Does Less H2O Mean More CO2?




(San Luis Reservoir, 2014)
 
The 2016-2017 winter season was California’s wettest on record. Since Governor Jerry Brown’s lifting of the state’s drought emergency in April, concerns over water scarcity, and more recently, flooding (ie. the Oroville Dam’s risk of collapse) have diminished significantly. Drought conditions have dropped from 95% of the state experiencing some form of drought a year ago to less than 25% presently. However, this has not been the case for long. For the previous five years (2012-2016), California had been in the grip of a severe and widespread drought. The lack of precipitation coupled with dwindling storage volumes put a major constraint on urban and agricultural water supplies. Water districts were forced to mandate conservation targets as the state struggled to preserve what water supplies remained. Effects such as these gained (deservedly) the most attention during the drought, but another byproduct of the drought – which will be discussed here – is the potential for increased CO2 emissions from thermal electric power generation.
 
In a drought, streamflows are typically significantly reduced. Over time, as reservoir levels in California declined, hydroelectric power output dropped.
 

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If hydroelectric generation fell off during the drought, what made up for this loss in generation? To understand the impact of this generation loss, the shares of the various fuel types must be examined, not their magnitudes. This approach will generally account for changes in demand, as well as the ongoing expansion of CAISO’s renewable generation. The change in CAISO’s generation mix between 2015 (a drought year) and 2016 (a wetter year with higher hydro output) is expressed in the charts below:
 

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As the graphs show, hydro and thermal power are inversely related – thermal’s share decreases more or less in line with the increase in hydro (and renewable) generation. Excluding renewables, this indicates it is likely that the generation needed to make up for a loss in CAISO hydro would have, at a minimum, the carbon intensity of natural gas – the least carbon intensive form of thermal power. From this, the carbon intensity of natural gas (1.22 lb CO2/kwh) was used to calculate the potential difference in CO2 emissions for 2015 vs. 2016. This analysis found that given these assumptions, an additional ~6.2 million US tons of CO2 was emitted in 2015 vs. 2016 as a result of lower CAISO hydro generation. This drop in CO2 emissions represents a significant portion of CAISO’s overall drop in CO2 emissions from 2015 to 2016.
 

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This analysis is a good example of another adverse effect of California’s drought. An important note: This “back of the envelope” analysis is intended to provide a glimpse at some of the interesting trends in the data. Various simplifications were made which should not change the overall findings, but this analysis should not be mistaken for a scientific study. For more definitive inferences, a more in-depth analysis is needed.
 
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Author:
 
Kyle Gellerstedt – Velocity Suite Intern