Precipitation measurement:

        An accurate assessment of water and nutrients coming in is essential in developing detailed water and nutrient budgets for each watershed.  Some of those incoming nutrients are from precipitation.  Measuring total precipitation volume and nutrient concentrations allows researchers to calculate total nutrient deposition from precipitation over a small area (the size of a rain gauge canister), which can then be scaled up to the size of the watershed.

        1. Precipitation amount
        2. Precipitation chemistry
        3. Snow
        4.  See the data
 
 
 

1. Precipitation amount


There are two types of rain gauges in use at the Hubbard Brook Experimental Forest.  The more common type is the standard rain gauge which is essentially a can that fills up with water until it is measured and emptied manually.  The other type is a more complicated weighing rain gauge which weighs water as it falls and records that information continuously on a chart.  Both types are described in detail below.  There are 24 standard gauges around the Hubbard Brook valley and 7 weighing gauges.  Each gauge is checked weekly, when the water in the standard gauges is measured and emptied and the chart paper from the weighing gauges is collected and the clocks reset.  Since precipitation in the standard gauges is only measured on a weekly basis, the amounts that fall at those locations are pro-rated for each day according to the ratio of daily weighed rain to the total weekly weighed rain at the nearest weighing gauge.  The tall platform, accessible by ladder, is necessary to raise the gauges above the level of winter snow accumulation.



The standard rain gauge consists of a metal canister open at the top.  Inside the top is a funnel which drains into a narrow cylinder.  The ratio (the funnel cross-sectional area divided by the cylinder cross-sectional area) is 10 to 1 so for every one inch of rain that falls from the sky, 10 is collected in the cylinder.  This allows accurate measurements down to 0.01 inches of rain (10 X 0.01 = 0.1 inches that actually collect in the cylinder).  Measurements are made with a specially calibrated ruler, in English or metric units, that takes into account this ratio.  The cylinder can hold 20" of water, so for rain events greater than 2", the cylinder overflows into the canister.  Once the full cylinder is emptied, this overflow water can be carefully poured into the cylinder and the measurements added together.  To prevent evaporation, a small amount (0.01 inches) of light gauge oil is added to the cylinder weekly.  The oil floats on the water surface, trapping water molecules that might otherwise evaporate.  The 0.01 inches is then subtracted from the weekly precipitation measurement.

        The weighing rain gauge consists of a metal canister that houses a funnel, a pail that sits atop a scale, and an analog recorder (a rotating drum with paper).  Once 0.01 inches of rain falls through the funnel into the pail, the weight of the water on the scale triggers the pen on the recorder to move upwards.  As more rain falls, the pen continues to move upwards creating a time series of rainfall amount on the chart paper of the rotating drum, which is turned by a clock mechanism.  Although precipitation is measured by its weight, it is converted to inches on the chart.  The chart paper is replaced weekly, but the pail is emptied only when it becomes too full, every two months or so.

        Wind screens are placed around all gauges.  These consist of a ring of 32 free-swinging metal "leaves" that prevent the development of strong updrafts around the canister which might change the trajectories of raindrops falling into or close to the gauge.  The leaves also create turbulent flow around the gauge opening facilitating the fall of rain straight down and avoiding drift to one side.

        The rain gauge canisters are the highest point within the clearing and thus become a preferred perch for songbirds.  When birds sit on the canister lip they always face out, and you know what that means!  Bird poop is frequently dumped out as the rainwater is measured, however since it generally floats, it does not add significantly to the measured volume of the water.  It is for this reason, and because of contamination from the metal of the canister, that we use a different collector to get water for chemical analysis.
 
 
 

2. Precipitation chemistry


Precipitation for chemical analysis is collected with a simple funnel apparatus we call a "bulk precipitation collector".  The collector is mounted on a post 2 m above ground level to avoid contamination by dust raised from the ground.  Water falls into the funnel and travels through a tube, filling the first bottle (the sample bottle).  Once the sample bottle is full, a tube carries any overflow to a second bottle that is already filled with water, and then out of a third tube to pour out onto the ground.  The purpose of the water-filled second bottle is to provide humid air in the tube draining the sample bottle, thus minimizing evaporation from the sample bottle. The loop in the tube leading from the funnel to the sample bottle is also designed to trap water and prevent evaporation from the bottle.  Preventing evaporation is critical to maintaining the original ion concentrations of precipitation.

        The bottles are collected weekly when the gauges are checked, and are replaced with clean bottles (and a clean funnel) even when there was no rain.  Cleanliness is of paramount importance in order to detect the low concentrations of ions present in precipitation.

       Weekly precipitation is analyzed for pH, base cations, anions, ammonium, aluminum, chloride and more (see data link below).  After analysis, the bottles of precipitation are saved and archived by the Forest Service.

        It was through decades of rainwater collection and analysis that researchers at Hubbard Brook were able to follow trends in acid deposition in New England.  To access a recent report on acid rain, click here.
 
 
 

3. Snow


During the winter months, November through March, the funnels and cylinders are removed from the rain gauges and a carefully measured volume of anti-freeze put in the canister.  Snow falls directly into the canister and melts in the anti-freeze.  Once a week, when the gauges are checked, the whole apparatus is weighed and put back in place without emptying.  Precipitation for the week is obtained by subtracting the current week's weight from the last week's weight, and converting weight to inches.  When the canister weight exceeds 20 lbs, it is emptied, recharged with anti-freeze, weighed, and put back in place.

        Snow depth and water equivalent of the snowpack are measured with a Mt. Rose snow tube.  Until 1979, snow courses were located within the forest near most of the rain gauge clearings.  Since then, in the interest of saving time, snow courses were reduced to one high elevation and one low elevation course on either side of the Hubbard Brook valley, and one course in the clearing at the Forest Service building.  Each week, ten samples are taken 2 m apart along a transect in each snow course.  The following week a parallel transect is used, 2 m from the last transect.  For each sample, the tube is driven into the snow to obtain a core.  Snow depth is measured and the entire snow core is weighed in the field using a specially calibrated scale to determine water content of the snow pack.

        For snow chemistry, the funnel and bottles of the bulk precipitation collector is replaced with a 21 liter bucket to collect falling snow.  These samples are collected weekly, melted, and analyzed and archived in the same manner as rainwater.
 
 
 

4. See the data

    Check the Datasets for over 40 years of precipitation data at Hubbard Brook.
 
 

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Web page created October 2001
by Ellen Denny and Thomas Siccama