The United States faces growing challenges to its water supply, and we need to know where there is not enough water, where we have more than we need, and the quality of the water. The USGS is charged with understanding and reporting on water availability including influences on water supply (how much water and of what quality) and water demand (how much water do humans and ecosystems need).
What is water availability?
Water availability refers to how much water is physically accessible (water quantity) and whether that water is safe to use (water quality). The differences in water quantity and quality across the landscape, and how they change through time, determine whether there is enough water for human and ecosystem needs. The amount and quality of available water is influenced by human activities, like using water for agriculture, drinking water supplies, and thermoelectric power generation, and by natural events like fires, tropical cyclones, and drought. Understanding water supply and demand will support better and more efficient water management for economic growth, protection of aquatic ecosystems, agriculture and energy production, and human population growth.
Because the concept of water availability is broad, many groups have made more specific definitions of water availability. Most include aspects of water quantity, water quality, and water timing (when water is available throughout the year). Some definitions of water availability focus on human water use, while others recognize the tradeoffs between human and ecological water needs. Other definitions address the idea that even though water may be physically present, it may not be accessible to people or ecosystems that need it.
The USGS Water Resources Mission Area defines water availability as the spatial and temporal distribution of water quantity and quality, as related to human and ecosystem needs, as affected by human and natural influences. In other words, we define water availability as how water quantity and quality change through time and space, while considering who needs the water and what might be affecting the quantity and quality of the water. This definition is consistent with the SECURE Water Act (more on this below) and aligns with the scope of water science in the USGS.
Assessing water availability is difficult because the things that affect water quantity, water quality, and water use (the removal of water for human needs) are complex. Because of this complexity, most research focuses on one part of water availability at a time. For example, one research group might study how the timing of snowmelt affects the amount of water in reservoirs, and another team might investigate how nutrients from agriculture influence toxic algal blooms. These studies are important, but they each only tell us about one piece of the water-availability puzzle. These different research areas need to be considered together, or integrated, to fully understand water availability and predict how water availability might change in the future.
What has Congress charged the USGS to do as part of the SECURE Water Act?
The U.S. Congress recognized the need for regularly updated information on water availability in the Omnibus Public Land Management Act of 2009 (Public Law 111-11). Subtitle F, also known as the SECURE Water Act, calls for the establishment of a national water availability assessment within the USGS. Specifically, the SECURE water act directs the USGS…
…(1) to provide a more accurate assessment of the status of the water resources of the United States; (2) to assist in the determination of the quantity of water that is available for beneficial uses; (3) to assist in the determination of the quality of the water resources of the United States; (4) to identify long-term trends in water availability; (5) to use each long-term trend to provide a more accurate assessment of the change in the availability of water in the United States; and (6) to develop the basis for an improved ability to forecast the availability of water for future economic, energy production, and environmental uses.
…[and to] conduct an ongoing assessment of water availability by (A) developing and evaluating nationally consistent indicators that reflect each status and trend relating to the availability of water resources in the United States, and (B) maintaining a national database of water availability data that (i) is comprised of maps, reports, and other forms of interpreted data; (ii) provides electronic access to the archived data of the national database; and (iii) provides for real-time data collection; and (C) developing and applying predictive modeling tools that integrate groundwater, surface water, and ecological systems.
In other words, the USGS is required by Congress to understand and report on:
-
how much water is available
-
whether that water is safe to use
-
human and ecosystem needs for water
-
how water availability has changed over the years
-
how we can expect water availability to change in the future
-
what causes these changes in water availability
To answer these questions, the USGS Water Resources Mission Area will collect and share regularly updated data on water availability with the public, decision makers, and the scientific community. We will analyze and interpret these data to provide national assessments of water availability every five years. We will also conduct more detailed assessments of water availability in ten regions across the United States.
How is the USGS Water Resources Mission Area assessing water availability?
We approach water availability as the difference between water supply and demand, determined using core components of water quantity, water quality, water use, and aquatic community health.
To quantify (provide measurements) of water availability, we will combine model predictions of these components into a calculation that compares the amount of available water (water supply) to the total demand for water. Models are crucial for estimating water availability on a national scale because it’s impossible to continuously monitor every water source. Models also allow us to forecast how water availability may change into the future. When calculating water supply, we will include information on whether the water is of high-enough quality for the specific demand. For example, the water-quality requirements for safe drinking water are different than those for the protection of fish populations. We will also use models to examine how extreme events and compounding stressors, like heat waves plus low streamflow, affect the amount of water and its suitability for specific uses. This can help us identify tipping points for water availability—when water no longer meets our standards for a given use —and discover more sustainable and resilient ways of managing water.
This information will help answer the following types of questions:
-
How much water do we have, and where is it? Do we have more or less than in the past? How might the amount of available water change in the future?
-
What is the quality of our water, what are the problems, and where do they occur? Is the quality better or worse than in the past? How might it change in the future?
-
How do we use water, and do we have enough for all uses? How have we changed the way we use water? How might our ways of using water change in the future, and how would that be affected by changes in water quantity and quality?
-
What is the potential vulnerability and resilience of human populations and ecosystems to water shortages (not enough water), surpluses (too much water), and worsening water quality?
