Turlock Subbasin Overview

The Turlock Subbasin (5-022.03) is located within the San Joaquin Valley in central California:

Water Budgets Overview

Water budgets in the GSP are divided into four modeling scenarios: historical water budget, current water budget, projected water budget, and climate change water budget; and provided for three interconnected systems including the land surface system, the stream and river system, and the groundwater system.

This review focuses on Turlock Irrigation District (TID) supplies to the Turlock subbasin in the land surface system and the groundwater system for historic and projected scenarios. Further, projects and management actions are summarized below and incorporated into a new scenario “Projected with PMAs”.

Historic Water Budget

Historical Groundwater and Surface Water Use Over the historical period (1991 to 2015), water supplies from the agency averaged 517,000 AFY to the Turlock Subbasin (subbasin). Of the agency water supplies, 15% consisted of groundwater supplies (79,200 AFY), and 85% consisted of surface water supplies (437,800 AFY). Water supplies from the agency are only used for agricultural water supply. Agency surface water is diverted from the Tuolumne River at La Grange Dam.

Agricultural Water Supplies Historically, agricultural water supplies acquired from the agency and via private groundwater pumping, are comprised of 437,800 AFY (agency surface water), 79,200 AFY (agency groundwater), and 269,700 AFY (private groundwater). For a total of 786,500 AFY of agricultural water supplies. Nearly all (97%) surface water (426,300 AFY) is delivered to West Turlock Groundwater Sustainability Agency (WTGSA) for agricultural irrigation; 3% of surface water (11,500 AFY) is delivered to East Turlock Groundwater Sustainability Agency (ETGSA), for agricultural irrigation as well. All (100%) of agency groundwater supplies (79,200 AFY) are utilized in WTGSA; 0% of groundwater supplies (0 AFY) are utilized in the ETGSA. Less than a third (21%) of private groundwater pumping supplies (57,500 AFY) are used in WTGSA; most (79%) of private groundwater pumping supplies (212,200 AFY) are used in ETGSA.

Urban Water Supplies
Historically, urban water supplies average 55,500 AFY for the subbasin. Urban areas chiefly rely on groundwater for this supply. The majority (97%) of urban water supplies (53,800 AFY) are utilized in the WTGSA, where the urban areas are primarily located; very few (3%) of urban water supplies (1,700 AFY) are utilized in the ETGSA. The agency does not provide groundwater or surface water for use in urban areas.

The following is a visual representation of the land surface system supplies and demands for the historic scenario:

Projected Water Budget

Projected Condition Water Budget The projected condition water budget is based on a simulation of future land and water use over a 50-year period (1969 to 2018) of historic hydrologic conditions. This scenario assumes 2015 land use development, urban supplies and demands are based on UWMP projections, and the agricultural land use and cropping mix remain at 2015 land use. The projected condition water budget does not include project and management action impacts.

Over the 50-year period (1969 to 2018) in the scenario, water supplies from the agency averaged 490,800 AFY to the subbasin. Of the agency water supplies, 10% make up groundwater supplies (51,300 AFY), and 90% make up surface water supplies (439,500 AFY). In comparison to historic water supplies from the agency (517,000 AFY), projected water supplies from the agency (490,800 AFY) decrease overall by 26,200 AFY.

Agricultural Water Supplies In the projected conditions water budget, agricultural water supplies acquired from the agency and via private groundwater pumping, are comprised of 439,500 AFY (agency surface water), 51,300 AFY (agency groundwater), and 287,000 AFY (private groundwater). For a total of 777,700 AFY of agricultural water supplies, a decrease of 8,800 AFY from historic conditions. Specifically, agency surface water increases 1,700 AFY, agency groundwater decreases 27,900 AFY, and private groundwater increased by 17,300 AFY. Most (98%) of surface water (429,100 AFY) is delivered to WTGSA; 2% of surface water (10,400 AFY) is delivered to ETGSA. All (100%) of agency groundwater supplies (51,300 AFY) are utilized in WTGSA; 0% of groundwater supplies (0 AFY) are utilized in ETGSA. Few (14%) of private groundwater pumping supplies (38,800 AFY) are used in WTGSA; 86% of private groundwater pumping supplies (248,200 AFY) are used in ETGSA. Relative to historic distributions in the subbasin, the supply of agency surface water in WTGSA increases by one percent and decreases by one percent in the ETGSA. Groundwater supplies distribution remains the same (100% of agency ground water is utilized in WTGSA). Private groundwater supplies distribution changes in the WTGSA and ETGSA from a 21% (WTGSA) 79% (ETGSA) distribution in historic conditions to a 14% (WTGSA) 86% (ETGSA) distribution in projected conditions.

