This section presents relevant results on runoff projections. Changes in monthly peak runoff volume and timing are reported first, followed by changes in seasonal and annual runoff. Next, potential trends in runoff projections are investigated and compared with their counterparts in the historical period.
3.1. Changes in Monthly Runoff
Changes in monthly runoff are examined in terms of monthly pattern, peak volume, and peak timing.
Figure 3 shows mean monthly runoff during the historical baseline period and two projection periods (mid-century and late-century), along with projected range (represented by 10 projections under RCP 4.5 or RCP 8.5, respectively) of future monthly runoff. Sacramento Four Rivers (SAC4;
Figure 3a,b) and San Joaquin Four Rivers (SJQ4;
Figure 3c,d) are used as examples in this case. Corresponding results for individual watersheds are provided in
Appendix C (
Figure A3 and
Figure A4).
Compared to the baseline conditions, mean runoff is expected to increase from January to April for both SCA4 and SJQ4 under both emission scenarios (RCP 4.5 and RCP 8.5). Higher increases are expected during the late-century than in the mid-century. In contrast, less runoff is projected for July–October in both future periods under both emission scenarios for SAC4 and SJQ4. The projected decreases in runoff are more significant in the late-century than its counterpart in the mid-century. There are significant variations between models in projected changes in runoff during the wet months (January–April) for both SAC4 and SJQ4 as well as during the major snowmelt months (May–June) for SJQ4. In comparison, there is more consensus among projections during the dry months (July–October) for both SAC4 and SJQ4 under both emission scenarios. This general pattern (wet months getting wetter and dry months getting drier) is also evident in eight individual watersheds (
Figure A3 and
Figure A4 in
Appendix C).
With respect to peak volume, on average, larger peaks are expected for SAC4 under both emission scenarios (
Figure 3a,b). This is particularly the case in late-century for which a 52% increase under RCP 4.5 and a 69% increase in peak runoff under RCP 8.5 are projected. This pattern is also evident for individual Sacramento Valley watersheds (
Figure A3 in
Appendix C). For SJQ4 (
Figure 3c,d), changes in peak volume are much smaller compared to the changes of SAC4. Under RCP 4.5, a 6% increase in peak runoff is projected on average during the mid-century; for late-century, a 5% increase is projected. Under RCP 8.5, a similar increase is expected in mid-century. However, for late-century, peak runoff is projected to decrease by about 14%. This indicates that snowpack would most likely become smaller in late-century under the higher emission scenario (RCP 8.5) due to significant warming expected in this period since snowmelt is a major contributor to the runoff of these watersheds (
Table 1). A similar change pattern is observed in individual San Joaquin Valley watersheds (
Figure A4 in
Appendix C).
In terms of peak timing, no changes are projected for SAC4 during mid-century; however, the peak is projected to shift one month earlier from March to February (
Figure 3a,b) by late-century. For individual Sacramento Valley watersheds (
Figure A3), on average, runoff is projected to peak earlier except for Sacramento River above Bend Bridge (SBB) during mid-century. For SJQ4, the peak timing is projected to remain unchanged in May in both future periods under both future emission scenarios (
Figure 3c,d). This is also the case for individual San Joaquin Valley watersheds (
Figure A4) with one exception that Stanislaus River is projected to have an earlier peak during late-century under RCP 8.5. While there are significant variations between projections of individual models, most projections indicate an earlier peak timing for SCA4 and an unchanged peak timing for SJQ4 with few exceptions. The monthly temporal scale in
Figure 3 is appropriate for water supply planning. Analyzing changes in peak timing at a finer time scale (e.g., daily) would be more suitable for other purposes including flood management and emergency responses.
Collectively, the projected increases in runoff during the wet season and decreases in runoff during dry season pose challenges to the current water storage management practice and threatens water supply reliability. Additional off-site storages may be required to accommodate projected increases in runoff during the flood season (i.e., before 1 April). The stored water can either be used to recharge depleted groundwater basins or released during the water supply season to meet water demands. The earlier peak runoff projected for the Sacramento Valley watersheds in late-century indicates that management changes might be necessary. For instance, modifications to reservoir operating rules for the flood season could allow greater storage volumes be maintained in order to accommodate the earlier peak runoff. However, such modifications must be made in connection with flood management considerations.
