Drought conditions in northwestern Nevada, Nov. 26th, 2014 and outlook

Drought conditions have changed significantly in some parts of Nevada, but not so much in others.  As you can see from the side-by-side comparison of drought severity in Nevada, provided by the U.S. Drought Monitor (see http://bit.ly/1xU7GW2), the crescent-shaped area of Exceptional Drought (depicted as D4 on the map, in a brownish-red) has more than doubled, now covering approximately 12% of Nevada, up from slightly more than 5%, estimated on January 7, 2014.  Remember that the D4 designation indicates likely long-term ecological change due to exceptionally dry conditions.  At the same time, the eastern half of the state has seen a slight improvement from severe to moderate drought conditions.  This has been reflected in comments from ranchers in Nevada's eastern counties, who note that rangelands have had plentiful forage in some areas.  Drought conditions are not uniform in the state and vary significantly for many reasons. 



Weather forecasters remain uncertain about our chances of having a good Sierra snowpack as we transition to winter.  The Mt. Rose SNOTEL site reports an accumulated total of 2.4 inches of liquid precipitation since the beginning of the water year (Oct. 1, 2014).  This includes about 0.7 inches that fell as snow.  In contrast, the accumulations of precipitation, as snow and total , in water year 2012 were 1.7 and 5.2 inches--slightly more than double the observed to date.

Overall, the water year has not had a promising start in the western portion of the Great Basin, with less than 10--30% of normal recorded at our weather stations, depending upon location.  This means that snowpacks in the Sierra Mountains are well short of normal relative to this date in other years. 

 

Dipping into the reserves

The discharge graph for the USGS gaging station just behind the City of Reno's solid waste transfer station (left) illustrates the loss of flow from Lake Tahoe, which occurred shortly after July 28th.  Note that the hydrograph has discharge presented on a log scale--shortly after July 30th flow decreased dramatically in the Truckee, reaching a minimum of about 60 cubic feet per second around mid-day, August 3rd.  Fortunately, several storms (see the graph below from a weather station located at the University of Nevada have increased flow for short periods of time (see precipitation record below). However, without more rain we will likely see a return to the low levels observed before our monsoon-driven storms brought some relief. 

Drought status in northern Nevada, July 30 2014

As the illustration below shows, the size of the regions of Nevada that are included in the four categories of drought severity (moderate severe, exceptional and extreme) have remained about the same and in the same locations for more than year.  However, northern Nevada residents are beginning to see the effects of several dry years, especially if they the right to withdraw water from surface water supplies such as the Truckee River.  As of the end of July, two sources of water for the Truckee will dwindle (Lake Tahoe and Boca Reservoir), which means that flows in the Truckee will also decrease.  This means that downstream users, such as the Truckee Meadows Water Authority, will begin releasing storage from other sources in a stages to meet their needs downstream.  This also has the benefit of maintaining some flow in the Truckee.  The Truckee Meadows Water Authority has a plan for meeting water demand by using different sources upstream, including Donner Lake.  Their plan relies partially on voluntary conservation (http://bit.ly/1oMc1q2).  The plan is designed to meet system demand for the remainder of the summer months--a critical period that corresponds with peak water use associated with irrigation and lawn maintenance. 

One of the consequences of this shortage is that many of the ditches that divert Truckee River water for what were historically agricultural purposes will be dry for the rest of the irrigation season.  This will have a big impact on those who use the water to irrigate pastures and crops. 

Signs of Drought

Signs of Drought:

One of the most obvious signs in lack of rainfall.  The National Drought Mitigation Center (http://droughtmonitor.unl.edu/Home.aspx) reports two indices related to drought--percent of normal precipitation (PNP) and the standard precipitation index (SPI).  PNP (Percent of normal precipitation) is an easy to calculate index, provided you have a definition of "normal" that is commonly accepted.  For example, normal could be the long term average amount of precipitation received at a location during a year, or a shorter time period like a month.  It

Status of the drought, May 15

Severe Drought Conditions Persist

The National Weather Service's Elko office provided a summary of drought conditions in the Great Basin, with an emphasis on Nevada, especially eastern Nevada (http://bit.ly/1oYbsat). The drought outlook for the state remains stable, which is to say that in parts of four counties (Churchill, Lyon, Humboldt and Eureka) exceptional drought conditions persist.  The Nevada Water Supply Outlook for May, 2014 (found through http://1.usa.gov/1hS4kHS) reports that the snowpack in the Upper Humboldt River Basin is well above the median amount observed in past years (159%) and precipitation received as of May 16th was very close to the average (92%)  In contrast, the snowpack in the lower Humboldt River Basin was at 9% of the median amount and precipitation received as of May 16th was approximately 77% for the water year. 


In the Truckee River Basin, the latest forecast from the Natural Resources Conservation Service (May 16, 2014) indicates a less than even chance (30%) that flow for May through July at the Farad gaging station will be at 35% of the 30 year average, an even chance (50%) chance that flow will be at 22%, and a much higher chance (70%) that flow will be at about 9% of what we expect. 