Urban Water Supplies In the projected conditions water budget, urban water supplies average 75,800 AFY for the subbasin. In comparison to the historic conditions water budget, urban water supplies (55,500 AFY, historic) increase by 20,300 AFY. Of the urban water supplies, nearly all (99%) of urban water supplies (75,000 AFY) are utilized in the WTGSA; 1% of urban water supplies (800 AFY) are utilized in the ETGSA. Relative to historic distributions, there is a two percent increase in urban water supplies in the WTGSA and a two percent decrease in the ETGSA.

The following is a visual representation of the land surface system supplies and demands for the projected scenario:

Land Surface System: Components

In this section, the focus is on comparing between the historic, projected, and projected with PMAs scenarios, for the land surface system.

Shown below is the distribution of components in the land surface water budget, the size of the rectangles corresponds to the proportion of the water budget in that larger rectangles correspond to a larger proportion of the water budget. The rectangles are color coded to represent the change from the historic scenario. The greatest changes correspond to red and green colors (decrease or increase from historic), while yellow indicates little change.

From the above chart we can determine that the largest decrease in supplies is agency groundwater (-27,900 AFY) and the largest increase in supplies is urban groundwater (20,300 AFY). For demands, the largest decrease is agricultural percolation (-29,300 AFY) and the largest increase in demand is agricultural et (28,800 AFY).

Similar to the above graph (Change Between Projected and Historic Scenarios), the difference between the Projected and Projected with PMAs scenarios are compared below:

From the above chart we can determine that the largest decrease in supplies is private groundwater (-63,300 AFY) and the largest increase in supplies is urban surface water (17,600 AFY). For demands, the largest increase in demand is urban runoff & return flow (480 AFY).

In the GSA, projected projects and management actions have “average” values which are the expected volumetric contribution of the project over 50 years, and maximum values which are expected in wet or above wet years (typically ~50%) of years. For this reason, volumetric benifits are applied to the projected scenario in two ways. The first being projected with average PMA values, and the second being projected with maximum PMA values. Four scenarios (Historic, projected, projected with average PMA values, and projected with maximum PMA values) are compared below:

As shown in the graph above, the proportion of each component is similar across scenarios. Generally, there is an increase in agency surface water, and urban surface water; as well as a decrease in private groundwater and urban grounfwater. The land surface system value represents a maximum values as there was little change to the demand values from the projected scenario due to the PMAs. In reality the land surface system may be lower.

Groundwater System: Components

In this section, the focus is on comparing between the historic, projected, and projected with PMAs scenarios for the Groundwater System.

Shown below is the distribution of components in the ground water budget, the size of the rectangles corresponds to the proportion of the water budget in that larger rectangles correspond to a larger proportion of the water budget. The rectangles are color coded to represent the change from the historic scenario. The greatest changes correspond to red and green colors (decrease or increase from historic), while yellow indicates little change.

From the above chart we can determine that the largest decrease in inflow is deep percolation (-27,800 AFY) and the largest increase in inflow is stream seepage (47,900 AFY). For outflows, the largest decrease is discharge to stream (-47,100 AFY)and the largest increase in outflows is urban groundwater production (20,300 AFY).

Similar to the above graph (Change Between Projected and Historic Scenarios), the difference between the Projected and Projected with PMAs scenarios are compared below:

From the above chart we can determine that the largest decrease in inflow is stream seepage and subsurface inflow as there was no change in these categories, and the largest increase in inflow is canal and reservoir recharge (23,600 AFY). For outflows, the largest increase in outflow is agency ag groundwater production (-41,300 AFY).

In the GSA, projected projects and management actions have “average” values which are the expected volumetric contribution of the project over 50 years, and maximum values which are expected in wet or above wet years (typically ~50%) of years. For this reason, volumetric benifits are applied to the projected scenario in two ways. The first being projected with average PMA values, and the second being projected with maximum PMA values. Four scenarios (Historic, projected, projected with average PMA values, and projected with maximum PMA values) are compared below:

As shown in the graph above, the most notable change is an increase in canal and reservoir storage (XXX,XXX AFY). Generally, there is an increase in agency surface water, and urban surface water; as well as a decrease in private groundwater and urban grounfwater. The land surface system value represents a maximum values as there was little change to the demand values from the projected scenario due to the PMAs. In reality the land surface system may be lower.

Periodic Groundwater Measurements (DWR)

The location of the Turlock subbasin, relative to CA counties. The north half of the Turlock Subbasin is located within Stanislaus County, and the south half of the Turlock Subbasin is located within Merced County, as shown below.

Groundsurface Elevation (GSE) in wells is measured periodically by DWR. GSE is shown below, deeper wells are in yellow and shallower wells in purple. It is reported in the GSP, and evident from GSP data, that in general the shallowest domestic wells are in the western subbasin and the deepest in the eastern subbasin. It is unclear what type of wells DWR is monitoring as this is marked “Unknown” for many.

Wells are located throughout the subbasin, and monitoried periodically by DWR.

PMA Applications

Where will volumetric benifits from projects and management actions be felt? [under construction]