# Presentation: 132817

## Rain, Snow and Snowmelt    

** ****	**** ****changes in recent decades  in western North America **

**                  ****Dan Cayan***** (1,2)       *****Mike Dettinger***** (2,1)      *****Iris Stewart***** (1)     Noah Knowles (2)***
***                                                                                  ******1  SIO     2  USGS    ***

**Climate of wet days is crucial to Western water.**
**An advance to earlier  spring runoff timing from western snowmelt-dominated watersheds is observed since 1950’s.**
**What caused it?   (****precip timing, earlier melt, more rain, PDO shift)?**
***	***

## Western US Annual Precipitation mountain precipitation is crucial resource

***Topography***

## In the West, many of the wettest days occur during 

**In the West, many of the wettest days occur during **
**WARM storms when temperatures are near rain/snow threshhold at middle (1000-2500m) elevations**

**PPT ****(mm)**

## In the Sierra Nevada, ~20 precip days provide 2/3 of annual precip

**In the Sierra Nevada, ~20 precip days provide 2/3 of annual precip**

***These days play out during a     short season when N Pac     storms are active.***

## Hydrological effect of  

- mm/day

**Sacramento Headwaters**

**San Joaquin Headwaters**

**Snowmelt**

**Hydrological effect of  **
**warmer mid-elevation and **
**cooler high-elevation zones**
**is seen in comparing **
**snowmelt runoff timing**
**btw Sacramento (lower elev)**
**and San Joaquin (higher elev)**

***Noah Knowles***

## If the Sierra

**If the Sierra**
**warms by +2C**
**Yosemite loses**
**about 20% of**
**snow covered**
**area**

*Mike Dettinger*

## With climate warming, California would lose much of  its late spring Snowpack   

- By the end of the century California could lose half of its  late spring snow pack due to climate warming.  This simulation by Noah Knowles is guided by temperature changes from PCM’s Business-as-usual coupled climate simulation.

***Noah Knowles ***

## Earlier spring flows last 2-3 decades

**“****Center Timing” of snowmelt watersheds**
** ****have advanced by 1-5 weeks earlier across West**

***Iris Stewart***

**center time**

## Earlier streamflow 

**Precipitation Timing Trends**

***Earlier streamflow ***
***in recent decades***
***Has occurred despite***
***Precip coming later***

***But in the West,***
***Precip has come later!***

**Streamflow Timing Trends**

## Atmospheric Circulation has driven   warmer spring temperature and ealier runoff 

**Earlier streamflow**

**Later streamflow**

**Atmospheric Circulation has driven   warmer spring temperature and ealier runoff **

**Warming Trend Spring Iemp**
**1950-1997**

## Warm phase of PDO yields earlier snowmelt

## But, streamflow timing correlations are even 

**But, streamflow timing correlations are even **
**stronger with spring temperature anomalies**

**TI is 4mo mean **
**  ****temp anom**
**preceding &**
**during month**
**    ****of CT**

## Partial correlations:

**Partial correlations:**
**streamflow  timing fluctuations are quite strongly affected by spring temperature (TI)**
**Warm TI anomalies **
**Produce earlier CT’s.**
**PDO has evidently had **
**an influence, but not**
**as much as TI  **

## Examining temperature anomalies more carefully:

**Winter and especially**
**Spring temperatures **
**were warmer in the western United States during  1977-1998**
**relative to 1950-1976**
*         **But this map reflects*
*dry days (ppt=0) only—this*
*Suggests that there was*
*Earlier snowmelt*

**Dec-Feb**

**Mar-May **

*Examining temperature anomalies more carefully:*
**                   ****1977-1998 minus 1950-1976 Tmin   ****dry days only**

## 	But, in addition to dry days

**Dry days**

**Wet days**

**Dec-Feb**

**	****But, in addition to dry days**
** ****wet days were  warmer during 1977-1998 **

## Changes in synoptic patterns were involved 

***Mike Dettinger***

**Changes in synoptic patterns were involved **
**	****in the shift toward earlier spring flows:**
**	****	****a different mix of wet days**

##   Peak temp changes        	occurred in

**  ****Peak temp changes        ****	****occurred in**
**     ****March and April**

**Dec-Feb**

**Jan **

**1977-1998 minus 1950-1976 Tmin   **
**	****wet days (>6mm precip)**

**Feb **

**Jan **

**Apr **

**Mar **

**May **

## 			Summary

**Rain *****vs.***** snow is crucial to water issues in the West.  In California’s**
**  ****Sierra Nevada, only 20-30 days deliver most of the year’s water.**
**Timing of spring runoff advanced 1-3 wks **
**in decades after 1977 r.e. decades before**
**  *****Causes:***
**	****Winter and *****esp***** Spring temp **
**              ****Certainly dry days but also wet days were involved—this ****	****	****	****suggests not only early snowmelt but more ****	****	****	****	****immediate rainfall runoff occurred.**
**	****PDO contributed, but other climate anomalies resulting in ****	****warmer winters & spring were evidently stronger influences.**
**	****Since 1998, PDO reversed but spring temp and spring flows ****	****did not reverted  **
**Need more & better monitoring at mid-high elevations**