Many of California’s reservoirs are now fuller than they’ve been in years thanks to an extraordinarily wet winter. Yet drought conditions are likely to return based on historical cycles, even without accounting for climate change. And California’s depleted groundwater is still in need of massive inputs if it is to be replenished.
A recent study in Geophysical Research Letters provides some potentially valuable insights for future water management. In it, the authors make one of the first attempts to quantify the effect that human water management has on the frequency and intensity of surface water drought in California–and in some cases, they found, management practices exacerbated drought conditions. The authors address “hydrological drought,” which has to do with the amount of water in the watershed, rather than the amount of precipitation falling from the sky.
“Most studies that looking into drought look from a top down viewpoint,” says Amir AghaKouchak, a hydrologist at University of California, Irvine, who was not involved in the study. Those studies look at changes in precipitation, or weather circulation patterns. But increasingly, he says, studies include the human dimension, such as water demand and water use. “This study includes both top down and bottom up [factors], with a historical viewpoint,” says AghaKouchak.
The research team analyzed the contribution of human water management in California during the years of 1979 – 2014. During the exceptional drought of 2014, water management significantly mitigated or intensified drought conditions, they found. In Southern California, through the operation of reservoirs, water management alleviated the drought deficit by about 50% during low-flow periods. In the Central Valley, however, water consumption—primarily through irrigation—intensified the drought duration by 50% and water deficit by 50–100%.
The model developed by first author Xiaogang He of Princeton’s Terrestrial Hydrology Research Group, and his co-authors, could become a tool for water managers and policy makers in the future. “With this framework you can take the projected climate future and implement it in this [model],” says co-author Niko Wanders, also part of the Terrestrial Hydrology Research Group. “It lets you say, ‘OK, what if we changed the policy in our reservoir?’”
In one scenario he outlines, managers might decide to reduce the water allotment of farmers located downstream by 20%. They’d use this model to estimate overall crop yield and crop loss. Or perhaps the manager prioritizes fishing needs, or energy production, such as in hydroelectric power. “Running through these scenarios,” he says, “you can find what is the sustainable solution, what is feasible, and what will the cost be of such a solution as well.” Cost and savings estimates of different water management scenarios could also, Wanders notes, help planners know when to build additional reservoirs.
To verify the accuracy of their system, the researchers modeled two scenarios. In the first, they didn’t include any human activities, whether irrigation, reservoir operation, or groundwater pumping. But for the second, they included all such human activities for which they could garner data. They then compared the model simulated river discharge with data for observed river discharge for past years from US Geological Survey stations. “And we found that by including those human water management and water use [activities], we can better reproduce observed river discharge,” says He.
Now He hopes to expand the scale of such “bottom-up” drought models beyond California, to examine larger regions. He is also working to fine tune the model, adding the ability to differentiate between the relative contribution of each kind of water management practice.
AghaKouchak, meanwhile, is creating a similar model focusing on Australia. “This is the direction I think the field needs to move in,” he says.