This post is part of our larger Comprehensive Guide To SGMA” 

Basin Setting

In Part 1 of Understanding GSPs we covered Introductory and Plan Area sections in a GSP. In this section we will cover the Basin Setting section that is full of important water budget, recharge, and soil information. Basin setting refers to hydrogeological models, current and historical groundwater conditions, water budget information, and whether there are designated management areas within the basin boundaries. The following highlight key points to pull from the Basin Setting section.


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Recharge requires a loo at hydraulic conductivity of soils for determining soils with higher permeability and infiltration rates, which means the path from surface water to groundwater is easier to travel and more closely interconnected. This could also indicate areas suitable for groundwater recharge activities. Many of the GSPs will likely use the Soil Agricultural Groundwater Banking Index (SAGBI) to assist in determining crucial areas for recharge. AQUAOSO includes SAGBI as a GIS layer as part of our research tool to better assist customers in locating potential recharge opportunities.


AQUAOSO Map showing SAGBI Layer for California.
AQUAOSO Map showing SAGBI Layer for California


Clay Layer


Indication of weather large deposits of clay, like the Corcoran Clay layer, may present issues with groundwater quality, perched aquifers, and groundwater supply depending on the depth and thickness of the clay layer. Many GSAs will treat the upper and lower aquifers, with the clay layer in-between, as separate for sustainable yield goals. AQUAOSO provides two Corcoran Clay GIS layers, depth and thickness, to better assist customers with identifying potential issues resulting from the clay layer.


AQUAOSO Map Showing Corcoran Clay Depth and Thickness.
AQUAOSO Map Showing Corcoran Clay Depth and Thickness


Surface Water Sources: Local and Imported Supplies


Local and imported water supplies. Important for understanding additional sources of water potentially available to a parcel within the basin. AQUAOSO also provides information on surface water sources for water districts allowing you read and view on our GIS map for comparison purposes.


Current and Historical Groundwater Conditions


GSPs include historical groundwater data based on past studies and reports. GSPs will update every five years and evolve as groundwater monitoring continues to increase in frequency. Below are some key points to pull out of the Groundwater Conditions section of a GSP.


Groundwater Elevation


Groundwater elevation provides evidence of groundwater extraction, difficulty of extraction, and an indicator of potential overdraft issues. Also, if there different parts of the aquifer, such as lower and upper, that have different characteristics, this section will account for those elevations separately. AQUAOSO provides publicly available groundwater depth data in an easy to view GIS map layer allowing a glimpse into recent groundwater depth readings for your area of interest.


AQUAOSO Map Showing Groundwater Depth
AQUAOSO Map Showing Groundwater Depth


Groundwater Hydrographs


Hydrographs can illustrate trends in groundwater depth over long periods of time. The GSP will provide those graphs and explain the trends in more detail. A trend in declining groundwater levels indicates a net loss of groundwater for the basin which may lead to subsidence, expensive groundwater extraction, and a likelihood of stricter restrictions on future pumping.


Groundwater Storage


Groundwater storage section of the GSP provides insight into the total amount of groundwater stored in a subbasin and the specific yield estimates. Specific yield assists in understanding the actual porosity, or how much space is available for the water to infiltrate, of a subbasin. Additionally, the change in groundwater storage is mentioned in a GSP. The change in storage indicates whether or not there is a consistent decline in groundwater in the subbasin. Knowing there is a consistent decline indicates a higher likelihood of groundwater pumping restrictions to ensure the subbasin reaches sustainable yield. Owning a property in an area with consistent decline in groundwater storage may present problems in relying on groundwater as a source of water for crops.


Water Quality


The amount of groundwater is important, but equally as important is the quality of the available groundwater. Contaminants such as nitrate and arsenic, as well as water salinity, can impact crop yields and soil health. The GSP will look at whether the specific contaminants analyzed, such as nitrate, exceeds the Maximum Contaminant Load (MCL) over a defined period of time. Such analysis is useful in understanding the water quality trends for the basin and planning appropriate measures to protect land and crops from high levels of contamination.


Land Subsidence


GSPs will also comment on land subsidence within a basin. GSPs will likely rely on various past subsidence studies showing historical trends in subsidence. Subsidence impacts groundwater quality, drainage, infrastructure, and other important agricultural assets. State and Federal government programs are analyzing subsidence with greater detail and more recent data should be forthcoming over the next few years.


Water Budget Information


The GSP water budget section will explain the methodology used to determine the water budget and then present the results. Water budget components are largely dictated by regulation. [Insert Image of Accounting Center and GSP Regs]. Interesting information coming from the water budget analysis includes groundwater extraction by water use sectors, inflows and outflows based on local surfaces water sources and canals, and impacts on changes to water storage.  Finally, the water budget section of the GSP will describe whether there are overdraft conditions are present.


Subbasin Sustainable Yield


GSP Regulations require the water budget to quantify the sustainable yield for the subbasin. Sustainable yield is defined as “the maximum quantity of water, calculated over a base period representative of long-term conditions in the subbasin and including any temporary surplus, that can be withdrawn annually from a groundwater supply without causing an undesirable result” (CWC Section 10721(w)). The sustainable yield will be a basis for groundwater pumping restrictions, fees, and other GSA rules governing a subbasin.


Taking Action


Armed with insight into the information available from the GSPs, you are ready to take action. Don’t got it alone. Let AQUAOSO gather important information and guide your organizations efforts to identify, understand, monitor, and mitigate water risk.


Contact us today! Look for our third post on Understanding GSPs in the near future.

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