Groundwater Ecosystem Findings
Last updated 5/1/2023 at 10:18am
Scientists from the University of California Irvine (UCI) presented initial findings of a three-year study of Groundwater Dependent Ecosystems (GDEs) in the Borrego Subbasin on March 27 at the Steele/Burnand Anza-Borrego Desert Research Center. Dr. Travis Huxman, Chair of UCI's Department of Ecology and Evolutionary Biology leads the study. He gave a brief history of water usage in the Borrego Valley and noted the current study is a joint venture of the Tubb Canyon Desert Conservancy, UCI, and the Department of Botany at the San Diego Natural History Museum. Dr. Huxman introduced the project co-managers, Dr. Laurel Brigham and Dr. Nikki Fiore.
GDE's are groupings of plants and animals whose survival depends upon accessing and using groundwater. Historically, the largest such assemblage of plants and animals in the Borrego Subbasin is several thousand acres of mesquite stretching from the Borrego Sink northward toward the airport. This area is often referred to as a mesquite forest or, using the Spanish word, a mesquite bosque.
Dr. Fiore explained the scientific team's first task of this three-year study was to review and analyze Appendix D4 of Borrego's Groundwater Management Plan (GMP). Appendix D4 is the part of the GMP that addresses GDE's; it asserts there were once several thousand acres of a GDE near the Borrego Sink, but that a decades long decline in groundwater levels led to the demise of all but a remnant of this historical GDE by 1985. Dr. Fiore noted the three arguments supporting Appendix D4's assertion of the demise of this GDE near the Borrego Sink exhibit substantive knowledge gaps and lack site-specific data. An examination of the evidence, and lack thereof, supporting D4's assertion regarding GDE's comprised the remainder of the presentation.
The map used to create Appendix D4 was the first knowledge gap addressed by Dr. Fiore. She explained her surprise upon examining the mapping of the "remnant GDE" described in Appendix D4. "The map of the remnant GDE contained a straight line. This caught my eye because plants growing in an ecosystem don't typically have straight edges. As I dug deeper into understanding the map used in D4, I discovered the straight line represented the political boundary between the State Park and the Borrego Springs Community Planning Area. The map used in D4 stopped at the boundary between the Park and the private property in Borrego Springs. The problem with this is that SanGIS, which maps vegetation on both sides of this boundary, shows there is another 2500 acres of mesquite bosque that exists in the Subbasin and is not taken into account by the Appendix D4."
Dr. Laurel Brigham addressed the second knowledge gap – the relationship of mesquite rooting depth and depth to groundwater. Dr. Brigham explained that Appendix D4 assigned 15.3 feet as the rooting depth for mesquite plants and that this number had been calculated rather than observed/measured. She noted the significant difference between this calculated rooting depth of 15.3 feet and the 1988 measured rooting depth of mesquite in the Borrego Sink at 39.8 ft. Parenthetically Dr. Brigham reported there are citations in the literature of observed mesquite rooting depth of 150 feet.
With respect to depth to groundwater, Appendix D4 cited a measurement of 55 feet from a well a mile away from the mesquite bosque. Combining this depth to groundwater measurement of 55 feet with a rooting depth of 15.3 feet, Appendix D4 asserts any remaining mesquite are not touching, or using, groundwater. However, when the measured rooting depth of 39.8 ft and the interpolated depth to groundwater of 13 feet from the Watermaster's Water Year 2022 Annual Report for the Borrego Springs Subbasin are used, the probability is quite different from that asserted by Appendix D4, as it is likely that mesquite are able to access groundwater. It is worth noting that the groundwater contours presented in the Watermaster's report, and which are used to estimate groundwater depth in the vicinity of the Borrego Sink, have uncertainty associated with them due to the limited amount of data used to create the contours. Dr. Brigham notes that isotopic analyses of water taken from plants will be conducted to quantitatively determine groundwater use by mesquite.
Dr. Fiore returned to the podium to address the third knowledge gap of Appendix D4–the use of remote sensing data, i.e., satellite imagery, to assess the health of the mesquite bosque ecosystem near the Borrego Sink. She described how satellite imagery has been used for decades to develop inferences as to the health of ecosystems by measuring the greenness of plants in a given area. She explained that Appendix D4's attempt to use satellite imagery as an indicator of ecosystem health was constrained by the fact that it used a map that excluded 95% of the mesquite bosque.
Additionally, Dr. Fiore noted the analysis of satellite imagery in Appendix D4 did not use appropriate time frames for assessing groundwater use by plants in the ecosystem. She clarified that, unlike plants reliant on surface water, plants accessing groundwater retain their greenness during periods of minimal precipitation, with May generally being the driest month in the Borrego Basin. Appendix D4 used other time frames that are less reliable in isolating use of groundwater by plants in the ecosystem.
The full details of the presentation can be found in the publicly available Technical Memorandum on the Tubb Canyon Desert Conservancy website.
This project is supported by a grant from the California Proposition 68 Funds. The grant proposal was created by the Tubb Canyon Desert Conservancy in 2022 and was awarded through a competitive process administered by the Borrego Water District and the California Department of Water Resources.