Evaporation stations are used to measure the amount of water that evaporates into the atmosphere over specified periods of time. Also measured are related atmospheric parameters. The purpose of this module is to describe this station and associated equipment as used by the National Weather Service (NWS).
Evaporation station observations measure the following parameters:
The site of the equipment should be fairly level, sodded, and free from obstructions. It should be representative of the principal natural agricultural soils and conditions of the area. Under no circumstances should the pan or instrument shelter be placed on a concrete slab or pedestal, over asphalt, or crushed rock.
Obstructions such as trees or buildings should not be closer than two (preferably four) times the height of the object above the pan. The exposure should be free from obstructions that may cast shadows over the pan during any part of the day.
The layout of the plot eliminates shadows in Northern Hemisphere stations. The plot is enclosed within a fence to not only protect the equipment but also to prevent animals from drinking from the evaporation pan. The height of the fence is 4 or 5 feet.
The following equipment are normally used for making evaporation measurements:
The following equipment may be found at an evaporation station:
The following sections provides descriptions, and installation and maintenance information for the equipment listed above (except for the water storage tank).
An evaporation pan is usually 46 to 48 inches in diameter and 10 inches deep. It is constructed of monel stainless steel or galvanized steel. Monel is preferable. However, the inordinate cost associated with monel over galvanized pans logically dictates galvanized steel. Through the NOAA Logistics Supply Center (NLSC) a monel pan costs in excess of $1400 whereas a galvanized pan can be fabricated for less than $200.
Mounting the Pan: The support for the pan should be of pressure treated wood. The wooden support should be installed perfectly level to assure a level water level in the pan. Once the support is installed, sufficient space should be allowed below the pan to inspect for leaks.
The pan should be routinely inspected for leaks since leaks will render the measurements useless. Report leaks on the observation form along with the date it was discovered and the date of repair or replacement.
The interior of the pans should not be painted. The pan should be cleaned as necessary to keep it free from sediment, scum, oil films and algae. If sediment, scum, oil films and algae are left in the pan, the rate of evaporation will be reduce. During freezing conditions, the pan should be emptied, cleaned, and stored upside down or in a shelter.
Control of Algae: The growth of algae in the pan can be discouraged by the addition of a small amount of Copper Sulfate in the water. Alternative additivess are also being explored.
Filling the Pan (Fixed Point): With the fixed point gage/stilling well installed, fill the pan to where the tip of the fixed point is the water level.
Filling the Pan (Hook Gage): Fill the pan to within 2 inches of the rim. The pan is to be refilled by the observer when the level drops to 3 inches below the rim.
Draining the Pan: Use a length of plastic water hose and siphon the water from the pan.
Do Not Tip the
When draining the pan, do not tip it, irreparable damage can result.
Fixed Point/Stilling Well: The fixed-point gage consists of a pointed rod mounted within a 2 1/2 to 3 1/2 inch diameter tube or stilling well of non-corrosive metal (object on left side of the image). The tube is about 10 inches long. This tube is attached to a metal base of sufficient size and weight so that its position in the pan is not subject to change by the wind. The point in the stilling well will be approximately 7 1/2 inches above the bottom of the pan when in position. Two small openings in the side of the tube near the base permit water to flow into the stilling well. The stilling well also restricts surging action of water at the point of measurement, hence its name.
Fixed Point Measuring Tube: The fixed point measuring tube is a 15 inch clear cylinder (object in the lower center of the image) with a cross sectional area equal to one hundredth of the area of the evaporation pan. It is used to measure the amount of water necessary to replace that lost from evaporation. The tube is graduated at 1 inch intervals to permit an accuracy of 0.01 inch.
Water Storage Tank: A water container holding approximately 30 gallons of water used as a reserve for replenishing the pan.
Place the fixed point gage about 1 foot from the north edge of the pan.
Ensure that the small holes in sides of the stilling well are clean and open. Also, the Measuring Tube should be inspected for legibility and leaks.
