Hydrology: Water Science, Use, Availability

Hydrology: Science of Water

• Hydrology focuses on hydrologic concepts to measure current conditions and predict future water availability changes
• Water is essential for life
• "Water Use" is the withdrawal of water from a source and transported to a specific location, like from a river, lake, or well
• Water consumption differs from water use because it refers to water used for drinking or combined with a product and is not directly available for reuse, it's considered "consumed water"
• Only a limited amount of water is usable

Water Availability and Distribution

• Approximately 70% of the Earth's surface is water
• Only about 0.3% of the total water on Earth is readily accessible and clean enough for human consumption
• Water resources are not evenly distributed across the planet
• Two main environmental challenges related to water are drought (not enough water) and floods (too much water in the wrong location)

The Hydrologic Cycle

• Water is constantly in motion on, under, and above the Earth's surface
• Water evaporates, turns into vapor, and rises into the atmosphere
• Constant circulation of water and water vapor is known as the hydrologic cycle
• The main processes are evaporation, evapotranspiration (transpiration + evaporation), condensation, precipitation, and runoff
• Precipitation follows three potential paths:
  • Interception by vegetation or surface depressions
  • Infiltration through the Earth's surface
  • Flow over the surface

The Urban Water Cycle

• A key aspect of environmental tech consisting of integrated water management
• Integrated water management involves managing and controlling the urban water cycle
• The goal is to protect public health, while conserving freshwater resources, and sustaining aquatic ecosystems

Rainfall: Depth, Volume, and Intensity

• Rainfall is measured by the depth of water accumulated in a rain gauge during a storm, usually in millimeters or inches
• Total volume of water from a storm is calculated by multiplying the land area by the rainfall depth
• The formula is volume = depth × area
• In SI Metric Units:
  • Volume is expressed in cubic meters
  • Rainfall depth is expressed in millimeters
• Area must be in square meters, and rainfall depth should be in meters to keep the unit consistent
• Convert hectares into square meters by multiplying by 10,000 (1 ha = 10,000 m²)

Rainfall Intensity

• Expressed as depth per unit of time (in./h, mm/min, or mm/h)
• Rainfall intensity computation determines how much rain would fall if the storm lasted for one hour at a steady rate
• Rainfall intensity isn't constant over a storm's duration
• The average rainfall intensity is useful in hydrology problems and applications
• Hyetograph: graph displaying rainfall intensity or volume over time

Recurrence Interval

• Hydrologic events like rainstorms don't occur with regularity
• It's possible to determine the average frequencies of storms with specific intensities and durations
• Prediction about how often a similar storm can be expected within the next year or several years can be made.
• Recurrence interval, also called the "return period," determined by the average number of years between storms
• N-year storm: "N" stands for recurrence interval in years
  • A storm with a 5-year return period is called a 5-year storm
  • This means the average time between storms of the same duration is 5 years over a long period of time
  • A similar storm won't occur exactly every 5 years
  • The probability of a 5-year storm occurring in any given 5-year period is less than 100%

Probability of Occurence

• Data about intensity, duration, and return period needed in the urban drainage project
• P= 1/N where P symbolizes the probability and N is the amount
• The more intense the hydrological event, the larger the recurrence interval
• Larger recurrence interval creates lower chance of occurrencen

Intensity, Duration, and Frequency (IDF)

• "Storm intensity," "storm duration," and "recurrence interval" are not independent
• "Frequency" is often used as a substitute for "return period"
• A storm's frequency is inversely related to its return period: A 100-year storm occurs less frequently than a 10-year storm
• Relationships can be represented using IDF curves

Rainfall Curves

• Rainfall intensity-duration-frequency (IDF) curves are used to illustrate a typical set of rainfall conditions
• Rainfall curves for a specific location should be obtained from the National Weather Service, state, or county agencies
• Shorter storms have higher average intensities than longer storms
• Given duration, the storms with higher intensities correspond to storms with longer recurrence intervals
• Rainfall curves are used by intersecting a storm duration on the horizontal axis with the return period on the curved lines, vertically, then moving horizontally to read the expected rainfall intensity on the vertical axis
• A 10-year storm with a 30-min duration would have an intensity of 100 mm/h (or about 4 in./h)