None of this is good news, especially because our typical precipitation patterns are dominated by snow accumulations in the mountains during the winter months and our typical water supply forecasts assume that snowmelt will be gradual.  However, other forecasts for the region (for example, the May, June, July temperature outlook provided by the National Weather Service (http://1.usa.gov/S0TeKB) indicates a good chance (about 67%) that the next three months will be warmer than what we expect based on the median temperature calculated for these months.

At the Farad gage, on May 16, flow (817 cubic feet per second) was approximately 46% of the average flow for this day (1770 cubic feet per second), based on 105 years of record.  This is more than 2.3 times the minimum flow observed on this day (347 cubic feet per second, observed in 1934. 

All of this means that drought, especially in the northwestern portion of the state, will be in the news for the summer.  Drought-related events, especially fires, will be prominent stories as the summer progresses. 

The Floriston Rate

The Floriston Rate:

 

Location of the U.S. Geological Survey Gaging Station at
Farad, CA. Off Interstate route 80

The rates of flow known as "Floriston Rates" in the Truckee River Operating Agreement represent minimum flow rates that have to be met with water flowing from Lake Tahoe, any creeks tributary to the Truckee River and the several upstream reservoirs that impound water.  A gaging station operated by the U.S. Geological Survey near Farad, CA (23 miles west of Reno, NV on interstate route 80) measures flow and provides upstream water managers with benchmark flow rates that change with the seasons.  Florison rates vary between 300 cubic feet of water per second to 500 cubic feet of water per second, dependent in part upon the season.  The rates, based on the federally established operating agreement, specify that average flow in the Truckee be 500 cubic feet per second between 1 March and 30 September.  For the rest of the year (1 October to 28 February) average flows must be at least 400 cubic feet per second.  These flows need to be maintained to meet the needs of downstream water users, including wildlife, farmers, municipalities and others.  If water from natural flows and the system of reservoirs is not sufficient to meet these targets, the Truckee River Operating Agreement allows Lake Tahoe to be drawn down to meet needs if water levels in the lake permit this. 

 

Note that as of 31 March, the flow rates at the Farad gage were less than the target (500 cubic feet per second).  Remember that the target is average flow, which means that sometimes flows can exceed or be less than the target. 

 

If you have Google Earth on your computer try going to the location of the Farad gage to see where it lies in course of the Truckee River.  The latitude and longitude of the station are Latitude  39°25'41", Longitude 120°01'59".  If you zoom in enough and look carefully you can see a cable that runs from the station across the river.  The cable is for a small cart that can be used to take measurements of flow as part of regular efforts to maintain the mathematical relationship used to estimate flow rates from stage height (called the "rating curve").  Given the importance of flow estimates from this gaging station, you can imagine that the U.S. Geological Survey checks the rating curve often and makes adjustments as needed. 

 

 

 

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March 1, 2014 State of Nevada Estimates Show Lack of Snow Accumulation

March 1, 2014 State of Nevada Estimates Show Lack of Snow Accumulation

Snow has accumulated since the beginning of March, 2014, but the storms have not been so significantly large that they have produced the overall amounts in the locations shown on this map.  The Sierra Crest shows accumulations of 20 to more than 90 inches on the ground at higher elevations on March 1, 2014.  As mentioned in earlier posts, this is important because snowpack serves as the sustaining source of water for the major systems originating in the Sierra Crest (including the Truckee, the Carson and the Walker Rivers). We rely on deep snowpack that does not warm and melt too quickly.  The snowpack serves almost as a reservoir without a dam. 

Looking eastward from the Sierra Crest we can see that accumulations across the state are very low, especially in the area that lies beneath the persistent area of exceptional drought shown in last week's drought report.  The April 1 water supply outlook, due out tomorrow, will have the latest information about snowpack accumulations throughout the state.  Given recent storm fronts sweeping to the east, our snowpack accumulations should improve.  However, it is important to remember that snowpack is only part of the story--the snow needs to melt slowly enough so that it is released over months to sustain flows and fill reservoirs. 

Current status of drought conditions.

Current status of drought conditions.

The latest drought report from the National Drought Monitor shows an expansion of the formerly crescent-shaped EXCEPTIONAL drought zone in the center of the state (up 2.87% in area assumed to be affected).  This represents a revision from March 18th based on state and federal data and consultation and consensus interpretation.

It is important to remember that the maps are compiled from a wide range of observations and that the depicted drought zones do not represent completely homogeneous conditions.  However, the general message is that drought conditions in Nevada's central northwestern counties (Churchill, Lyon, Lander and now part of Humboldt) persist and the outlook has not improved over the past week. 

