No water budget would be complete without accounting for evaporation and related processes, such as transpiration and sublimation. Evapotranspiration, or "ET," refers to the combined flux of plant transpiration and evaporation from the adjacent soil. It is especially important for understanding water used by irrigated crops, and is related to crop productivity. Consumptive water use for irrigation represents the largest percentage across all water-use categories and is concentrated in the more-arid western U.S. where precipitation is not sufficient to grow many crops.
Water Census • Streamflow • Groundwater • Water Use • Environmental Flows • Evapotranspiration • Focus Area Studies
These fundamental mechanisms of evaporation and transpiration are a major part of the water cycle and have an important influence on water availability. Being able to quantify water lost or used through these processes can even have implications for administration of water rights and river basin compacts. Therefore, several studies are underway to improve techniques for quantifying these processes as part of the National Water Census. This will enable water managers to more accurately account for water through its entire cycle, improving the accuracy of water budgets.
Remote Sensing Research on Evapotranspiration
Historically, scientists have used empirical formulas calibrated with specialized instruments to measure crop-specific ET or reference crop ET to estimate other areas where crops are grown. The downside of this approach is that it results in local calibrations of crop-specific ET that may or may not be accurate to other areas with different climate, irrigation and conservation practices. This has made it difficult to quantify ET over broad areas such as irrigation districts, river basins, or States – which is commonly the scale at which modern water management decisions are coordinated or negotiated. Hence, there is a need to ensure accurate estimates of ET at the landscape scale.
In response to this challenge, the National Water Census has developed methods to use remote sensing as a method of estimating ET. USGS EROS has produced evapotranspiration estimates using satellite land surface temperature imagery using the Simplified Surface Energy Balance (SSEB) described in Senay et al. (2010), Senay et al., (2008), and Senay et al. (2007). Monthly actual ET (ETa) estimates at 1 km2 resolution are available for 2000 to present using MODIS thermal imagery for the conterminous U.S. Additional ETa products at 90 meter resolution are available using USGS Landsat data and has been applied to National Water Census Focus Area Study locations and as a pilot for a single year (2015) for the conterminous U.S. The MODIS results (1 km-resolution) are also used to produce anomaly maps as data become available, as part of an early warning system for drought. Due to the availability of satellite imagery that is repeated on a regular scale (depending on resolution), the SSEBop method results in ETa estimates that incorporate the spatial and temporal variation that occurs naturally throughout the growing season. This national coverage of ETa is scientifically defensible, is consistently generated on the basis of remotely sensed data, and can be used as a foundation for other projects, most notably the 2015 compilation of irrigation consumptive use. This analysis has shown the importance of the addition of both a detailed GIS layer of irrigated lands throughout the United States and implementation of the increased spatial resolution provided by the use of LANDSAT thermal-band imagery (90-meter) for future project efforts.
In addition, USGS is also researching remote sensing approaches to better quantify crop type, including fallowed lands, and crop water productivity.
Below are other science components of the National Water Census.
National Water Census
National Water Census: Streamflow
National Water Census: Groundwater
National Water Census: Water Use
National Water Census: Environmental Flows
National Water Census: Focus Area Studies
- Overview
No water budget would be complete without accounting for evaporation and related processes, such as transpiration and sublimation. Evapotranspiration, or "ET," refers to the combined flux of plant transpiration and evaporation from the adjacent soil. It is especially important for understanding water used by irrigated crops, and is related to crop productivity. Consumptive water use for irrigation represents the largest percentage across all water-use categories and is concentrated in the more-arid western U.S. where precipitation is not sufficient to grow many crops.
Water Census • Streamflow • Groundwater • Water Use • Environmental Flows • Evapotranspiration • Focus Area Studies
These fundamental mechanisms of evaporation and transpiration are a major part of the water cycle and have an important influence on water availability. Being able to quantify water lost or used through these processes can even have implications for administration of water rights and river basin compacts. Therefore, several studies are underway to improve techniques for quantifying these processes as part of the National Water Census. This will enable water managers to more accurately account for water through its entire cycle, improving the accuracy of water budgets.
