3.1.1. Precipitation

Figure 4 shows the percent differences between historical precipitation and mean (of 10 individual RCP 4.5 projections) projected precipitation in mid-century (Figure 4a,b) and late-century (Figure 4c,d), respectively, on both the annual scale (Figure 4a,c) and during the wet season (Figure 4b,d). It is evident that all regions are expecting increases in precipitation during the wet season, with increases

ranging from 2.8% (1.5%) to 9.8% (10.5%) in mid-century (late-century). This observation implies that future storms in the wet season would likely become more frequent, which is in line with the findings of previous studies [25,28]. On the annual scale, most regions are also projected to receive more precipitation, except for the driest two regions: South Lahontan and Colorado River. This suggests that those two regions are expecting much less precipitation in the dry season, although more precipitation is projected for them during the wet season. Typically, summer monsoons are a major contributor to dry season precipitation in these two regions [73,74]. This finding denotes that future monsoons over both regions are likely to become weaker or more sporadic. Across all regions, the San Francisco Bay and the South Coast generally have the highest and lowest increases in precipitation in late-century, respectively, on both temporal scales. This indicates that they are the most and least prone to changes in future storms during this period, respectively, yet they are not the wettest or driest regions. Comparing two future periods, the late-century period is generally expecting a higher increase in precipitation than the mid-century period except for the dry regions including Colorado River, South Lahontan, and South Coast.

**Figure 4.** Percent differences (%) between historical and mean RCP 4.5 projections on: (**a**) annual precipitation in mid- century; (**b**) wet season precipitation in mid-century; (**c**) annual precipitation in late-century; and (**d**) wet season precipitation in late-century.

The differences between historical precipitation and mean RCP 8.5 precipitation projections are also explored (Table 3). Similar to what Figure 4 indicates, wet season precipitation is expected to increase in both mid-century and late-century across all regions. Increases are expected for annual precipitation for most regions except for three dry regions (i.e., Colorado River, South Lahontan, and South Coast) in mid-century and one region (i.e., Colorado River) in late-century. The increases in late-century are higher. Comparing annual precipitation and wet season precipitation, changes in the latter is more significant in terms of magnitude, which is in line with the RCP 4.5 results as illustrated in Figure 4. Comparing two future periods, changes in the late-century is more pronounced compared to those of the mid-century. Comparing differences of the mean RCP 4.5 projections from the historical baseline and that of the mean RCP 8.5 projections, the latter are more notable. Those are expected since the late-century (compared to mid-century) and the RCP 8.5 scenarios (compared to RCP 4.5 ones) are both expecting higher increases in temperature (Section 3.1.2). A warmer atmosphere can hold more water moisture, indicative of more water available for precipitation.



In addition to looking at the mean of PRC 4.5 and RCP 8.5 projections, individual projections are also investigated (Figure 5) to provide insights on the potential range of precipitation changes. Overall, on both temporal scales, there is no consensus that all projections show increases or decreases consistently for any region in mid-century or in late-century. This finding is also reported in previous studies using old climate projections [22,26–28]. The changes mostly range from −25% to 50%, with a few outliers showing more than 50% increases in precipitation. Those outliners come from a single wet climate model under the higher greenhouse-gas emission scenario (RCP 8.5). The variation range is generally larger for late-century (compared to mid-century) and dry regions (compared to wet regions). Additionally, wet season precipitation shows larger change ranges compared to annual precipitation. These results indicate more uncertainties in the projections for the dry regions, in the wet season, and in late-century.
