Hydrology: Science of Water

Hydrology: Science of Water

• This chapter introduces basic concepts for measuring current hydrologic conditions
• Chapter helps to estimate future changes in water availability.
• Water is essential for life
• Water use involves taking water from a source like a river, lake, or well, and moving it to a specific place
• Water use is different than water consumption
• Consumed water is water that one has used for drinking or combined with a product, such that it is not directly available for reuse

Water Availability and Distribution

• About 70% of the Earth's surface is covered by water
• Only about 0.3% of the water on Earth is easily accessible and clean enough for humans
• Water resources are not distributed evenly
• Two main environmental challenges are:
• Drought: Not enough water
• Floods: Too much water

The Hydrologic Cycle

• Water continuously moves on, under, and above the Earth's surface
• Water evaporates and turns into vapor and moves into the atmosphere
• The constant circulation of water and water vapor is the hydrologic cycle
• Main processes of the hydrologic cycle:
• Evaporation
• Evapotranspiration, which combines transpiration and evaporation
• Condensation, the opposite of evaporation
• Precipitation
• Runoff
• Water from precipitation can follow multiple paths:
• Being intercepted by vegetation or small surface depressions
• Infiltrating the Earth to seep downwards into the ground
• Flowing over the ground's surface

The Urban Water Cycle

• Integrated water management is a main part of environmental technology
• The urban water cycle aims to protect public health, conserve freshwater, and sustain aquatic ecosystems.
• Management and control of the urban water cycle involves: wastewater collection, reclamation, and treatment

Rainfall: Depth, Volume, and Intensity

• Rainfall is measured by the depth of accumulated water in the rain gauge during a storm (mm or inches)
• The total volume of water that falls on an area during a storm can be calculated
• The volume is computed by multiplying the land area with the rainfall depth
• The formula is: volume = depth × area

Measurement Units

• Volume is expressed in cubic meters
• Rainfall depth is expressed in millimeters
• To ensure consistency, convert area to square meters, and rainfall depth to meters
• Relatively large areas in hectares should be converted to m² (1 ha = 10,000 m²)

Rainfall Intensity

• Rainfall intensity refers to the depth of rainfall per unit of time
• Rainfall intensity can be expressed as in./h, mm/min, or mm/h
• Can still compute storm intensity even if the storm lasted less than an hour
• Computed in terms of mm/h, finding how much rain would have fallen if the storm lasted an hour at a steady intensity
• The rainfall intensity might not be constant over the duration of a storm
• Average intensity is a useful number in hydrology
• Hyetograph is a graph of rainfall intensity (or volume) versus time

Recurrence Interval

• The average number of years between storms of specific intensities and durations
• Also known as the return period of the storm
• 'N' in N-year storm stands for the recurrence interval in years
• For example, a storm with a return period of 5 years is called a 5-year storm
• The more extreme the hydrologic event, the larger its recurrence interval
• Over a long period, the average time between storms of that intensity and duration is 5 years
• It is not a guarantee the storm will occur once exactly every 5 years

Probability of Occurence

• Data on storm intensity, duration, and return period are important in the design of urban drainage structures
• Storm intensity is useful for predicting peak flows in rivers
• Can use this formula P = 1/N
• The larger the recurrence interval N, the lower the probability of occurrence P
• This is due to because of the inverse relationship between the two

Intensity-Duration-Frequency Relationships

• "Storm intensity,” “storm duration,” and “recurrence interval" must be considered
• The term "frequency" is used intead of "return period."
• The frequency of a storm or other event varies inversely with its return period
• A 100-year storm is less frequent than a 10-year storm
• Relationships among rainfall intensity, duration, and frequency may be shown graphically in IDF Curves

Rainfall Curves

• A typical set of rainfall intensity-duration-frequency (IDF) curves
• Appropriate rainfall curves for a location should be from the National Weather Service
• Storms of shorter durations have, on average, higher average intensities than longer storms
• Higher intensities correspond to storms with longer recurrence intervals, for a given duration
• Rainfall curves are used by finding horizontal axis with a preselected storm duration
• Move vertically to an intersection with a specific storm return period
• Move horizontally to the vertical axis, where expected rainfall intensity can be read