Monday, June 21, 2021

Groundwater Dependent Ecosystems and SGMA


Is this a Groundwater Dependent Ecosystem?

People enjoy the last of a drying river on Memorial Day,
Ventura River Preserve 5-31-2021 

Flows rapidly dropped through the Ventura River Preserve this year, leaving the riverbed dry by June.  While this can be a common occurrence in dry years, surface flows are directly related to the groundwater below.  Under the State Groundwater Management Act (SGMA), the local Upper Ventura River Groundwater Sustainability Agency (UVRGSA) has been formed to develop a plan to sustainably manage groundwater into the future.  Draft documents have been released in preparation for a Draft Sustainability Plan later this year.

The question of how to determine whether a reach of the river is "connected" to groundwater, and therefore a "Groundwater Dependent Ecosystem" (GDE) under SGMA, is critical to the future sustainability of our water supply.  When a Groundwater Dependent Ecosystem suffers due to groundwater overdraft, the capacity of the landscape to store water can be compromised.    

The following comments were submitted to the UVRGSA regarding this concern.  The technical documents and other references are linked below. 

DATE: 6-18-2021 RE: Early Comments on Draft Supporting Documents for Upper Ventura River Groundwater Sustainability Plan

This memo is a follow up from our conversation regarding development of the Groundwater Sustainability Plan (GSP). The primary concern we discussed is the elimination of large portions of the basin from SGMA oversight through the assumption that surface water is somehow “disconnected” from groundwater. Apart from the fact that there are fundamental flaws in the methodology used to make this determination, the resulting conclusions and management criteria are not consistent with avoiding undesirable results.

The primary Sustainable Management Criteria (SMC) for the UVRGB is the Depletion of Interconnected Surface Water. The analyses presented to date do not adequately assess the groundwater/surface water interactions within and between the different reaches of the basin, or even acknowledge the impact of groundwater pumping on surface flows.

Screening Groundwater Dependent Ecosystems (GDEs)

The Upper Ventura River Groundwater Basin is a shallow alluvial aquifer integral to the riparian floodplain ecosystem of the main stem Ventura River. Throughout these reaches of the river, groundwater and surface water are connected, and to suggest they are not is to undermine the intent of the Sustainable Groundwater Management Act.

Figure 2. Confirming whether an ecosystem is connected to groundwater, TNC

The Riparian Groundwater Dependent Ecosystems Assessment Report characterizes the Robles reach as a “Losing reach with generally disconnected groundwater- surface water.” This categorization eliminates the majority of this Groundwater Dependent Ecosystem from consideration under SGMA by assuming that it is “disconnected” and thus has too great a depth to groundwater to support riparian habitat. Other reaches are similarly dismissed.

Figure 2 from Riparian Groundwater Dependent Ecosystems Assessment

The analysis presented relies heavily on the Nature Conservancy “Natural Communities (NC) Dataset,” using vegetation communities to eliminate GDE polygons from the Upper Ventura River Groundwater Basin. The NC dataset is a statewide geographic computer database that maps vegetation types in all potential GDEs throughout the State of California. The large geographic scope of this map does not accurately represent current on-the- ground conditions, and more robust ground truthing should be undertaken. Even the aerial photos presented tell a different story than is acknowledged in the narrative (i.e. Figure 6 North Robles Habitat Area Photographs, Aquatic GDE Characterization report)

Figure 6 North Robles Habitat Area Photographs

Unfortunately, the UVRGSA analysis does not fully implement the Best Practices for using the NC Dataset guidance provided by the Nature Conservancy, which presents six best practices for using local groundwater data to confirm whether mapped features in the NC dataset are supported by groundwater. (Best Practices for using the NC Dataset, TNC July 2019)

According to this guidance:  

While depth-to-groundwater levels within 30 feet of the land surface are generally accepted as being a proxy for confirming that polygons in the NC dataset are supported by groundwater, it is highly advised that fluctuations in the groundwater regime be characterized to understand the seasonal and interannual groundwater variability in GDEs. (see Best Practice #2.)

one of the key factors to consider when mapping GDEs is to contour depth-to- groundwater in the aquifer that is supporting the ecosystem (see Best Practice #5).

