Snow lysimeter runoff complete

Mature cedar lysimeter site

The snowpack has now completely melted at all the lysimeter sites, so today we could download the data and make an initial analysis. Total runoff during Dec-13 to Apr-4 was 808mm at the cedar site, and 874mm at the larch site. Actual runoff at the larch site would have been about 40mm greater, as a rain-on-snow event on Dec-24 caused the drainage to back-up resulting in some missing data at the larch site. The reason for the significantly greater runoff at the larch site is the relative lack of canopy interception and evaporation loss under the deciduous canopy. Time-lapse photography showed us that the larch site melt-off was completed on Apr-2, two days earlier than for the cedar site on Apr-4.

Daily runoff summary for the cedar and larch lysimeter sites

The summary chart of daily runoff for the lysimeter sites shown above tells the story of snowpack dynamics in a maritime climate, with frequent runoff events throughout the winter season. However, you can see that the snowmelt runoff begins to increase and become more consistent from the middle of February. The largest runoff events are caused by rain-on-snow on Dec-24 and Feb-12. On Dec-24 the precipitation recorded locally was 59.5mm which is almost the same as the runoff measured at the cedar site. During the main snowmelt season the daily runoff only reaches 20-25mm/day. You can see the hourly runoff values in the chart below, showing peak values of about 2-3mm/hour during fine weather melt, and 4-5mm/hour during rain-on-snow.

Diurnal melt pattern in the discharge hydrograph with rain-on-snow events Apr-3 and Apr-6

Snowmelt season is early

Stage = 59.5cm

Today was a warm day with spring-like conditions. Snowmelt season is well under way, and the water level is up in the river. Snow survey at each of the lysimeter sites confirmed that there has been significant melt over the past 3 weeks, despite some fresh snow accumulation during last week. The chart below shows that peak snow accumulation occurred in early February. The snowpack will likely completely melt in the lower elevations over the next 2-3 weeks. At the larch and young cedar sites the snow depth averaged about 60cm, with relative densities of 0.4 and snow water equivalents of 25cm.

Snow survey at the larch site

The chart below shows the stage hydrograph for Takiya River over the 2 months from mid-January to mid-March. We can see that flows have been elevated since late February, due to snowmelt and rain-on-snow events, and we can even see the diurnal snowmelt pattern which occurs during fine weather conditions.

Snow lysimeter runoff

The chart above shows the hourly runoff pattern from the snow lysimeters in the mature cedar and larch locations during mid-January to the end of February. There are many similarities, but also some interesting differences between the forest sites. All of the large peaks are associated with rain-on-snow events and these give similar runoff amounts for both locations.

The largest event starting on 11 February brought strong winds and rainfall of up to 4.5 mm/h, although temperatures remained only just above freezing point in the forest (larch site: 0.6 C, cedar site 0.2 C). With these cool temperatures we might assume the precipitation was snowfall, but the lysimeter runoff response shows a large peak that could only occur with rainfall. The local Murakami weather station nearer the coast shows a warm front moved in and temperatures rose to over 5 degrees Celsius.

In early February there are several small peaks at the cedar site which are not recorded at the larch site. This is most likely due to melting of intercepted snow in the cedar canopy.

The last two weeks of February show elevated levels of runoff (even night-time melting) due to rising temperatures and higher amounts of radiation in the energy balance. In this period we can see the runoff from the larch site generally exceeds that in the cedar site, where there is greater shading of the snowpack.

Snow survey

Mid-winter snow conditions at Takiya River today. Recent snowfalls have left large amounts of snow held in the forest canopy, but today temperatures reached 1-2 degrees Celsius, and so the snow in the canopy was melting and sliding off. The river channel shows no signs of any recent high flows. Mid-winter baseflow continues, and you can see large snow accumulation on the gravel bar to the right in the photo above.

Snow survey revealed that a significant amount of snow had accumulated since our last visit on 14 January. Snow depth at the larch site averaged 117cm, while the neighbouring cedar site had depths averaging only 79cm. This difference is due to interception and melt of snow in the cedar forest canopy, such as on days like today when temperatures are positive after snowfall. The relative snow density (compared to water) was around 0.31, and the snow water equivalents are given in the chart below. This is the depth of water that would result from the complete melting of the snowpack. So far this winter the snowpack size is moderately large, and the larch site has the most snow at 37cm of snow water equivalent.

Early winter hydrograph

Stage = 40.5cm, Ta = 2.0 degrees C

Above average snowpack has accumulated at Takiya River so far this winter. Today conditions were fine, allowing us to download the river water level data and the snow lysimeter data, as well as undertaking snow surveys. The hydrograph below shows the typical pattern for early winter, where increasingly heavy rains during December cause several moderate peaks, followed by a period of flow recession as rain turns to snow and snowpack accumulates in the basin.

The chart above shows the hourly lysimeter runoff for the mature cedar (evergreen) and larch (deciduous) study sites between 12 December and 14 January. During this season, all of the large peaks in runoff are due to rain-on-snow events. If you look at the hydrograph above, you will see that the peaks for lysimeter runoff are linked to peaks in the basin runoff, as for example the rain-on-snow event on December 24-25 (note: horizontal time-axes are not the same range). The small peak in lysimeter runoff at the very end (January 13-14) is due to melting of intercepted snow in the cedar forest canopy, while interception melt is minimal at the larch site.

Larch lysimeter site

Mature cedar lysimeter site

Heavy snow interception

Heavy snows have already fallen this month and today the forest canopy was filled with snow. Branches sagged under the weight of the snow, and some of the cedar trees were completely bent over under the snow load. Heavy snows can cause a lot of damage to cedar trees, especially younger trees. Below is a series of photos to illustrate the interception of snow in the cedar forest.

We also checked the precipitation accumulation in the winter precipitation storage gauges, and completed snow surveys at the locations of the snow lysimeters.

Winter precipitation storage gauge

Snow lysimeter at the larch site

Snow lysimeter at the mature cedar site

Snow lysimeter at the young cedar site

Winter season prep

Winter season arrived early in December this year. Already significant snowfall by mid-month, and today we had to work hard to complete all the winter season preparations - cleaning and setting up the snow lysimeters and charging the winter precipitation storage gauges. We also took one more discharge measurement to check the stage-rating curve before the winter season really sets in.