff becomes streamflow once it enters a stream channel

Hydrologic Cycle Aspects

• Water transportation
• Temporary water storage
• Change of state

Water Balance

• Defines mass conservation across hydrological cycle components like atmosphere, water bodies, and vegetation

Total Water Quantity

• Approximately 1386 million cubic kilometers.

Precipitation Types

• Rain: Water drops > 0.5mm in diameter
  • ​Light: up to 2.5mm/h
  • Moderate: 2.5-7.5mm/h
  • Heavy: > 7.5mm/h
• Snow: Ice crystals combined as flakes, density 0.1 g/cm3
• Drizzle: Rain-droplets < 0.5mm, intensity < 1mm/hr
• Glaze: Freezing rain forms when rain touches ground at 0°C
• Sleet: Transparent, frozen raindrops that form when rain falls through subfreezing air
• Hail: Showery precipitation with irregular ice lumps exceeding 8 mm

Atmospheric Water Vapor Reporting Methods

• Vapor Pressure (e): Partial pressure of water vapor molecules
• Relative Humidity: Ratio of actual vapor pressure to saturation vapor pressure
• Mixing Ratio: Grams of water vapor per kilogram of air
• Dew Point Temperature: Temperature at which vapor in cooled air starts to condense

Precipitation Conditions

• Condensation requires temperature equal to or less than the dewpoint
• Supersaturation can occur without dust, requiring >10-4 mm dust to form cloud droplets
• Typical CCN is 0.2 µm and cloud droplets are 20-100 µm
• Raindrop size is 2000µm – 2 mm
• Precipitation needs downward velocity that exceeds uplift velocity to fall

Precipitation Processes

• Collision and Coalescence: Varying sizes of droplets collide, merge, and fall as rain
• Ice-Crystal Process: Ice crystals and water droplets exist, precipitation will fall as snow if it is cold, or as rain if it is warm enough

Air Mass Definition

• A large body of air having consistent temperature and humidity characteristics

Front

• Interface between contrasting air masses

Frontal Lifting

• Warmer air goes above cooler air, where they are in equilibrium with a cooler surface

Cold vs Warm Fronts

• Cold: Cold air advances towards warm air
• Warm: Warm air advances and overrides cold air.

Frontal Precipitation

• Precipitation linked to extratropical cyclones and associated with uplift, cooling, and condensation in warm, cold, and occluded fronts

Orographic Lifting

• Air rises due to elevated land, leading to more precipitation on windward slopes

Orographic Precipitation

• Occurs in mountains and is sometimes connected to frontal systems

Convective Lifting

• Warm air rises from a warm surface and progressively cools. This occurs as the air near the ground is heated by the earth, rises and cools causing precipitation

Convective Storms

• Vertical instability, cooling, condensation, precipitation

Cyclonic Lifting

• Forms in large, low-pressure systems when warm and cold air masses collide

Tropical Weather Depressions

• Active depressions over warm oceans absorb moisture and raise energy, causing intense rainfall over extended periods

Tropical Cyclones

• Gained energy through evaporation and latent heat release

Winds

• Circulate heat and moisture from heating to cooling areas

Climate in the Philippines

• Type I : Dry and Wet seasons
• Type II : No dry but rainy season
• Type III: Relatively dry and wet seasons
• Type IV: Evenly distributed rainfall

Precipitation Intensity

• Rate over specified time

Isohyet

• Maps of constant precipitation

Types of Time Represented Rainfall

• Hyetograph plots depth or intensity of rainfall as a function of time
• Cumulative Rainfall plots increments of summative rainfall as a function of time
• Rainfall intensity measures depth per unit of time

Lost Rainfall Records

• Records are occasionally lost or damaged

Interpolation of Rainfall

• Use simultaneous records from three nearby stations
• If precipitation is within 10%, calculate simple average
• If precipitation differs by >10%, use normal ratio method

Averaging rainfall

• Arithmetic average is useful if gages are evenly placed and relief is unimportant
• Theissen Method assigns weighting based on area
• Isohyetal average assigns area weights based on topographic contours
• Hypsometric relies on complex area calculation methods

Infiltration Rate

• The rate water enters the soil, if the water is on the surface, the infiltration occurs at a constant rate

Infiltration Capacity

• Capacity is the maximum rate for a specified soil type at a specified time

Cumulative Infiltration

• Cumulative depth of infiltrated water over specified time

Infiltration Equations

• Horton's: Infiltration starts at some rate fo and decreases to fc
• Philip's: Richard's equation solved for less restrictive conditions
• Green-Ampt: Another model for ifiltration

Ponding Time

• The amount of time between rainfall starts, and water starts to pool

Precipitation Gauges

• Recording rain gauges record intensity vs. time
• Non-recording gauges store total rainfall

Rainfall Frequency

• Relationship between rainfall intensity, duration, and frequency

Hydrology

• Study of water aspects on Earth

Hydrologic Cycle

• Water moves continuously where evaporation is conversion to vapor, releases water or recharge other sources

Water Balance Equation

• Includes precipitation, runoff, evaporation, transpiration, groundwater recharge, and any changes

Precipitation

• Can be measured as rainfall with gauges

Runoff

• Portion of precipitation flowing over land

Factors Affecting Runoff

• Climatic, catchment or antecedent moisture

Runoff Calculations

• Rational method: Runoff coefficient * Rainfall intensity * Catchment area = Peak runoff
• SCS number: Calculates maximum rain retention

Hydrographs

• River discharge over time

Hydrograph Components

• Include a rising limb, peak flow, and recession limb

Unit Hydrographs

• Helps estimate floods and watershed modeling
• It represents direct runoff relating from excess units of rainfall

Groundwater

• The water that exists beneath Earth's porous surfaces
• Aquifers store groundwater

Equations

• Discharge = hydraulic conductivity * gradient and area

Flood Routing

• Calculation of flood flow in water ways

Flood Control

• Structural includes building dams or levies
• Non-structural includes zoning and warning systems