Mapping and Monitoring River-Sourced Recharge with Satellite Data
Figure 1: The InSAR Recharge Signal: Showing the pressure pulse from the river-sourced recharge moving from near the mountain front, out into the Valley. It appears as uplift in the InSAR data. The color shows the month of the peak uplift. This was seen in the winters of 2017 and 2019; shown here are the data from 2017.
The Place
California’s San Joaquin Valley
The Problem
Groundwater use in California’s agricultural San Joaquin Valley satisfies up to two-thirds of water needs during very dry years. Increasingly frequent and persistent droughts have led to declining water levels, significant land subsidence, water quality degradation, permanent storage loss, and wells running dry. This has motivated state-wide legislature to ensure sustainable management of groundwater supplies by 2040 through the Sustainable Groundwater Management Act (SGMA).
The math describing the problem in the San Joaquin Valley is simple: “water taken out” far exceeds “water going in” and there are concerns that the imbalance will get worse over time - “water going in” will further decline with climate change and water demand with increase. To achieve sustainability goals, there is a critical need to understand and monitor how recharge occurs in these groundwater systems – how new water reaches and refills the systems. Natural recharge in the San Joaquin Valley is currently presumed to occur through a combination of 1) direct infiltration from mountain-sourced rivers and streams coming from the Sierra Nevada, 2) lateral subsurface flow from groundwater in the Sierras, and 3) infiltration of “local” water from precipitation. The timing, volume, and mechanisms for these types of recharge at local to regional scales are poorly understood with a critical lack of data and/or models.
Our Approach
Snowmelt from the Sierra Nevada is a major component of the freshwater supply in the Valley. With climate change threatening to have a major impact on the timing and volume of snowmelt, of particular interest is finding new ways to map and monitor recharge sourced in the Sierras. We used satellite data - surface displacements measured using interferometric synthetic aperture radar (InSAR) - to track recharge from mountain-sourced rivers. Resistivity data from airborne electromagnetic (AEM) surveys allowed us to determine how and where we could use this satellite-tracking of recharge.
We built on previous research (Neely at al., 2021) that showed striking uplift patterns during wet years, referred to as the “InSAR signature”, which was attributed to recharge. We focused our analysis on two study sites where high density AEM data were available and river-sourced recharge is determined to have occurred during wet years: 1) near Fresno, California and 2) near Visalia, California.
Results
The results are shown in the two figures below. There are recharge pathways at both sites, but we see no InSAR signature at the Fresno site, and a clear InSAR signature at the Visalia site. What is required in order to observe an InSAR signature: sufficiently high clay content. The clay creates the confining conditions that trigger a pressure pulse associated with the recharge, and is also compressible, so an uplift will be seen in the InSAR data.
Our conclusion: InSAR provides a powerful way to map and monitor mountain-sourced recharge. This could transform the way we study mountain-bounded alluvial aquifer systems around the world, where there are concerns about how changes in snowpack can impact the freshwater supply.
Figure 2: The Fresno Site. No InSAR recharge signal was seen at the Fresno site. Looking at the slice into the ground at this location, the subsurface image of electrical resistivity shows the reds and oranges that correspond to areas where there is sand and gravel – materials through which water from the ground surface moves downwards along recharge pathways to reach the groundwater system.
Figure 3: The Visalia Site. An InSAR recharge signal was observed at the Visalia site. What we see in the subsurface – the resistivity values moving along the color scale to yellows and greens which correspond to much more clay than at the Fresno site. The movement of water along recharge pathways through sediments with higher clay content will trigger a recharge signal due to the confined conditions and the properties of the clay.
Project Sponsors
The Gordon and Betty Moore Foundation
Project Leads / Contacts
Rosemary Knight - rknight@stanford.edu, 650-785-4150