Under RCP4.5, mean annual precipitation in the GYA is projected to increase 7% by mid century (2041-2060) and 8% by the end of century (2081-2099) relative to the 1986-2005 base period. Under RCP8.5, the projected increases are 9 and 15% for these periods, respectively. [medium confidence, >80% model agreement and SNR >1]
The projected increase in mean annual precipitation is attributed to increases during the December through April cold season, particularly in March and April when the snow-rain transition occurs. [high confidence, >80% model agreement and SNR >1]
By the end of the century (2081-2099), the wettest month shifts from May to April in the Big Horn, Upper Green, and Snake Headwaters watersheds. These shifts occur by mid century (2061-2080) and are amplified under RCP8.5. [medium confidence, 60-80% model agreement]
In the HUC6 watersheds, statistically significant positive trends in mean annual precipitation range from 0.17-0.23 inches/decade (0.43-0.58 cm/decade) under RCP4.5, and 0.35-0.52 inches/decade (0.89-1.3 cm/decade) under RCP8.5. Given the spread in the models, RCP4.5 and RCP8.5 trends are not significantly different over the 21st century. [medium confidence, significance in trends]
In this chapter, we analyze projected changes in mean annual, seasonal, and monthly precipitation in the GYA and the HUC6 watersheds. We summarize the main points of the projections and provide the details of the projections through time and space with interrelated maps, graphs, and checkerboard plots.
Annual and Seasonal Precipitation Over the GYA
The distribution of precipitation over GYA is influenced by the direction from which the moisture arrives, which varies seasonally and topographically (see Chapter 2). As evident in Figure 6-1, that influence is particularly strong during the winter and spring when most precipitation falls as snow at higher elevations. Under RCP4.5, projected mean annual precipitation over GYA increases by 1.4 inches (3.6 cm; 5.4%) over the 2021-2040 period to 2.4 inches (6.1 cm; 9.0%) in 2080-2099. Under RCP8.5, the increases for these periods are 1.6 inches (4.1 cm; 6.0%) and 3.9 inches (9.9 cm; 14.6%). Throughout the 21st century, the largest increase for both RCPs is in spring (MAM) followed by winter (DJF). During summer, the changes range from small increases (0.1 inches [0.3 cm]; 2.2%) to small decreases (-0.2 inches [-0.5 cm]; 2.8%). Fall precipitation increases somewhat until 2060 and decreases thereafter. A 1‑inch (2.5-cm) change in precipitation over the entire GYA amounts to roughly 1,000,000 acre-ft (123,348,000 m3) of water.
There is little change in the projected maximum length of wet spells under either RCP4.5 or RCP8.5 across the GYA (Figure 6-2). There is also little projected change in the maximum length of dry spells. As indicated by the increased upper portion of the shaded bands, after 2050 some models simulate an increase in the number of days for the maximum dry spell length under RCP8.5.
Precipitation Over the HUC6 Watersheds
For the 1986-2005 base period, mean annual precipitation in the GYA ranges from 22 inches (56 cm) in the Upper Green watershed to 31 inches (79 cm) in the Snake Headwaters watershed (Figure 6-3 and Table 6-1). The positive trend between 1950 and 2005 (here and in Chapter 3) continues under both RCPs. The trends are statistically significant at the 95% confidence level in all HUC6 watersheds under both RCPs. Although the amount of annual precipitation varies among the HUC6 watersheds, as indicated by the inset numbers, the trends (in inches/decade) are similar. Under RCP8.5, the projected trends are roughly twice those of RCP4.5. The precipitation trends are more gradual than those of temperature described in Chapter 5 and the annual values of the RCPs are not statistically different, as indicated by overlap of the medians and the range and overlap of the shaded bands.
Relative to the 1986-2005 base period, under RCP4.5 projected mean annual precipitation in the GYA is 7% greater by mid century (2041-2060) and 8% greater at the end of century (2081-2099) (Table 6-1). Under RCP8.5, the projected increases are 9 and 15% for these periods, respectively. The increases are essentially uniformly distributed over the HUC6 watersheds. Again, the absolute changes are relatively small but represent a substantial amount of water when totaled over the area of a HUC or the GYA.
