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
Below are publications associated with the National Water Census.
Uncertainty analysis of the Operational Simplified Surface Energy Balance (SSEBop) model at multiple flux tower sites
Evaluation of the Global Land Data Assimilation System (GLDAS) air temperature data products
Accuracy assessment of NOAA gridded daily reference evapotranspiration for the Texas High Plains
On the downscaling of actual evapotranspiration maps based on combination of MODIS and landsat-based actual evapotranspiration estimates
Evaluating the SSEBop approach for evapotranspiration mapping with landsat data using lysimetric observations in the semi-arid Texas High Plains
Actual evapotranspiration (water use) assessment of the Colorado River Basin at the Landsat resolution using the operational simplified surface energy balance model
A comprehensive evaluation of two MODIS evapotranspiration products over the conterminous United States: using point and gridded FLUXNET and water balance ET
Analysis of long-term trends (1950–2009) in precipitation, runoff and runoff coefficient in major urban watersheds in the United States
Actual evapotranspiration modeling using the operational Simplified Surface Energy Balance (SSEBop) approach
Operational evapotranspiration mapping using remote sensing and weather datasets: A new parameterization for the SSEB approach
- 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.
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
Below are publications associated with the National Water Census.
Uncertainty analysis of the Operational Simplified Surface Energy Balance (SSEBop) model at multiple flux tower sites
Evapotranspiration (ET) is an important component of the water cycle – ET from the land surface returns approximately 60% of the global precipitation back to the atmosphere. ET also plays an important role in energy transport among the biosphere, atmosphere, and hydrosphere. Current regional to global and daily to annual ET estimation relies mainly on surface energy balance (SEB) ET models or statEvaluation of the Global Land Data Assimilation System (GLDAS) air temperature data products
There is a high demand for agrohydrologic models to use gridded near-surface air temperature data as the model input for estimating regional and global water budgets and cycles. The Global Land Data Assimilation System (GLDAS) developed by combining simulation models with observations provides a long-term gridded meteorological dataset at the global scale. However, the GLDAS air temperature producAccuracy assessment of NOAA gridded daily reference evapotranspiration for the Texas High Plains
The National Oceanic and Atmospheric Administration (NOAA) provides daily reference evapotranspiration (ETref) maps for the contiguous United States using climatic data from North American Land Data Assimilation System (NLDAS). This data provides large-scale spatial representation of ETref, which is essential for regional scale water resources management. Data used in the development of NOAA dailyOn the downscaling of actual evapotranspiration maps based on combination of MODIS and landsat-based actual evapotranspiration estimates
Downscaling is one of the important ways of utilizing the combined benefits of the high temporal resolution of Moderate Resolution Imaging Spectroradiometer (MODIS) images and fine spatial resolution of Landsat images. We have evaluated the output regression with intercept method and developed the Linear with Zero Intercept (LinZI) method for downscaling MODIS-based monthly actual evapotranspiratEvaluating the SSEBop approach for evapotranspiration mapping with landsat data using lysimetric observations in the semi-arid Texas High Plains
The operational Simplified Surface Energy Balance (SSEBop) approach was applied on 14 Landsat 5 thermal infrared images for mapping daily actual evapotranspiration (ETa) fluxes during the spring and summer seasons (March–October) in 2006 and 2007. Data from four large lysimeters, managed by the USDA-ARS Conservation and Production Research Laboratory were used for evaluating the SSEBop estimated EActual evapotranspiration (water use) assessment of the Colorado River Basin at the Landsat resolution using the operational simplified surface energy balance model
Accurately estimating consumptive water use in the Colorado River Basin (CRB) is important for assessing and managing limited water resources in the basin. Increasing water demand from various sectors may threaten long-term sustainability of the water supply in the arid southwestern United States. We have developed a first-ever basin-wide actual evapotranspiration (ETa) map of the CRB at the LandsA comprehensive evaluation of two MODIS evapotranspiration products over the conterminous United States: using point and gridded FLUXNET and water balance ET
Remote sensing datasets are increasingly being used to provide spatially explicit large scale evapotranspiration (ET) estimates. Extensive evaluation of such large scale estimates is necessary before they can be used in various applications. In this study, two monthly MODIS 1 km ET products, MODIS global ET (MOD16) and Operational Simplified Surface Energy Balance (SSEBop) ET, are validated over tAnalysis of long-term trends (1950–2009) in precipitation, runoff and runoff coefficient in major urban watersheds in the United States
This study investigates the long-term trends in precipitation, runoff and runoff coefficient in major urban watersheds in the United States. The seasonal Mann–Kendall trend test was performed on monthly precipitation, runoff and runoff coefficient data from 1950 to 2009 obtained from 62 urban watersheds covering 21 major urban centers in the United States. The results indicate that only five out oActual evapotranspiration modeling using the operational Simplified Surface Energy Balance (SSEBop) approach
Remote-sensing technology and surface-energy-balance methods can provide accurate and repeatable estimates of actual evapotranspiration (ETa) when used in combination with local weather datasets over irrigated lands. Estimates of ETa may be used to provide a consistent, accurate, and efficient approach for estimating regional water withdrawals for irrigation and associated consumptive use (CU), esOperational evapotranspiration mapping using remote sensing and weather datasets: A new parameterization for the SSEB approach
The increasing availability of multi-scale remotely sensed data and global weather datasets is allowing the estimation of evapotranspiration (ET) at multiple scales. We present a simple but robust method that uses remotely sensed thermal data and model-assimilated weather fields to produce ET for the contiguous United States (CONUS) at monthly and seasonal time scales. The method is based on the S