The myth of normal river flow: Drought, floods, and management of California’s rivers
Is California still experiencing drought? Even after a winter of record rainfall and snowpack, followed by a tropical storm, this is still an important question. And if you read the headlines, the answer is…yes and no. Although drought has been declared officially over, unsustainable groundwater pumping and overallocation of surface water leads to water deficits that persist, stressing rural communities, urban water supplies, and ecosystems. So even in this year of abundant rainfall and snowpack, water managers and river ecologists are still thinking about drought. In fact, drought conditions can be thought of as the base case, or the more common of two extremes that tend to drive management action in California. As climate change increases frequency and severity of both drought and flood in California (Swain et al. 2018), water managers must continuously plan for both very dry and very wet conditions.
What makes water management in California so challenging?
California’s patterns of rainfall and river flow are defined by variability and extreme events. Precipitation and streamflow in California are more variable from year-to-year and within a year than any other part of the U.S. (Dettinger 2011). The dry season can last for 6 months or more in many parts of California, with many rivers relying on groundwater to keep flowing, and species relying on the ability to migrate from drying rivers to survive. Freshwater species are adapted to natural variability in river flow, but not to the alterations in flow caused by people – including water extraction, river regulation from dams, and climate change. The vast majority of rivers in California experience altered flow conditions (Zimmerman et al. 2018) and human water use, management, and habitat loss have worsened drought conditions (AghaKouchak et al. 2015). Human activities have at least doubled the probability of occurrence of extreme drought compared to natural conditions (He et al. 2017). This pattern would only intensify if climate change were included in the analysis.
A result is that freshwater biodiversity is in crisis, in California and around the world. Declines in biodiversity in freshwater habitats are happening faster than any other habitat type. Freshwater covers less than 1% of the earth’s surface, but holds 10% of the earth’s species, including one-third of all vertebrates (Tickner et al. 2020). The Living Planet Index (www.livingplanetindex.org) indicates population declines for freshwater species of 81% between 1970 and 2012, the greatest decline over all habitat types, with the main threats including habitat loss and degradation from dams and unsustainable water use. Freshwater biodiversity loss in California follows – or even leads – the global trend. In 2021, 73% of California’s freshwater fishes were extinct, listed under the Endangered Species Act, or considered species of special concern (Leidy and Moyle 2021). Beyond fishes, nearly half of California’s freshwater species are threatened with extinction, and that number is far higher – 90% for species found only in California and nowhere else on earth (Howard et al. 2015).
When are California rivers experiencing drought, and when is it a problem?
Here’s what we know: drought conditions are occurring with more frequency, greater severity, and longer duration. Human water use compounds the effects of drought, further stressing the state’s ecosystems and impacting farms, rural communities, and urban water supplies. Water use and management in California has become unsustainable in many watersheds, contributing to the drastic loss of freshwater biodiversity, decimating California’s iconic rivers, and triggering legal battles over a limited public resource with far too many demands. The problems are complex and overwhelming.
Where do we begin? One of our most critical needs is the ability to identify where drought conditions are likely, so that planning and action can occur in advance of the driest months and years. The U.S. Drought Monitor produces weekly maps of drought status based on precipitation, soil moisture, and other factors, as a way to determine drought impacts across the country – influencing agriculture, water supply, and terrestrial ecosystems. However, the U.S. Drought Monitor does not assess drought status in rivers and streams or potential drought effects on freshwater ecosystems. To fill this need, the Salmon and Steelhead Coalition, comprising The Nature Conservancy, Trout Unlimited, and California Trout, developed a Drought Flows Monitor web tool (https://rivers.codefornature.org/#/apps) that identifies watersheds likely experiencing critically dry conditions. This tool can be used as a trigger to act quickly and efficiently to mitigate the effects of drought on freshwater species, regionally as well as in crucial watersheds. The Drought Flows Monitor can help guide decisions by identifying California watersheds with historically low natural flows where ecological risk from human water use is very high.
How does it work?
The Drought Flows Monitor relies on data in the Natural Flows Database (NFD) that can be accessed at https://rivers.codefornature.org/. The NFD models a range of natural stream flows for every stream reach in California at the monthly time step for 1950 to the present and is extended monthly. The most downstream reach of the largest river in each large watershed was selected to summarize statewide drought effects. Mean monthly natural flow predictions were downloaded from the NFD for each reach, and the likely presence of drought was assessed by comparing that monthly flow to the historical range of flows for that month and location. Drought severity was then characterized using U.S. Drought Monitor categories (https://droughtmonitor.unl.edu/).
Categories include exceptional drought (lowest monthly flow for the model period); extreme drought (monthly flow in the 2-5th percentile of the range for the model period); severe drought (6-10th percentile); moderate drought (11-20th percentile); abnormally dry (21st-30th percentile) and average/ wet (31st-100th percentile). Figure 1 illustrates how the flow predictions for a given month and watershed are assigned to these categories.