The Seasonal Cycle of Precipitation
Projected mean monthly precipitation across the HUC6 watersheds, like mean annual precipitation across the GYA, shows the influence of topography and varies by season (Figure 6-4). For the 1986-2005 base period, May is the wettest month in the GYA. Over the northern and eastern watersheds (Missouri Headwaters, Upper Yellowstone, and Big Horn), precipitation increases throughout the winter and peaks in May before declining to summer minima (Figure 6-4). The southern and western watersheds (Upper Green, Snake Headwaters, and Upper Snake) receive more-or-less uniform precipitation throughout winter and spring before it declines to summer minima after May. As shown in Figure 6-4, under RCP4.5 an increase in January through April precipitation becomes greater through the century and, by the end of the century (2081-2099) the wettest month shifts from May to April in the Big Horn, Upper Green, and Snake Headwaters watersheds. These shifts occur by mid century (2061-2080) and are amplified under RCP8.5. This projected change in the seasonality of precipitation contributes to altering the timing of future runoff.
[U]nder RCP4.5 an increase in January through April precipitation becomes greater through the century and, by the end of the century (2081-2099) the wettest month shifts from May to April in the Big Horn, Upper Green, and Snake Headwaters watersheds. These shifts occur by mid century (2061-2080) and are amplified under RCP8.5. This projected change in the seasonality of precipitation contributes to altering the timing of future runoff.
Checkerboard plots for the HUC6 watersheds and the GYA (Figure 6-5) further illustrate the nature of the projected 21st-century precipitation changes. As in Figure 5-6, each rectangular grid in Figure 6-5 illustrates the differences (anomalies) between a given period and the base period (e.g., 2021-2040 minus 1986-2005) broken down by monthly and annual means, for the GYA and each HUC6 watershed.
Changes in mean monthly precipitation are more variable both among HUC6 watersheds and between RCPs than is the case with temperature (Figures 5-5, 5-6, 5-7). The four time periods all show increases in cold season (November through April) precipitation. The number of boxes displaying model agreement and SNRs >1 increases through time as the magnitude of future changes become greater. Subtle differences across the HUC6 watersheds for a given month reflect spatial differences in precipitation shown in Figure 6-1.
Changes in mean monthly precipitation are more variable both among HUC6 watersheds and between RCPs than is the case with temperature. … [However,] increases in cold season (November through April) precipitation are clear.
From June through October, precipitation changes are mixed in sign and vary by HUC6 watershed. Slightly more drying is evident in the northern and eastern watersheds. There is less agreement of the projected change among the models than there is during the cold season and no boxes display SNR >1. Lack of significance and model agreement is attributed to the wide range of summer precipitation simulated by the 20 GCMs (Figure A6-2). While projected increases in winter and spring are consistent among models, projections for the warm season (June through October) are a mix of increases, decreases, and no change that vary by climate model and watershed. Decreased precipitation in summer and increased precipitation in fall in some HUC6 watersheds are consistent with observed trends since 1950 (see Chapter 3). Seasonal contrasts in model agreement and model spread suggest that the underlying mechanisms of winter precipitation (e.g., changes in storm tracks and greater capacity for a warming atmosphere to hold moisture) are shared among the models, whereas the primary form of summer precipitation (convection) is more challenging to model and less consistent among models. It also reveals limitations in the ability to statistically downscale convective precipitation.
Summary of Projected Precipitation Changes
Under both RCP4.5 and RCP8.5, there is a high level of model agreement in the projected increase in mean annual precipitation over the GYA and the HUC6 watersheds. The increase is attributed to increases in winter and spring. [85 to 100% model agreement and SNRs >1]
Under both RCP4.5 and RCP8.5, the models project a mix of increases and decreases in summer precipitation with generally less than 60% model agreement. There are no SNRs >1 in the projected changes in summer precipitation.
There is little change in the projected length of wet spells under either RCP4.5 or RCP8.5 across the GYA (Figure 6-2). There is also little projected change in the maximum length of dry spells; however, after 2050 some models simulate an increase in the length of dry spells under RCP8.5.
Statistically significant positive trends in mean annual precipitation are projected for all HUC6 watersheds under both RCP4.5 and RCP8.5, but the trends for the RCPs are not statistically different.
Chapter 6 Appendix
Table and figures supporting Chapter 6
[NPS] National Park Service. [undated]. Water [webpage]. Available online https://www.nps.gov/yell/learn/nature/water.htm#:~:text=Yellowstone%20L…, December%20to%20May%20or%20June. Accessed 30 Apr 2021.