The GIS Spatial Analysis of Maximum Rooting Depth and Groundwater Level presented in the Riparian GDE document does not present such contour depth-to-groundwater mapping or account for temporal variability.

Figures from Best Practices for using the NC Dataset, TNC

Furthermore, TNC guidance acknowledges that;

In many situations, the hydrologic connection of NC dataset polygons will not initially be clearly understood if site-specific groundwater monitoring data are not available. If sufficient data are not available in time for the 2020/2022 plan, The Nature Conservancy strongly advises that questionable polygons from the NC dataset be included in the GSP until data gaps are reconciled in the monitoring network. Erring on the side of caution will help minimize inadvertent impacts to GDEs as a result of groundwater use and management actions during SGMA implementation.

Many of California’s GDEs have adapted to dealing with intermittent periods of water stress, however if these groundwater conditions are prolonged, adverse impacts to GDEs can result.

Therefore, it is likely that the NC vegetation mapping is representative of conditions in which groundwater levels have been frequently and repeatedly pumped beyond the reach of riparian tree roots. Meanwhile, field observations over the past few wetter years show that the riparian vegetation has rebounded, illustrating how the ecosystem responds with the variation in water years. Receding groundwater levels and corresponding loss of surface flows in the current drought will likely reverse this recent trend, with the potential loss of the many young sycamores.

Determining Groundwater/Surface water interactions

TNC guidance for determining GDEs recognizes the importance of surface flows;

In addition, SGMA requires that significant and undesirable adverse impacts to beneficial users of surface water be avoided. Beneficial users of surface water include environmental users such as plants or animals, which therefore must be considered when developing minimum thresholds for depletions of interconnected surface water.

The Model Results and SMC Implications Presentation (March 25, 2021) reaches the conclusion that:

  • Basin water budget is dominated by streamflow percolation into the Basin and groundwater discharge to Ventura River
  • GW pumping averages only ~10% of the GW Budget As low as 4% in wet years 
Up to 31% in dry years
  • Basin GW levels will be lower in dry seasons, but Basin will still re-fill in normal to wet years

The conclusion that there is no impact from pumping based on the fact that the basin rapidly refills in the wet season points to the likelihood that the surface water is in fact “connected” to groundwater during these periods. Moreover, the fact that pumping represents up to 31% of the budget in the critical dry years raises many questions.

figure from Model Results and SMC Implications Presentation (March 25, 2021)

The Model Results identify four areas of concentrated pumping, three of which directly impact groundwater levels in the “Robles Reach.” This reach is the area with the most storage in the basin, and should be considered as the “primary sub-basin” for water supply. Pumping in this reach directly affects conditions throughout the basin.

The analyses and graphs presented in the Model Results do not provide information on the spacial and temporal surface flow conditions as they relate to groundwater levels. Because the downstream reaches are largely dependent on surface and groundwater flows out of this sub-basin, further analysis is needed to more clearly define the relationship between groundwater levels and surface flows. The analyses should, at a minimum, determine threshold groundwater levels at which surface flows are diminished or eliminated, both in the reach being monitored and downstream.

Groundwater/Surface water interactions
Conjunctive Use Study 1978 

This relationship was established decades ago in the Ventura River Conjunctive Use Report (1978) which states that;

Flows in the live stretch are affected by both the rate of recharge of the upper part of the Ventura River groundwater basin and by the rate of groundwater extraction from wells in the river.

Investigations published in the Conjunctive Use Report identified groundwater elevation thresholds in the upper basin at which flows in the live reach will cease;

when the water level in well 4N23Wl6C4 falls below Elevation 495, surface flow in much of the live stretch stops although some pools remain. A flow of 1 cfs or more in the live stretch corresponds with a water level in this well of greater than about Elevation 507.

Groundwater levels also affect surface flows in the Robles Reach, which frequently dries up despite constant inflows. Unfortunately, the Aquatic GDE Impact Analysis is quick to dismiss the effect of groundwater elevation on surface flows;

No monitoring is recommended at either of the critical riffle aquatic GDEs or the Robles Habitat Area, as impacts from pumping in these areas were determined to be minimal or non-existent.

