NRW105: 7.1 Importance of Water

Introduction

• Water, air, and food constitute people's most important natural resources
• Humans survive only a few minutes without oxygen, less than a week without water, but can survive about a month without food
• Water is essential for oxygen and food supply in the plant photosynthesis process
• Water is the most essential compound for all living things, including the human body
  • Human babies are approximately 75% water
  • Adults are 60% water
  • The brain is about 85% water
  • Blood and kidneys are 83% water
  • Muscles are 76% water
  • Bones are 22% water
• Constant water loss occurs through perspiration
  • About 2 liters of water per day should be consumed in temperate climates
  • Up to 9.5 liters of water per day should be consumed in hot desert climates
  • A Loss of 15% of body water usually causes death
• Earth is known as the Water Planet due to the abundance of liquid water
• Oceans cover about 70% of Earth's surface
• Approximately half of the Earth’s surface is obscured by clouds
• The planet holds about 1.4 billion cubic kilometers (km3) of water, equivalent to about 200 billion liters per person
• Earth's water could cover the United States to a depth of 145 km
• Over 97% of Earth’s water is seawater, which is too salty for drinking or irrigation
• Rivers and lakes contain less than 0.01% of the world’s water, but are the most commonly used water sources
• One crucial environmental goal is ensuring clean water access for everyone
• Water is a renewable resource that is difficult to destroy
• Evaporation and precipitation replenish freshwater supplies
• Water availability is complicated by its uneven distribution across the Earth
• Arid climates and densely populated areas cause water shortages, projected to worsen due to population growth and climate change
• Human activities worsen shortages by overuse and pollution
• Hundreds of millions lack safe drinking water
• Billions lack access to improved sanitation
• About two million people die every year from diarrheal diseases, 90% of these deaths occur among children under 5, which are easily preventable

Water Reservoirs and Water Cycle

• Water is the only common substance existing naturally as a solid, liquid, or gas
• Water is distributed in water reservoirs
  • Oceans are the largest reservoirs at 97% of all water, but too saline for human uses
  • Ice caps and glaciers are the largest freshwater reservoirs, inconveniently located in Antarctica and Greenland
  • Shallow groundwater is the largest reservoir of usable fresh water
• Rivers and lakes are the most heavily used water resources but represent a tiny amount of the world’s water
• If all the world’s water was the size of 1 gallon
• Freshwater would be about 1/3 cup
• Readily usable fresh water would be 2 tablespoons
• The hydrologic cycle shows the movement of water through oceans, atmosphere, glaciers, groundwater, lakes, rivers, and biosphere
• Solar energy and gravity drive the water cycle
• The water cycle involves water moving from bodies of water to the atmosphere through evaporation, forming clouds, and falling back to water and land as precipitation
• Land water returns to the ocean through surface runoff, rivers, glaciers, and subsurface groundwater flow, or back to the atmosphere by evaporation or transpiration

Water Salinity

• An important part of the water cycle concerns salinity, which is the abundance of dissolved ions in water
• Saltwater in oceans has high salinity, about 35,000 mg of dissolved ions per liter of seawater
• Evaporation is a distillation process that produces nearly pure water with almost no dissolved ions
• Water vaporizes and leaves dissolved ions in the original liquid phase
• Condensation forms clouds and precipitation
• Rainwater that falls onto land dissolves minerals in rock and soil which increases salinity
• Most lakes, rivers, and near-surface groundwater are freshwater

Primary Fresh Water Resources: Precipitation

• Precipitation levels vary globally, affecting freshwater availability
• More precipitation falls near the equator, while less precipitation tends to fall near 30 degrees north and south latitude, where the world’s largest deserts are located
• Rainfall and climate patterns are related to global wind circulation cells
• Intense sunlight at the equator heats the air, causing it to rise and cool, which decreases the air mass's ability to hold water vapor, resulting in rainstorms
• Around 30 degrees north and south latitude, descending air produces warmer air, which increases its ability to hold water vapor, resulting in dry conditions
• Both dry air conditions and warm temperatures favor evaporation
• Global precipitation and climate patterns are affected by the size of continents, major ocean currents, and mountains

Surface Water Resources: Rivers, Lakes, Glaciers

• Flowing water from rain and melted snow on land enter river channels by surface runoff and groundwater seepage
• River discharge describes the volume of water moving through a river channel over time
• The relative contributions of surface runoff vs. groundwater seepage to river discharge depend on precipitation patterns, vegetation, topography, land use, and soil characteristics
• River discharge increases soon after a heavy rainstorm due to surface runoff
• The steady normal flow of river water is mainly from groundwater discharging into the river
• Gravity pulls river water downhill toward the ocean
• The moving water of a river erodes soil particles and dissolves minerals
• Groundwater contributes a large amount of dissolved minerals in river water
• The geographic area drained by a river and its tributaries is called a drainage basin or watershed
• The Mississippi River drainage basin includes approximately 40% of the U.S
• Rivers are an important water resource for cropland irrigation and drinking water for many cities around the world
• Rivers with international water supply disputes include the Colorado, Nile, Euphrates, Ganges, and Jordan
• Lakes are a freshwater source, replenished from surface runoff and groundwater discharge
• Lakes are short-lived geologically because they are constantly filling with sediment supplied by incoming rivers
• Lakes form from:
  • Glaciation
  • Recent tectonic uplift
  • Volcanic eruptions
  • Artificial lakes (reservoirs) formed by damming rivers
• Climate changes can significantly modify a lake's size
• During the last Ice Age, the climate in the western U.S. changed and caused more than 100 large lakes to disappear
• The Great Salt Lake in Utah is a remnant of Lake Bonneville
• Glaciers represent the largest freshwater reservoir, but are not used as a water source due to their remote locations
• Melting glaciers provide a natural source of river water and groundwater
• During the last Ice Age, glaciers contained 50% more water, leading to about 100 m lower sea levels
• Sea level has been rising over the past century due to melting glaciers
• Continued warming will cause additional sea level rise

Groundwater Resources

• Groundwater is a much larger usable fresh water reservoir than rivers and lakes combined
• Groundwater is a particularly important resource in arid climates
• Groundwater is the primary water source for rural homeowners, providing 98% of the US water demand
• Groundwater is located in pore space between mineral grains and fractures in subsurface earth materials
• Most groundwater originates from rain or snowmelt infiltrating the ground and moving downward to the saturated zone
• Other sources of groundwater include seepage from surface water, surface water pumped into the ground, irrigation, and wastewater treatment systems
• Recharge areas are locations where surface water infiltrates the ground rather than running into rivers or evaporating
  • Wetlands are an example of recharge areas
• A large area of subsurface, porous rock holding water is called an aquifer
• Aquifers are commonly drilled, with wells installed for agriculture and personal use

Water Use in the U.S. and World

• People require water to produce food, energy, and mineral resources
  • One tomato = 11.4 liters
  • One kilowatt-hour of electricity = 79.5 liters
  • One loaf of bread = 568 liters
  • One pound of beef = 6056.6 liters
  • One tonne of steel = 238,480.9 liters
• Humans require only 3.8 liters per day to survive
• A typical Canadian uses approximately 466 liters per day, including cooking, washing, flushing, bathing, and commercial/industrial uses provided by public utilities
• An area's water demand is a function of its population and other uses of water