Looking at changes in peak volume for all study locations across all 20 projections (
Figure 4), Sacramento Valley watersheds are generally projected to experience larger peaks, particularly for Sacramento River above Bend Bridge (SBB), Feather River (FTO), and Sacramento Four Rivers (SAC4). A few extreme values show over a 150% increase in peak volume during late-century. The extreme values originate from the climate model which projects the most significant increases in future precipitation. For San Joaquin Valley watersheds, median change is generally near or slightly above zero during mid-century, while by late-century there is greater variation among models and more cases of projected decreases in peak volumes. Extreme values for SJQ4 watersheds are all below an increase of 100%. Overall, the range of projected change in peak volume in late-century is larger than mid-century for all study locations, indicative of less agreement among climate projections further into the future because of increasing uncertainty. In addition, the projected change ranges for Sacramento Valley watersheds by late-century are much wider than those of the San Joaquin Valley watersheds, indicative of more uncertainty in the projections of peak runoff in these watersheds. This finding suggests that more adaptive capacity (e.g., additional storage facility) is needed in the Sacramento Valley to accommodate this uncertainty.
In addition to looking at changes in peak volume of 20 individual projections altogether (
Figure 4), the changes are further examined under two different emission scenarios separately. Specifically, the mean of 10 RCP 4.5 projections and mean of 10 RCP 8.5 projections in both future periods are compared with the baseline conditions for all study locations (
Figure 5a). For Sacramento Valley watersheds, increases in peak runoff volume are projected in both future periods under both emission scenarios. Increases over the Feather River (FTO) are the most significant. Increases in late-century are higher than their counterparts in mid-century. Comparing two emission scenarios, RCP 8.5 tends to expect higher increases than RCP 4.5. As noted in the Introduction section, atmospheric river (AR) storms contribute largely to the total wet season precipitation in California and serve as a dominant cause of flooding in the state [
3]. Particularly, the climatologic peak of AR landfall is at latitudes that impact the Feather River, Yuba River, and American River watersheds. Projections show that the frequency of landfalling ARs in California may increase by about 30% by the end of the 21st century [
54]. Increases in the length of AR season, storm temperature, and peak AR intensity along the California coast are also projected [
54]. These increases projected in the frequency, duration, and strength of ARs likely lead to increases in peak runoff of Sacramento Valley watersheds. In comparison, changes in projected peak runoff of San Joaquin Valley watersheds are relatively smaller (
Figure 5a). The change magnitudes are generally less than 10%. In addition, by late-century under RCP 8.5, peak volumes in the San Joaquin Valley watersheds are projected to decrease. One likely cause of this projected decrease is that peak snowpack under the RCP 8.5 emission scenario is expected to be smaller than its historical counterpart. Those findings are generally in line with what
Figure 4 illustrates.
Changes in peak timing on a monthly basis are investigated in a similar way (
Figure 5b) as applied to changes in peak volume (
Figure 5a). The peak timing of the mean RCP 4.5 and RCP 8.5 projections are compared to their counterparts in the historical baseline period. A negative difference indicates earlier peak while a positive difference means later peak. No difference means no changes in peak timing. Projected changes for San Joaquin Valley watersheds are limited: Under RCP 4.5, no changes in peak timing are projected during both future periods, and under RCP 8.5, only two watersheds (Stanislaus and Merced) tend to have earlier peaks on average. In contrast, changes in Sacramento Valley watersheds are significant. Projected warming likely increases the rain-to-snow ratio during the winter over these watersheds, leading to generally earlier runoff peak. For Feather (FTO), Yuba (YRS), and American River (AMF) watersheds, earlier peaks are projected, particularly for late-century under RCP 8.5 scenario. For Sacramento River above Bend Bridge, however, later peaks (one month later) are projected during mid-century while no changes are projected during late-century. Looking at Sacramento four rivers together (SAC4), no changes are projected in mid-century while earlier peaks are projected during late-century. Potential causes of these changes are discussed in the Discussion section (
Section 4).
Figure 5b sheds light on changes in peak timing by comparing the mean of individual projections to the baseline. The changes associated with individual projections are also examined to give a more complete picture of the number of projections showing changes in peak timing, cataloged by change magnitude (in months), analysis period, and emission scenario (
Table 2). For San Joaquin Valley watersheds, no RCP 4.5 or RCP 8.5 projections show earlier peaks with one exception (one RCP 8.5 projection for SNS) during mid-century. Most projections show no changes in peak timing during this period except for the San Joaquin River (SJF) where about half of projections are projected to peak one month later. During late-century, all but one (for SJF under RCP 4.5) projections show no changes. Under RCP 4.5, a majority of projections indicate no changes in peak timing from the baseline. However, under RCP 8.5 during the same period, a number of projections show earlier peaks particularly for the Stanislaus River (SNS) where the peaks of all projections are expected to occur one-to-four months earlier. Looking at four rivers together (SJQ4), most projections show peak timing to be in the same month as the historical baseline.
