Changes in land use, climate, and population demographics are placing unprecedented demands on water supplies in the United States. Add frequent droughts to that list, and mapping water use in the western United States is becoming increasingly important.
California’s San Joaquin Valley is one of the world’s most productive agricultural regions. Much of that productivity depends on the availability of water for irrigation. Recent prolonged droughts in California have underscored the importance of accurately monitoring changes and trends in water use in order to make well-informed water management decisions.
Landsat images show some change to the farmland in this valley over time. Not visible is exactly how much water was used to irrigate those crops. That’s where evapotranspiration (ET) comes into play.
ET is the combined effect of evaporation from the soil and transpiration from plants. ET is a major part of the hydrologic budget of a watershed. It varies with different climate, vegetation types, and land use.
With ET estimates derived from satellite data, scientists at EROS estimated how much water is being used to water crops. They used a computer model that incorporates Landsat imagery, including the Landsat 8 thermal band, along with climate data to estimate ET for every Landsat scene of the San Joaquin Valley from 1984 to 2014.
They then used the ET results along with precipitation and runoff data to create maps that show water use in the valley over that 31-year time period. The maps show seasonal crop water use in millimeters with enough detail to show individual fields and reveal which crops are using the most, or least, water.
In the southern portion of the San Joaquin Valley is the Tulare Lake Basin, which covers a significant portion of the valley. Tulare Lake was once the largest freshwater lake west of the Mississippi River. The lake used to appear naturally every winter as rainfall and snowmelt from the nearby Sierra Nevada Mountains flowed down and filled the basin.
By the 1920s, the basin was drained and today remains largely dry. Rivers that once flowed into the basin are diverted to agriculture. Agricultural drainage is sometimes stored in several evaporation ponds. These ponds are visible as the dark shapes in many of the Landsat images.
Some fields are flooded after harvest to remove salts accumulated during irrigation, control crop disease, and provide soil moisture for the next planting. For these purposes, water sits in the field for a week or less and is then drained. In some cases, a locally common fungal cotton pest needs to be controlled and a deeper and longer duration flood treatment is used. Water sits for over a week or many weeks and is then pumped out or drained by cutting through a levee. These flooded fields provide valuable habitat for waterfowl.
With these water use maps, land managers and farmers can tell which crops use the most, or least, water at the field scale. From this, they can determine whether water could be used more efficiently. At the basin scale, large-area maps can be used to evaluate aquifer depletion and quantify groundwater pumping; or resolve water rights allocation disputes.
The accompanying pair of seasonal ET maps (May–September) shows crop water use in the San Joaquin Valley in 1990 and in 2014. The colors correspond to millimeters of water returned to the atmosphere through ET. Fields that are green and blue show the highest ET values. Relatively more water has been used on those irrigated fields. Orange hues are areas that have very little ET, such as sparsely vegetated desert.
Comparing the maps reveals changes in irrigation patterns during this period. Notice, for example, that water use intensified in many places (increase in blue areas) and some irrigated lands (green in 1990) transitioned out of agricultural production (reddish brown) by 2014.
These water use maps can show not just seasonal ET but water used in a single day, or even over the course of decades thanks to the extensive Landsat archive. View the .gif animation below that shows an annual ET map for every year from 1984 to 2014. This map series shows flooding events around Tulare Lake Bed in 1984 and 1997.
Accurate information on water availability and usage is necessary for planning sustainable use of water, particularly in an arid region like the southwestern United States. The study done for the San Joaquin Valley can become the basis for planning, monitoring, and assessing water use across the country. These maps can help farmers and land managers optimize and conserve water resources.