by Jay Lund

People often have strange ideas about how water works.  Even simple water systems can be confusing.  When water systems become large complex socio-physical-ecological systems serving many users and uses, opportunities for confusion become extreme, surpassing comprehension by our ancient Homo sapien brains.

When confused by conflicting rhetoric, using numbers to “follow the water” can be helpful.  The California Water Plan has developed some such numbers.  This essay presents their net water use numbers for 2018, by California’s agricultural, urban, and environmental uses by hydrologic region. 

Net water use is the amount by which a water use deprives water from other uses.  This differs from gross water use (a.k.a. applied water use) which includes both the net use plus any water returned after use which is available downstream for other uses.  The biggest net water uses, which deplete the most available water, are evapotranspiration from crops, urban landscapes, and wetlands, as well as required flows to the ocean.  Even large instream environmental or hydropower flows high in a watershed can have little net water use if reused downstream. 

The water accounting for agricultural and urban net water use is fairly strong here, but accounting for environmental flows remains primitive, and should probably be a lower bound.  Much past accounting conveniently (and sloppily) quantified “environmental water use” as all water not consumed by agriculture and cities, which inflated environmental water use.  The environmental water accounting here includes only evapotranspiration from interior environmental purposes (mostly wetlands) and outflows to the ocean required by law and regulation.

Here are the raw regional net water use numbers for 2018 by hydrologic region (arranged mostly north to south) by major purpose.  Details and data are available at https://data.cnra.ca.gov/dataset/water-plan-water-balance-data

If the average water availability in California is about 75 million acre-ft per year, clearly there will be more “surplus” ocean outflows and some greater water use in wetter years, and less “surplus” outflow and water use in drought years.  In this large, diverse, and complex water system, it is hard to catch every drop before it reaches the ocean.  Even in very dry years, some water escapes the clutches of water managers and users.

Table 2. Percent of net water use in each hydrologic region by major water use

California’s tremendous hydrologic and water use diversity jumps out from Table 2.  In the North Coast, net water use is 94% environmental (mostly outflow from wild and scenic rivers), with very little other uses (agricultural use here might be a bit more from illegal agriculture).  Most of California’s hydrologic regions have less than 10% of their water use being environmental, including the major urban regions and three of the largest the agricultural use regions.  All regions could be called unbalanced, individually, in different ways, which makes the state on average seem more balanced than it really is. 

When presented as percent of net water use, agriculture is the largest overall water use in California, and urban use is a distant third, as least in 2018.  Urban water conservation is good and merits some attention, but we clearly obsess with it disproportionately from a statewide perspective.  Agriculture is the big water use, and it is important how this use is managed (and largely reduced) for the future and for droughts.  Slash and burn approaches to water conservation hurt people and often ecosystems.

Other DWR data would show that total environmental use varies wildly from dry to wet years because much of it is for wild and scenic river flows. 

96% of net environmental water use is in just two northern regions. Most regions in California have less than 1% of total state net water use for environment.  89% of agricultural water use is concentrated in just four regions.  73% of urban water use occurs in just two regions.  We often talk about how important or unimportant water uses are for California overall, while neglecting how even small uses statewide can dominate in some regions, and how some uses of large statewide concern are almost absent from many regions.

We should make more use of numbers in water policy. For example, Greg Gartrell et al. (2017 and 2022) have done insightful analyses of Delta inflows and outflows, which dispel several myths and put Delta water balance in perspective.  Grafton et al. (2018) show how water conservation based on gross water use is often ineffective and misleading for saving water.  A recent Delta Independent Science Board (2022) report reviews how we might make better water numbers and put them to better use for current and future challenges.  Implementing the Sustainable Groundwater Management Act will require far better and more available numbers for groundwater, surface water, and water demands than we have ever had.

We need insightful numbers to challenge and improve our current conceptions and prepare a more common basis for the difficult water discussions needed for a better future.  

Jay Lund is a Professor of Civil and Environmental Engineering and Co-Director of the Center for Watershed Sciences at the University of California – Davis. He is easily confused, so numbers help him think things through.

Further reading

Delta Independent Science Board. 2022. Review of Water Supply Reliability Estimation Related to the Sacramento-San Joaquin Delta. Report to the Delta Stewardship Council. Sacramento, California.

DWR, California Water Plan water use data https://data.cnra.ca.gov/dataset/water-plan-water-balance-data

Gartrell, G., J. Mount, and E. Hanak (2022), “Tracking Where Water Goes in a Changing Sacramento–San Joaquin Delta, Technical Appendix: Methods and Detailed Results for 1980–2021,” PPIC, San Francisco, CA.

Grafton et al. (2018), “The Paradox of Irrigation Efficiency”, Science, https://www.science.org/doi/10.1126/science.aat9314