At the same time, the Truckee Meadows Water Authority recently issued a statement that water supplies from Lake Tahoe and the reservoir system will be sufficient to meet peak demands through the summer months (until the end of August).  This is based on the best projections from information and analyses.  As an example, the Federal Water Master for the Tahoe-Truckee system issues weekly reports that indicate the probability of specific flow rates at different points, from Tahoe to Pyramid Lake.  These represent potential wet conditions (indicated by the 30% trace on the graph below) and potential dry conditions (indicated by the 70% trace on the graph below).  This graph (projected mean monthly flow rates at the U.S. Geological Survey stream gaging site at Farad (just upstream of the Nevada/California border) estimates that with all flows combined (that means natural flows in the Truckee and releases from the upstream reservoirs) supplies may be sufficient to meet the Floriston rate (more on what that is in the next post) under the wet (30%) and medium wet (50%) scenarios.  However, if our snowpack and reservoir storages remain low and we have a dry (70%) scenario, this may lead to some difficulties meeting the Floriston rate as the summer progresses.  It is important to remember that the projections on this graph represent current observations of the snow pack and reservoir storage--winter snow accumulations may continue, which means the projections will likely change as conditions change. 

Conditions and drought assessment remain about the same

Conditions and drought assessment remain about the same


The area designated as D4: Exceptional drought and the surrounding areas, designated as D3:  Extreme drought, despite recent storms (see below).  The 10 day weather forecast (until 3/22/14) for Truckee, CA forecasts a 40% chance of snow showers on May 20th (next Thursday), with warm and windy weather between now and then.  Currently (3/12/14) Lake Tahoe's water level is at 6224.2 ft above mean sea level (http://bit.ly/1iD2gcw).  This is between the levels observed in 2010 and 2011on this date. 

Soil Moisture Content Improving in Sierras

Soil Moisture Content Improving in Sierras

The last several storms in the Sierras have increased the moisture content of soils on the Sierra Crest.  The graph below (from the Big Meadow SNOTel site, north of the Mt. Rose Ski resort) shows soil moisture trends for the past 30 days at 2, 8 and 20 inches below the soil surface.  A storm several days before the increase in soil moisture on Feb 14th seems to have left enough snow to change soil moisture content.  This is good--remember the agricultural definition of drought?  It is related to soil moisture availability to support plant growth.  Also, the graph appears to show that moisture is sinking into the soil, changing the soil moisture availability at 20  inches below the soil surface as well as at 2 inches below the soil surface.  Note the lag between the rise on Feb 14th at 2 inches (green line) and the rise at 20 inches (red line).  It looks like it takes about a day for the effects seen at 2 inches to reach 20 inches. 




 

Snow water equivalent measurement

Snow water equivalent measurement:






Photo of Dr. James Church--l
ikely in the vicinity of Mt. Rose

Our information about the amount of water in a snowpack comes from at least two sources--the Natural Resources Conservation Service's Snow Telemetry (SNOTel) system and snow surveys (see http://www.wcc.nrcs.usda.gov/snotel/SNOTEL_brochure.pdf).  Dr. James Church, a professor of classic languages at the University of Nevada, developed techniques for measuring the water content of snowpacks based on the mass of a core extracted using a tube.  The technique is based on the mass of a cubic centimeter of frozen water, which is slightly less than the mass of a cubic centimeter of liquid water. 

Church developed the Mt. Rose sampler--a coring tool with a scale that translates mass into inches of water.  See a photo below of Church at work (courtesy of the Natural Resources Conservation Service's Snow Measurement web site (http://www.wcc.nrcs.usda.gov/snowcourse/sc-hist.html).

The Mt. Rose sampler is in common use for estimating the water content of the snowpack in snow courses.  Snow courses consist of many cores taken in a line for some distance.  Nevada's Mt. Rose Ski Resort has one of the original snow courses from early surveying work and a SNOTel site.  For recent data from the Mt. Rose Ski Resort snow course see http://1.usa.gov/1gJ0RM4.

Precipitation and drought effects are not evenly distributed in Nevada

Precipitation and drought effects are not evenly distributed in Nevada

Precipitation and drought effects are not evenly distributed in Nevada.  The graph below (from the Natural Resources Conservation Service's SNOTeL sites in northeastern Nevada, for the upper Humboldt River Basin) shows that as we reach the end of February, the amount of water estimated to be in the snowpack is approximately two inches less than what we observed last year at this time.  A few definitions:  first, the water year begins on October 1st and ends on September 30th of the following year.  This corresponds with theoretical completion of a hydrologic cycle that spans a single year period, after evaporation and plants have depleted the maximum expected amount of water from the soil and deeper in the soil profile.  The water year designation always corresponds with the calendar year in which it ends (for example, we are in water year (WY) 2014, which began on October 1, 2013 and will end on September 30, 2014).   Another aspect of this that is important is the concept of snow water equivalent.  Snow is not 100% water--it contains air pockets and the crystals themselves don't allow for even packing. This means that 12" of snow is not the equivalent of 12" of water.  If we had a column of snow that was, for example, 12 inches deep and we melted it, the amount of water released would not rise to 12" above the measuring surface.  As you can see from the graph below, the Upper Humboldt River Basin appears to have more water in the snowpack this year than we saw in WY 2012 at this time and for the remainder of that year.   

Snow pack water equivalents for water year 2011 through current year. 

Currently nine counties in Nevada have been declared to be in a state of drought emergency (see previous posts and http://1.usa.gov/1fs5Dkx.