Mapping Water Use From Space: Landsat and Water Resources in the United States

"A vision to observe Earth for the benefit of all..." — Interior Secretary Stewart Udall, 1966

Water is one of the country's most important natural resources, long thought inexhaustible. Changes in land use, climate and population are placing unprecedented demands on it; droughts are becoming more frequent, and water levels in aquifers and reservoirs are falling. Is there enough water for all our needs? Landsat satellites help answer that question.

Water used in a single day by crops in irrigated fields in southern Arizona, mapped from Landsat. USGS.

Water-use mapping

USGS scientists have used Landsat data to refine evapotranspiration (ET) mapping, which measures how much water crops use across a landscape and over time. A computer model combines Landsat and weather data, and the key ingredient is Landsat's thermal (infrared) band. With its 100-metre resolution, water use can be mapped field by field, anywhere in the world.

From one day to decades

ET maps can show water use for a single day or a whole growing season, and the vast Landsat archive makes it possible to map decades and reveal long-term trends. That archive might never have existed without the support former Interior Secretary Stewart Udall gave, in the 1960s, to observing the Earth from space.

Figure 1. Seasonal (May–September) evapotranspiration in California's San Joaquin Valley in 1990 (left) and 2014 (right). Water use intensified in many places (more blue), and some irrigated land (green in 1990) had left farming by 2014 (reddish brown). USGS.

What the maps are for

Field-scale maps help estimate and manage water use on irrigated cropland, answering questions such as:

• Where is water being used, how much, and by whom?
• Which crops use the most water, or the least?
• Can water be used more efficiently without hurting crop yields?

Basin-scale maps cover large areas and help in understanding the water balance and availability of river basins and watersheds, for:

• estimating water use by different sectors within a watershed;
• settling disputes over water rights and allocations;
• evaluating aquifer depletion and quantifying net groundwater pumping.

Planning for tomorrow

In a 2014 Government Accountability Office report, 40 of 50 state water managers expected water shortages in their states before 2023. Addressing such concerns starts with a clearer understanding of water availability and trends in water use. Landsat-based water-use mapping has shown immense potential at local and regional scales, and could become the basis for planning, monitoring and assessing water use across the country.

Sources

• Rebecca Johnson, Mapping Water Use—Landsat and Water Resources in the United States, USGS Fact Sheet 2016–3037, version 1.1, September 2019. https://doi.org/10.3133/fs20163037
• U.S. Government Accountability Office, Freshwater: Supply Concerns Continue, and Uncertainties Complicate Planning, GAO-14-430, 2014. https://www.gao.gov/products/GAO-14-430
• G. B. Senay and others, "Evaluating Landsat 8 evapotranspiration for water use mapping in the Colorado River Basin," Remote Sensing of Environment, 2016.
• The maps are taken from the fact sheet's PDF.