*3.3. Analysis of Distributions for Individual California Streamflow Stations (Historical and Future)*

Figures 4 and 5 depict the distribution of historical and projected drought deficit quantities for select stations (Yuba and San Juaquin), as compared to Figures 2 and 3 which show drought deficits over time. The median of Warm Dry and Other projection models at RCP 4.5 and 8.5, appear higher than historical. The median of Average and Cool Wet projections at RCP 4.5 and 8.5, appear to vary from being far above historical to slightly below. Outliers tend to be more extreme with Warm Dry and Other projection models than for Average and Cool Wet projection models. The ranges of drought deficit quantities vary across all projection models. For the Yuba River (Figure 4), only the Warm Dry RPC 4.5 and 8.5 scenarios have drought scenarios that are significantly different (i.e., larger deficits) from the historical distribution (as determined from the two tailed *t*-test at *p* = 0.05). For the San Joaquin River (Figure 5), only the Cool Wet RPC 4.5 scenario had a significantly different (i.e., smaller deficit) from the historical distribution.

Tables 3–5 display the largest deficit, longest duration, and highest intensity of each drought under historical conditions and under model scenarios. Out of all the droughts that were historically recorded or projected, this table displays the highest values of the three drought categories. Streamflow deficit (Table 3) increases by as much as three times larger than historical in the worst-case future scenario. For instance, in the American River, the largest drought deficit in the historical record is 30 MAF and the in the future scenario Warm Dry RCP 8.5 the drought deficit is 89 MAF.

Table 4 displays the duration (length) of droughts for each station and scenario. It is noteworthy that future droughts may increase in duration two to three times larger than historical. For instance, in the American River the largest drought duration in the historical record is 4 years and the in the future scenario Warm Dry RCP 8.5 the drought duration is 11 years.

Table 5 displays the intensity (deficit divided by duration) of droughts for each station and scenario. In general, there are fewer differences in drought intensity between the historical record and future period. This could be a result of droughts being longer in duration (see Table 4) and this would lead to less intense droughts in a given year. This is a limitation of the analysis as we are not evaluating the individual yearly deficits as isolated droughts and intensity is defined based on extended drought periods.

**Figure 4.** Box plot of Yuba River's drought deficit quantities historical (1950–2015) under all projected models (2020–2099). Bottom bar and top bars extended from the box represent the lowest and highest usable values. The line in the middle represents the median value in the data. Bottom half of the box represents the lower quartile, and the top half represents the upper quartile. The highlighted boxes are significant (*p* = 0.05) in terms of different from the historical distribution based on the *t*-test. (Note: 1 MAF = 1233 million cubic meters).

**Figure 5.** Box plot of San Joaquin River's drought deficit quantities historical (1950–2015) under all projected models (2020–2099). Bottom bar and top bars extended from the box represent the lowest and highest usable values. The line in the middle represents the median value in the data. Bottom half of the box represents the lower quartile, and the top half represents the upper quartile. The highlighted boxes are significant (*p* = 0.05) in terms of different from the historical distribution based on the *t*-test. (Note: 1 MAF = 1233 million cubic meters).

*Water* **2021**, *13*, 3211

**Table 3.** Largest drought streamflow deficit (MAF) by projection model for each streamflow station. Each of the values represent the largest drought deficit out of all projected droughts under each climate scenario or historical records. Red highlighting indicates droughts with a larger deficit than historical. Blue highlighting indicates droughts with a smaller deficit than historical. Bold values are the largest projected deficit for each station. (Note: 1 MAF = 1233 million cubic meters).


*Water* **2021**, *13*, 3211

**Table 4.** Longest drought duration (years) by projection model for each streamflow station. Each of the values represent the longest drought out of all projected droughts under each climate scenario or historical records. Red highlighting indicates a longer drought than historical. Blue highlighting indicates a shorter drought than historical. Bold values are the longest projected drought for each station. (Note: 1 MAF = 1233 million cubic meters).


*Water* **2021**, *13*, 3211

**Table 5.** Highest drought intensity by projection model for each streamflow station (MAF/year). Each of the values represent the highest drought intensity out of all projected droughts under each climate scenario or historical records. Red highlighting indicates droughts with a higher intensity than historical. Blue highlighting indicates droughts with a lower intensity than historical. Bold values are the highest projected intensity for each station. (Note: 1 MAF = 1233 million cubic meters).


#### **4. Discussion**

Climate change will likely result in greater precipitation and runoff, but also more years of drought [2], as reflected by the climate models used in this study. The results of this drought analysis indicate that under three of the four climate change model scenarios, there are increased streamflow deficits, greater intensity, and longer duration of droughts with both RCP conditions (4.5 and 8.5). Warm Dry and Other simulations are projected to have larger droughts (2–3 times larger) than the historical record. Recent studies propose that longer droughts may become more prevalent in future years [13], and the study presented here suggests this is most likely to occur under the Warm Dry or Other RCP 4.5 or 8.5 scenarios. A limitation in this analysis is the aggregation of the streamflow into a water year value which does not allow for the analysis of changes in seasonality of flows as was show might lead to higher winter flows and lower summer values [4]. This was also shown in monthly drought analyses at Shasta Dam [5]. Regardless, from a water supply and planning perspective, the analyses presented here allows for enhanced planning of drought scenarios. Based on this analysis of the eight projection models, future scenarios may be used for improved water management, including drought impacts on groundwater usage and flood potential.

#### **5. Conclusions**

It is commonly understood that with climate change, climate related events will become more extreme. Californians will need to adapt appropriately if this happens. In times of prolonged drought, the western United States tends to use groundwater reserves to fulfill water needs [14]. If drought deficit, duration, and intensity increase as the Warm Dry and Other models project, groundwater may become a more prevalent water source, resulting in potentially negative consequences. As groundwater is consumed, the level drops and wells must be dug deeper, consequently raising the cost of groundwater access [15]. Socio-economic issues, due to unequal groundwater access and the associated costs needed to dig deeper wells and purify lower quality water frequently found deeper in aquifers [14]. Aside from economic effects, seawater intrusion, wetland devastation, land surface abatement, spring bereavement [15], regional climate feedback-loops, and other unintended consequences [14] may occur. Appropriate investments in infrastructure may be needed to mitigate changes in future water availability. Analyses conducted in this paper intend to help California resource managers understand the implication of the projected climate models on future California river streamflow, allowing policy for preparation of the worst-case scenarios.

**Author Contributions:** Conceptualization, L.L. and T.P.; methodology, L.L. and T.P.; formal analysis, L.L.; writing—original draft preparation, L.L.; writing—review and editing, T.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data for this study was access and available at California Energy Commission, Cal-Adapt website at https://cal-adapt.org/tools/streamflow/ (accessed on 12 May 2021).

**Acknowledgments:** I would like to thank Chapman University for providing the opportunity to author this research as part of the Research and Creative Activity Course (491 in Spring 2021). Piechota acknowledges the sabbatical support from Chapman University.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **List of Acronyms**


#### **References**