Nearly all projections show changes in peak timing for Sacramento Valley watersheds (
Table 2). During mid-century, all RCP 8.5 projections show earlier peaks in Feather, Yuba, and American River watersheds, which is also the case for most (9 out of 10) RCP 4.5 projections. During late-century, all RCP 4.5 and RCP 8.5 projections are projected to peak even earlier in these three watersheds. In comparison, for Sacramento River above Bend Bridge, only one RCP 8.5 projection tends to peak one month earlier during both future periods. Most projections (7 RCP 4.5 and 6 RCP 8.5 projections) show a change in peak timing to one month later for this watershed during mid-century and show no changes in peak timing during late-century. For Sacramento Four Rivers (SAC4), no changes are expected under most projections during mid-century; however, during late-century, more than half of the runoff projections are expected to peak earlier. Overall, those observations are largely in line with what
Figure 5b illustrates.
In short, notable increases in peak runoff volume are projected for Sacramento Valley watersheds. Increases in late-century under RCP 8.5 are higher than their counterparts in mid-century under RCP 4.5. Increases are also projected for San Joaquin Valley watersheds. However, the amount of increase in San Joaquin Valley watersheds is generally within the range of model biases (
Table A2 in
Appendix B). For Sacramento Valley watersheds, except for the Sacramento River above Bend Bridge, runoff is projected to peak earlier particularly during late-century under RCP 8.5. In comparison, no changes in peak timing are projected for San Joaquin Valley watersheds under most projections.
3.2. Changes in Seasonal and Annual Runoff
This section examines changes in projected April–July runoff and annual runoff from the baseline condition.
Figure 6 illustrates the full range of the differences between 20 individual RCP 4.5 and RCP 8.5 projections and the baseline for each study location in two future periods. For April–July runoff (
Figure 6a), a majority of projections during late-century show decreases. This is particularly true for Feather (FTO), Yuba (YRS), American (AMF), and Sacramento four rivers (SAC4) where all 20 projections consistently exhibit decreases in April–July runoff. The largest decrease (around −65%) is projected for the Yuba watershed (YRS). For mid-century, most projections exhibit increases in April–July runoff over Sacramento River above Bend Bridge (SBB), Stanislaus (SNS), and Merced (MRC); it is the opposite for other study locations. The largest increase (slightly above 50%) is projected for the Merced (MRC) watershed. Overall, the variation ranges are generally large for all study locations in both future periods. The ranges in late-century are even larger than their counterparts during mid-century, which is expected since uncertainty in climate projections tends to become larger further into the future. This is also evident for annual runoff (
Figure 6b). Comparing Sacramento Valley watersheds to San Joaquin Valley watersheds, variations in the latter are generally higher. This suggests that climate models tend to agree less with each other on potential future climate over San Joaquin Valley watersheds which are more dominated by snowmelt (versus Sacramento Valley watersheds). The largest increase (over 100%) in annual runoff is projected for Merced (MRC). On average, except for Tuolumne (TLG) and San Joaquin (SJF), other locations are expected to experience more runoff on the annual scale. These findings are further discussed in the Discussion section (
Section 4).
In addition to looking at the potential range of future changes in April–July runoff and annual runoff, the number of projections showing positive changes is summarized to illustrate the level of consensus among different climate projections on the change signal (
Table 3). For Feather (FTO), Yuba (YRS), American (AMF), and Sacramento Four Rivers (SAC4), the increasing signal is generally weak in April–July runoff particularly during late-century when no projections show increases. For Sacramento River above Bend Bridge (SBB) and Merced (MRC), however, a majority of projections show increases in April–July runoff during mid-century. In general, fewer projections indicate increases in late-century (versus mid-century) under RCP 8.5 (versus RCP 4.5). Particularly, no projections show increases in April–July runoff over any Sacramento Valley watersheds during late-century under RCP 8.5. For annual runoff, most projections exhibit increases in Sacramento Valley watersheds as well as Stanislaus (SNS) and Merced (MRC), while the consensus is lower for Tuolumne (TLG) and San Joaquin (SJF). This is generally consistent with what
Figure 6 illustrates.
In summary, large variations in future April–July runoff and annual runoff are projected. Variations are even larger during late-century compared to their counterparts in mid-century. However, most projections indicate decreases in April–July runoff for Feather River (FTO), Yuba River (YRS), American River (AMF), and San Joaquin River (SJQ); most projections show slight increases in annual runoff for all watersheds except for Tuolumne River (TLG) and San Joaquin River (SJQ). The change rates (% change from the baseline) are generally higher during late-century (versus mid-century) under RCP 8.5 (versus RCP 4.5).
