InSAR remote sensing improves groundwater monitoring

The satellite-based InSAR (interferometric synthetic aperture radar) is used to make high-resolution maps of land surface motion in space and time, including measurement of subsidence (or sinking).

The study, published in the journal Environmental Research Letters, took advantage of the incredibly fine-scale resolution of InSAR to evaluate subsidence patterns according to crop type, revealing surprising results.

Corn and soy show greatest rates of subsidence

For example, despite reports of high water consumption by fruit and nut crops in California, the crop types with the greatest rates of subsidence, and by association the greatest rates of groundwater use, were field crops such as corn and soy, followed by pasture crops like alfalfa, truck crops like tomatoes, and lastly, fruit and nut crops like almonds and grapes.

Because displacement is a response to groundwater storage change in locations with varying geology, soils and vegetation, the interpretation of InSAR varies across locations, unlike satellite measurements of climate that have the same interpretation in any location. Therefore, InSAR must be combined with other sources of geophysical data to achieve location-specific insight into groundwater use.

Subsidence occurred at much higher rates in irrigated cultivated land

By combining InSAR with other land surface datasets including land cover, potential evapotranspiration (a measure of plant water demand), and the location of surface water supply networks, UC San Diego researchers found that between 2015 and 2017, subsidence occurred at much higher rates in irrigated cultivated land compared to undeveloped land, and in dry surface water-limited years relative to wet years.

Over the study period, there was a median 272 millimeters (or 16 inches) of total cumulative subsidence for field crops (like corn and soy), and a dry water year subsidence rate of 131 millimeters (5 inches) per year. For fruit and nut crops, (like almonds and grapes) there was a median 62 millimeters (2.5 inches) of total subsidence over the study period, and a dry water year subsidence rate of 31 millimeters (1 inch) per year.

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Almond trees next to an irrigation canal in California’s Central Valley. – Photo: Canva

Field and pasture crops use less efficient irrigation methods

“The outcome might be explained by two things. First, on average fruits and nuts require less water physiologically, compared to field and pasture crops. Second, field and pasture crops tend to use irrigation methods that are less efficient and higher-volume than those used by fruit and nut crops,” said said study lead author Morgan Levy, an assistant professor with a joint appointment with UC San Diego’s Scripps Institution of Oceanography and School of Global Policy and Strategy. “However, fruits and nuts may still consume greater total volumes of water because they occupy more land area, even if their groundwater use intensity is less.”

“Our results suggest the possibility that transitions to fruit and nut cultivation might be desirable, at least from a groundwater sustainability perspective, although more research is needed to confirm this,” Levy said.

Advance global groundwater monitoring and management

California is an example of a semi-arid and irrigation-dependent climate for agriculture. Coordinated efforts from the UC San Diego team of climate scientists and geophysicists to link subsidence, groundwater and surface water use, and crop production data across comparable time and space scales has tremendous potential to advance groundwater monitoring and management in agricultural regions in other parts of the world, said the authors of the study.