The tool can be used to look at historical and current drought conditions in California’s watersheds. Figure 2 shows drought status of California’s watersheds for the wettest (2017) and driest (1977) years from 1950 to 2022, according to the Northern Sierra 8 station index. One pattern is that drought conditions in 1977 were widespread and severe, but tended to be more widespread and severe in March and April than in August. This doesn’t mean conditions weren’t dry in August – streams were going dry and water was scarce – but three important insights can be gleaned from this pattern.
First, drought effects are not synonymous with dry streams. Abnormally low flows during the wet season are common during drought and can have big impacts even if a river doesn’t go dry. A river that might have 2,000 cfs in March of a wet year might only have 200 cfs in March of a drought year. That difference can have vast ecological consequences for species that rely on high flows to inundate rearing habitat and support migration in March.
Second, drought effects often appear in late winter and early spring and will likely persist until the start of the following year’s wet season. The reduction in August drought severity in 1977 shown in Figure 2 is likely because many streams have very low or no flows in August in most years, rather than because drought severity has lessened. Surface flow assessments can’t distinguish an average year from a drought year when flow is zero in both cases, even if impacts on riparian species and groundwater levels might be quite different. We know this because few areas of California are likely to get significant rain after April, and during dry years the end of significant storms often happens earlier, in March. Longer dry seasons are likely to become more common with climate change (Swain et al. 2018). We don’t have to wait until summer to start thinking about changing water management. We can confidently begin drought management actions much earlier, evaluating any rare late-season storms that may improve conditions.
Third, human and ecological experiences of a drought are based on observed flow – or what actually occurs in a river – rather than natural flow. These maps don’t include the effects of dams, diversions, or discharges. Streams often go dry during a drought because of the interaction of natural drought effects and human use – which is why human water use should be managed during drought years to avoid exacerbating drought effects that further degrade or dry up perennial streams and rivers.
How can the Drought Flows Monitor improve water management decisions?
The Drought Flows Monitor captures current and historical drought conditions that occurred throughout California. Drought conditions can be detected for any month, but patterns of two or more months of drought by March or April result in drought conditions likely to persist until the start of the next wet season. This means we can tell fairly early in a year if water will get scarce during drier and hotter months.
At least two types of management decisions can be made using this information: 1) immediate water conservation efforts for priority streams and rivers as a watershed enters drought conditions, according to natural flow estimates, and 2) planning for longer-term drought actions over the dry season, once a watershed has been in drought conditions for two or more months by April. The drought categories in the Drought Flows Monitor provide a useful framework for tailoring drought actions as drought severity increases, potentially beginning with voluntary water conservation efforts in the abnormally dry category, and progressing to water restrictions or curtailments as watersheds enter severe, extreme, and exceptional drought. Advanced drought planning is lacking for most of California’s watersheds, but this tool provides data helpful in closing that gap, providing advance notice, and addressing water scarcity before it becomes an emergency.
What about human water use?
The Drought Flows Monitor only considers natural flow conditions, as an indicator of natural drought stress. It is not a comprehensive indicator of drought conditions experienced by freshwater species as it does not account for additional human modifications to flow and habitat. In some locations with long-term gages, results from the Drought Flows Monitor can be compared to gage data to confirm observed flow conditions are indeed critically dry, and identify locations where human water use is likely further stressing freshwater species. The Monitor includes links to USGS gages and visualizations of current flow observations compared with historical discharge to help users assess whether drought categories based on natural flows are consistent with observed data. But because gage locations are very limited, other approaches to assess actual flows and ecological stress are still needed. To help fill these data gaps, The Nature Conservancy is currently working with collaborators to model actual flows in all stream reaches in California, to provide a dataset of flows that include human modifications and can be compared with natural flow conditions and enable alteration assessments, even where gages are not present. You can learn about our work on actual flows modeling on the California Water Blog: https://californiawaterblog.com/2021/09/26/developing-tools-to-model-impaired-streamflow-in-streams-throughout-california/.
The Drought Flows Monitor can be used to trigger drought actions directly, and as a tool to identify watersheds to verify instream conditions and stress to freshwater species through site visits or collection of field data. Collection of site-specific data is resource-intensive and cannot be applied across large spatial scales; so, a hierarchical approach of identifying priority watersheds using the Drought Flows Monitor that are further assessed using site-specific empirical methods can help protect rivers across large areas. Used together, statewide assessment of drought severity using the Drought Flows Monitor, combined with empirical observations at targeted watersheds, can help guide decisions to protect freshwater species in the rivers and streams with the highest ecological risk of water use. That said, knowing actual flows is not necessary for action when drought conditions are expected. When a watershed experiences drought, any additional decrease in flow risks harm to freshwater species, and drought actions are warranted. The Drought Flow Monitor is a tool for developing more comprehensive management that is responsive to changing conditions, fast to implement, and widespread – the type of approach needed to protect freshwater biodiversity in a changing climate.