Integrated Water Availability Assessments Fundamentals
National Water Availability Assessment Data Companion
Groundwater and Streamflow Information Program
Water Availability and Use Science Program
National Water Quality Program
The United States faces growing challenges to its water supply, and we need to know where there is not enough water, where we have more than we need, and the quality of the water. The USGS is charged with understanding and reporting on water availability including influences on water supply (how much water and of what quality) and water demand (how much water do humans and ecosystems need).
What is water availability?
Water availability refers to how much water is physically accessible (water quantity) and whether that water is safe to use (water quality). The differences in water quantity and quality across the landscape, and how they change through time, determine whether there is enough water for human and ecosystem needs. The amount and quality of available water is influenced by human activities, like using water for agriculture, drinking water supplies, and thermoelectric power generation, and by natural events like fires, tropical cyclones, and drought. Understanding water supply and demand will support better and more efficient water management for economic growth, protection of aquatic ecosystems, agriculture and energy production, and human population growth.
Because the concept of water availability is broad, many groups have made more specific definitions of water availability. Most include aspects of water quantity, water quality, and water timing (when water is available throughout the year). Some definitions of water availability focus on human water use, while others recognize the tradeoffs between human and ecological water needs. Other definitions address the idea that even though water may be physically present, it may not be accessible to people or ecosystems that need it.
The USGS Water Resources Mission Area defines water availability as the spatial and temporal distribution of water quantity and quality, as related to human and ecosystem needs, as affected by human and natural influences. In other words, we define water availability as how water quantity and quality change through time and space, while considering who needs the water and what might be affecting the quantity and quality of the water. This definition is consistent with the SECURE Water Act (more on this below) and aligns with the scope of water science in the USGS.
Assessing water availability is difficult because the things that affect water quantity, water quality, and water use (the removal of water for human needs) are complex. Because of this complexity, most research focuses on one part of water availability at a time. For example, one research group might study how the timing of snowmelt affects the amount of water in reservoirs, and another team might investigate how nutrients from agriculture influence toxic algal blooms. These studies are important, but they each only tell us about one piece of the water-availability puzzle. These different research areas need to be considered together, or integrated, to fully understand water availability and predict how water availability might change in the future.
What has Congress charged the USGS to do as part of the SECURE Water Act?
The U.S. Congress recognized the need for regularly updated information on water availability in the Omnibus Public Land Management Act of 2009 (Public Law 111-11). Subtitle F, also known as the SECURE Water Act, calls for the establishment of a national water availability assessment within the USGS. Specifically, the SECURE water act directs the USGS…
…(1) to provide a more accurate assessment of the status of the water resources of the United States; (2) to assist in the determination of the quantity of water that is available for beneficial uses; (3) to assist in the determination of the quality of the water resources of the United States; (4) to identify long-term trends in water availability; (5) to use each long-term trend to provide a more accurate assessment of the change in the availability of water in the United States; and (6) to develop the basis for an improved ability to forecast the availability of water for future economic, energy production, and environmental uses.
…[and to] conduct an ongoing assessment of water availability by (A) developing and evaluating nationally consistent indicators that reflect each status and trend relating to the availability of water resources in the United States, and (B) maintaining a national database of water availability data that (i) is comprised of maps, reports, and other forms of interpreted data; (ii) provides electronic access to the archived data of the national database; and (iii) provides for real-time data collection; and (C) developing and applying predictive modeling tools that integrate groundwater, surface water, and ecological systems.
In other words, the USGS is required by Congress to understand and report on:
-
how much water is available
-
whether that water is safe to use
-
human and ecosystem needs for water
-
how water availability has changed over the years
-
how we can expect water availability to change in the future
-
what causes these changes in water availability
To answer these questions, the USGS Water Resources Mission Area will collect and share regularly updated data on water availability with the public, decision makers, and the scientific community. We will analyze and interpret these data to provide national assessments of water availability every five years. We will also conduct more detailed assessments of water availability in ten regions across the United States.
How is the USGS Water Resources Mission Area assessing water availability?
We approach water availability as the difference between water supply and demand, determined using core components of water quantity, water quality, water use, and aquatic community health.
To quantify (provide measurements) of water availability, we will combine model predictions of these components into a calculation that compares the amount of available water (water supply) to the total demand for water. Models are crucial for estimating water availability on a national scale because it’s impossible to continuously monitor every water source. Models also allow us to forecast how water availability may change into the future. When calculating water supply, we will include information on whether the water is of high-enough quality for the specific demand. For example, the water-quality requirements for safe drinking water are different than those for the protection of fish populations. We will also use models to examine how extreme events and compounding stressors, like heat waves plus low streamflow, affect the amount of water and its suitability for specific uses. This can help us identify tipping points for water availability—when water no longer meets our standards for a given use —and discover more sustainable and resilient ways of managing water.
This information will help answer the following types of questions:
-
How much water do we have, and where is it? Do we have more or less than in the past? How might the amount of available water change in the future?
-
What is the quality of our water, what are the problems, and where do they occur? Is the quality better or worse than in the past? How might it change in the future?
-
How do we use water, and do we have enough for all uses? How have we changed the way we use water? How might our ways of using water change in the future, and how would that be affected by changes in water quantity and quality?
-
What is the potential vulnerability and resilience of human populations and ecosystems to water shortages (not enough water), surpluses (too much water), and worsening water quality?