This conclusion is inconsistent with the guidance provided in Monitoring Networks and Identification of Data Gaps BMP (DWR 2016) which states:

23 CCR §354.34(c))(6): Depletions of Interconnected Surface Water.

Monitor surface water and groundwater, where interconnected surface water conditions exist, to characterize the spatial and temporal exchanges between surface water and groundwater, and to calibrate and apply the tools and methods necessary to calculate depletions of surface water caused by groundwater extractions. The monitoring network shall be able to characterize the following:

(A) Flow conditions including surface water discharge, surface water head, and baseflow contribution.

(B) Identifying the approximate date and location where ephemeral or intermittent flowing streams and rivers cease to flow, if applicable.

(C) Temporal change in conditions due to variations in stream discharge and regional groundwater extraction.

(D) Other factors that may be necessary to identify adverse impacts on beneficial uses of the surface water.

DWR guidance provides detailed information on developing a monitoring network to accurately assess these concerns.

Establishing Minimum Flow Thresholds

As described above, the current GSP analysis incorrectly concludes that groundwater pumping has little to no effect on surface flows throughout the majority of the basin. But even for the identified groundwater dependent “Habitat Areas,” the development of minimum flow thresholds is inadequate. For example;

For the Foster Park Habitat Area, while the City’s low-flow thresholds are based on only one HSI score evaluated in the Padre study (average thalweg depth), we understand this currently provides the best available information to establish minimum thresholds for the depletion of interconnected surface water sustainability criteria.

This statement ignores best available science, including the recently published CDFW Draft Instream Flow Recommendations (2021) as well as the NMFS Draft Biological Opinion for Foster Park Wellfield (2005).

Implications for the UVR Groundwater Sustainability Plan

According to the Brownstein Water Group, the Cuyama Valley Basin and the Paso Robles Area Subbasin GSPs were recently deemed incomplete for deficiencies in their definitions of sustainable management criteria (SMC), including minimum thresholds and undesirable results. Some of the concerns cited by DWR are that the GSP;

  • provides insufficient detail for how it determined that the selected minimum thresholds . . . are consistent with avoiding undesirable results
  • does not relate different minimum thresholds for different portions of the basin to conditions that could cause undesirable results
  • does not sufficiently discuss expected impacts and therefore “precludes meaningful disclosure to, and participation by, interested parties and residents in the Basin.

It is clear from these recent DWR determinations that much more work is needed to develop and present a clear understanding of the workings of the Upper Ventura River Groundwater Basin, the potential impacts from groundwater pumping, and a plan to better manage the limited resource to ensure future sustainability and a healthy ecosystem.


These initial comments are provided as requested, in good faith, prior to the release of the Draft GSP in the interest of stakeholder engagement and with the hopes that the UVRGSA is able to augment the current analysis and develop a meaningful assessment of the impact of groundwater pumping on surface flows in the Ventura River. It is clear that this will be necessary to successfully develop the Groundwater Sustainability Plan to a level that satisfies the objectives of the Sustainable Groundwater Management Act (SGMA) in order to gain the support of local stakeholders and approval by the California Department of Water Resources.


Upper Ventura River Groundwater Sustainability Agency (UVRGSA) 

Mapping Indicators of GDEs, Groundwater Resource Hub, The Nature Conservancy

IDENTIFYING GDEs UNDER SGMA; Best Practices for using the NC Dataset, The Nature Conservancy, July 2019

Monitoring Networks and Identification of Data Gaps BMP , CA Dept of Water Resources,

DRAFT ENVIRONMENTAL IMPACT REPORT, VENTURA RIVER CONJUNCTIVE USE AGREEMENT,  Report on the Environmental Impacts of the Proposed Agreement Between Casitas Municipal Water District and the City of-San Buenaventura for Conjunctive Use of the VENTURA RIVER - CASITAS RESERVOIR SYSTEM, Prepared for CMWD and the City,  June 1978

GSAs Shooting 50% on GSPs—DWR Releases First GSP Assessment Results for High Priority Basins, Brownstein Water Group, June 4, 2021

More info:

Sustainable Groundwater Management Act (SGMA)California Department of Water Resources

Groundwater Resource Hub, The Nature Conservancy

Natural Communities Commonly Associated with Groundwater, California Department of Water Resources

Groundwater Dependent Ecosystems - How can we manage groundwater to benefit both people and nature? The Nature Conservancy,

On this blog:

Understanding CDFW Instream Flow Recommendations