Inspector Gregory: Is there any other point to which you would wish to draw my attention?
Sherlock Holmes: To the curious incident of the dog in the night-time.
Gregory: The dog did nothing in the night-time.
Holmes: That was the curious incident.

By Josué Medellín-Azuara and Richard Howitt, UC Davis Center for Watershed Sciences

California water analysts – us included – have long assumed that building a peripheral canal to carry exported water around the Sacramento-San Joaquin Delta would significantly harm local farming. Diverting so much fresh Sacramento River water would make the Delta too salty for irrigating high-value crops, we thought.

Delta farmers have assumed as much in opposing Gov. Jerry Brown’s plan to build a pair of giant tunnels beneath the Delta to transport water to the San Joaquin Valley and Southern California.

Yet, for all the pessimistic prognostications, no one had rigorously analyzed how a peripheral conveyance system would likely affect Delta water salinity and crop production. The forecasts were not based on formal hydrodynamic models, which can simulate the movement of water and salt under different conditions.

To remedy this, we recently ran hydrodynamic models to examine changes in water salinity, crop yields and crop revenues under various combinations of water export management and sea level rise in different parts of the Delta.

The study did not consider the specific changes in the state’s proposed Bay Delta Conservation Plan, which includes the governor’s “preferred alternative” to tunnel exported water under the Delta. The modeled tunnel operated under 1981—2000 water conditions with a capacity of 7,500 cubic feet per second – enough to transport up to 59 percent of average annual exports (4.9 million acre-feet), with the remainder continuing to be pulled through Delta channels to the export pumps.

Once we established the salinity changes in irrigation water, we used a detailed model of the Delta’s farming economy to estimate the effects of those changes on crop yields and revenues. (The model includes the role of salinity in farmers’ cropping decisions.) Finally, we ran these revenue changes through an economic model to see their effects on the Delta economy as a whole.

The result: no barking dog, as Sherlock Holmes might say.

Our study showed that the cost of salinity changes under a peripheral tunnel and a range of other conveyance scenarios would be less than 1 percent of the Delta’s total crop revenue – an estimated $2.3 million a year.

That’s a far cry (or bark) from previous estimates. A recent study for the state Delta Protection Commission put the annual revenue loss at $28 million to $54 million. Our earlier study in 2007 also had much higher estimates.

How do we account for such large cost reductions? Four factors explain the difference.

First, our study – Transitions for the Delta Economy, released earlier this year – is the first to forecast salinity levels with hydrodynamic models, which were built by fellow UC Davis researcher William Fleenor. Second, we conducted the analysis island-by-island, which enabled us to show that the islands at risk are those that grow the lowest value crops. Third, these low-value crops are the most salt-tolerant. Fourth, salinity tends to be highest in the late summer and fall, when most irrigation is finished except for relatively low-value pasture and hay.

The modeling scenarios included sea level rise, which would also affect Delta salinity and farm revenues. Many western islands that serve as barriers to sea water intrusion are subsided and expected to become permanently flooded in the coming decades.

As with the introduction of peripheral export infrastructure, Delta water interests and analysts have assumed that sea level rise would cause substantial salinity-related losses to the local farming economy. Our modeling runs, however, showed that sea level rise would have little effect on salinity during the irrigation season — even with the three-foot increase projected for 2050.

The same holds true with the loss of the sea water barriers – namely Bradford, Brannan-Andrus, Jersey, Sherman and Twitchell islands. (Bar graph does not include this scenario.) In addition, most higher value crops are not located in the parts of the Delta that would see the highest salinity increases. Again, no barking dog.

The combined application of hydrodynamic, water quality and agro-economic modeling is a considerable improvement over past analyses, which simply assumed a particular level of salinity change and applied that to the Delta farm economy.

Improved modeling details seem unlikely to change the conclusion that salinity costs of peripheral tunnels or canals are minor, even with sea level rise.

It doesn’t take an elaborate computer modeling exercise, however, to confirm a far bigger threat to Delta farmland: its fragile network of levees. The islands have sunk well below the level of surrounding waterways, and their levees are predicted to fail — with high costs of repair and likely abandonment — causing a loss of farm acreage. This dog is barking, and it could take a bigger bite out of the Delta farm economy.

 Further reading and references

Delta Protection Commission (2012), Economic Sustainability Plan for the Sacramento-San Joaquin Delta.

Fleenor, W., Hanak, E., Lund, J.R., Mount, J., 2008. Delta Hydrodynamics and Salinity Conditions. Public Policy Insititute of California, p. 32.

Hoffman, G. J. 2010. Salt Tolerance of Crops in the Southern Sacramento-San Joaquin Delta. Report for the California Environmental Protection Agency.

Medellin-Azuara, J., Hanak, E., Howitt, R., and Lund, J. R. (2012a). Transitions for the Delta Economy. Public Policy Institute of California, San Francisco, California. (February 2012).

Suddeth, R., Mount, J., Lund, J.R., 2010. Levee Decisions and Sustainability for the Sacramento-San Joaquin Delta. San Francisco Estuary and Watershed Science 8(2).

Van Genuchten, M.T., Hoffman, G.J., 1984. Analysis of Crop Salt Tolerance Data, In: Shainberg, I., Shalhevet, J. (Eds.), Soil Salinity under Irrigation, Processes and Management. Springer: Berlin, pp. 258-271(Ecological Studies, 251).