3.3. Trend Analysis
Aside from relative changes from the baseline, the overall change pattern (increasing, decreasing, or no change) is also investigated during the entire projection period 2020–2099.
Figure 7 illustrates all projections on April–July runoff and annual runoff, mean of these projections, and the trend lines of the mean values for Sacramento Four Rivers (SAC4;
Figure 7a,b) and San Joaquin Four Rivers (SJQ4;
Figure 7c,d). Large year-to-year variations are evident under both emission scenarios. Large variations also exist across different projections. It is also evident that April–July runoff accounts for a larger portion of the annual runoff for San Joaquin Four Rivers other than Sacramento Four Rivers. For the mean projection on annual runoff, the changing trend is not pronounced for San Joaquin Four Rivers. For Sacramento Four Rivers, an increasing trend can be observed. For the mean projection on April–July runoff, a decreasing trend is notable for both study locations under both emission scenarios particularly under RCP 8.5.
The specific changing rate (slope of the trend line) is determined for SJQ4 and SAC4 as shown in
Figure 7 along with individual watersheds in Sacramento Valley and San Joaquin Valley (
Table 4). April–July runoff tends to decrease under both emission scenarios for all study locations. These decreasing trends are all statistically significant at a significance level of 0.05 except for one case (Merced River,
p = 0.06). The decreasing rates are generally higher under RCP 8.5 than their RCP 4.5 counterparts. Comparing to San Joaquin Valley watersheds, Sacramento Valley watersheds generally have higher decreasing rates. For annual runoff, a slight decreasing trend is detected for all San Joaquin Valley watersheds. In comparison, all Sacramento Valley watersheds except for American (AMF) show increasing trend. However, none of these trends in annual runoff is statistically significant (
p > 0.05). The trend in the April–July runoff over annual runoff ratio is also examined. The ratio tends to decrease significantly in the projection period under both emission scenarios across all study watersheds, suggesting less snowmelt contribution to annual runoff further into the future. The decreasing rates under RCP 8.5 are higher than the decreasing rates under RCP 4.5.
In addition to trends in mean projections (
Table 4), trends in individual projections and observed runoff are also assessed. For annual runoff, no statistically significant trends are identified in observed runoff for those study locations except for Sacramento River above Bend Bridge (SBB; slope: 31.8 million cubic meters (MCM)/year;
p-value: 0.05) during a history period (1920–1999) with the same length as the projection period (2020–2099). No RCP 4.5 projections show any trends on annual runoff over the projection period. However, one RCP 8.5 projection (GFDL-CM3) indicates significant increasing trends at all study locations. Another RCP 8.5 projection (ACCESS1-0) shows a decreasing trend for all study locations except for San Joaquin River watershed (SJF). A third RCP 8.5 projection (CESM1-BGC) exhibits slight increasing trends at Merced (MRC) and San Joaquin River watershed (SJQ). Other RCP 8.5 projections on annual runoff have no significant trends.
For April–July runoff (
Figure 8a), there are slightly increasing or decreasing trends in the historical (1920–1999) values at all study locations. However, none of them is statistically significant. More than half of the projections (12, 14, 14, 13, and 12 out of 20 for SBB, FTO, YRS, AMF, and SAC4, respectively) for Sacramento Valley watersheds show significant decreasing trends. In comparison, only about one third to half of the projections (11, 7, 7, 6, and 7 for SNS, TLG, MRC, SJF, and SJQ4, respectively) have significant decreasing trends for San Joaquin Valley watersheds. This implies that the impacts of warming on snowpack (and thus April–July runoff) are expected to be more pronounced in Sacramento Valley watersheds rather than in San Joaquin Valley watersheds. The latter are located in higher elevations with higher resilience to warming than the lower-elevation Sacramento Valley watersheds. Comparing decreasing rates in historical and projection periods, the latter are generally higher. For April–July runoff over annual runoff ratio (
Figure 8b), it has a significant decreasing tendency during the historical period for all watersheds. During the projection period, most projections (ranging from 14 for SBB to 19 for YRS) show significant decreasing trends. The decreasing rates in the projection period are generally larger compared to their historical counterparts.
In a nutshell, statistically significant trends are not evident in projected annual runoff for most watersheds. On average, however, April–July runoff is projected to decrease particularly under RCP 8.5. April–July runoff over annual runoff ratio is also projected to decrease. The decreasing rates are expected to be higher than their corresponding historical decreasing rates, indicating less contribution of snowmelt to total annual runoff further into the future.