Remote Sensing Research on Evapotranspiration
Historically, scientists have used empirical formulas calibrated with specialized instruments to measure crop-specific ET or reference crop ET to estimate other areas where crops are grown. The downside of this approach is that it results in local calibrations of crop-specific ET that may or may not be accurate to other areas with different climate, irrigation and conservation practices. This has made it difficult to quantify ET over broad areas such as irrigation districts, river basins, or States – which is commonly the scale at which modern water management decisions are coordinated or negotiated. Hence, there is a need to ensure accurate estimates of ET at the landscape scale.
Sources/Usage: Public Domain.Example of SSEBop Eta product: Spatial distribution of (A) annual evapotranspiration (millimeters per year) and (B) evapotranspiration (millimeters per day) on the day of satellite overpass (July 6, 2013) in a selected part of the southern Willcox irrigation district in Arizona. Modified from G.B. Senay, USGS, written commun., Oct. 26, 2015. [ETa, actual evapotranspiration; mm, millimeter] In response to this challenge, the National Water Census has developed methods to use remote sensing as a method of estimating ET. USGS EROS has produced evapotranspiration estimates using satellite land surface temperature imagery using the Simplified Surface Energy Balance (SSEB) described in Senay et al. (2010), Senay et al., (2008), and Senay et al. (2007). Monthly actual ET (ETa) estimates at 1 km2 resolution are available for 2000 to present using MODIS thermal imagery for the conterminous U.S. Additional ETa products at 90 meter resolution are available using USGS Landsat data and has been applied to National Water Census Focus Area Study locations and as a pilot for a single year (2015) for the conterminous U.S. The MODIS results (1 km-resolution) are also used to produce anomaly maps as data become available, as part of an early warning system for drought. Due to the availability of satellite imagery that is repeated on a regular scale (depending on resolution), the SSEBop method results in ETa estimates that incorporate the spatial and temporal variation that occurs naturally throughout the growing season. This national coverage of ETa is scientifically defensible, is consistently generated on the basis of remotely sensed data, and can be used as a foundation for other projects, most notably the 2015 compilation of irrigation consumptive use. This analysis has shown the importance of the addition of both a detailed GIS layer of irrigated lands throughout the United States and implementation of the increased spatial resolution provided by the use of LANDSAT thermal-band imagery (90-meter) for future project efforts.
In addition, USGS is also researching remote sensing approaches to better quantify crop type, including fallowed lands, and crop water productivity.
- Science
Below are other science components of the National Water Census.
National Water Census
The USGS National Water Census (NWC) is designed to systematically provide information that will allow resource managers to assess the supply, use, and availability of the Nation’s water. The goal of the NWC is to provide nationally-consistent base layers of well-documented data that account for water availability and use nationally.National Water Census: Streamflow
The USGS National Water Census complements the USGS national network of more than 8,000 streamgages by estimating streamflow for ungaged locations throughout the country, by analyzing streamflow records, and by providing tools for analysis of streamgage data to end users. The USGS National Water Information System (NWIS) makes the actual streamgage data available to the public, most of it in "near...National Water Census: Groundwater
The National Water Census (NWC) is leveraging a long history of groundwater studies and is accelerating ongoing regional studies to assess the Nation's groundwater reserves, studies that formerly were conducted under the USGS Groundwater Resources Program. The NWC is also increasing the ability to integrate groundwater and surface-water analyses into watershed-level assessments of water...National Water Census: Water Use
Through the National Water Census, USGS will provide national information on withdrawal, conveyance, consumptive use, and return flow by water-use category at spatial and temporal resolutions important for risk-informed water management decisions. Water-use data provide a foundation for water managers to analyze trends over time, plan more strategically, identify, and ultimately quantify...National Water Census: Environmental Flows
Environmental water studies refer to understanding the quantity, timing, and quality of water flows, as well as the water levels and storage required to sustain freshwater and estuarine ecosystems and the human livelihoods that depend on these ecosystems. The concept of ‘environmental flows’ in stream ecology are the basis of these studies, but they go beyond the understanding of surface flows and...National Water Census: Focus Area Studies
Focus Area Studies are stakeholder-driven assessments of water availability in river basins with known or potential conflict. They contribute toward ongoing assessments of water availability in large watersheds, provide opportunities to test and improve approaches to water availability assessment, and inform and ground truth the National Water Census with local information. Common to each of the... - Publications