Hook Gage/Stilling Well: This gage consists of a hook in the end of a stem that is graduated to tenths of inches over a range of several inches. The stem's threads have a pitch of one-tenth of an inch. The three-legged SPIDER and adjusting nut assembly support the hook and provide for adjustment of the height of the hook when the gage is installed on the top of the stilling well. The adjusting nut is threaded to screw onto the stem of the hook. After assembly, the Spider gage is placed on the top of the stilling well with the three legs of the spider resting on the top rim of the well with the hook centered in it. The threaded stem of the spider gage should be vertical and the adjusting nut must turn easily to adjust the height of the hook within the well. The relative height of the hook in the well is indicated by the scales on the horizontal surface of the spider gage.
The hook and spider gage must be kept clean. After thorough cleaning, oil the threads on the stem about twice a year with a single drop of light-viscosity machine oil. Care must be taken to remove any excess oil that may transfer to the surface of the water. If disassembled for cleaning, care must be taken during reassembly to determine that the threads on the stem and adjusting nut properly match by turning the adjusting nut counterclockwise (as viewed from the top) until the top of the nut coincides with one of the graduations on the stem. If the scales has been properly assembled, the index line on the ring of the spider will coincide with "0" on the circular scale. If not, it will be necessary to unscrew the adjusting nut and reassemble the gage by matching the alternate combination of threads.
The F-104 anemometer is a wind accumulator that measures the number of miles of wind that has passed over the surface of the pan through the readings from an odometer. The five digits appearing in the window of the meter indicate the total wind movement to a tenth of a mile for any total from zero to 10,000 miles. The right hand digit indicates the tenths of a mile.
The wind anemometer is mounted on the wooden support of the evaporation pan in the Northwest corner of the support. The anemometer is usually mounted on a special display pintle with the cups are about 6 to 8 inches above the rim of the pan. In this position, shadows from the cups rarely fall on the pan. The F-104 is held on the pintle by a knurled set screw at the bottom end of the anemometer housing.
Routinely remove the cups and spindle. Clean and lube the upper spindle bearing thoroughly as follows:
Nota Bene: Enter "Anemometer cleaned" on the form corresponding to the date.
The 8 inch Standard Rain Gage consists of an overflow can, measuring tube, collection funnel, measuring stick and support. The top portion of the collector funnel is exactly 8 inches in diameter and funnels rainfall into the measuring tube.
In order to provide rainfall measurements to the hundredth of an inch, the measuring tube has a cross-sectional area that is one-tenth the cross-sectional area of the funnel. Therefore, when 1 inch of rain falls into the funnel, it fills the measuring tube to a depth of 10 inches. Accordingly, the scale of the measuring stick used with the gage is graduated to hundredths of an inch. The measuring tube is 20 inches high and holds exactly 2.00 inches of water.
Any additional rainfall will cause the tube to overflow into the outer can. Since rainfall depths in the overflow can are not increased 10 times, the measuring stick cannot be used in the overflow can. Instead, the accumulated water in the overflow can must be measured in the 20 inch measuring tube using the stick. This overflow amount is then added to the 2 inches which originally caused water to flow into the overflow can.
When the measuring tube and funnel are removed, the overflow can serves as a snow gage for winter. Measurements made directly in the overflow can, without being transferred to the measuring tube, are a direct one inch to one inch. The overflow can also be used to take core samples of accumulated snow to determine water equivalent.
The 8 inch Standard Rain Gage is installed using an aluminum mounting stand which is assembled and held in place by 3 stakes which are driven into the ground.
The 8 inch Standard Rain Gage must be check thoroughly for leaks during each visit. Both the measuring tube and the overflow can must be checked. Minor leaks can be repaired with Silicone Sealant available at most hardware stores. More serious repairs may require solder work or replacement.
The gage must be level. A spirit level across the top of the funnel is the ideal method. Releveling is accomplished by driving in the mounting stakes as necessary.
For additional information on the 8 inch Standard Rain Gage, check our web module on the 8 inch gage.
The sixes thermometer is a U shaped mercury maximum-minimum thermometer mounted on a clear piece of plastic with a metal handle. The thermometer is used to determine the max and min temperature of the evaporation pan water during the observation period. The mercury column moves blue "dumbbells" at each end of the U which provides the max and min temperatures of the water. To reset the sixes thermometer, a magnet is used to move the dumbbells back into contact with the mercury columns.
The sixes thermometer is secured to the plastic mount by clips held in place by screws. Simply remove the clips, install the new sixes thermometer, and replace the clips and screws.
Each visits requires inspection of the sixes thermometer. It is notorious for separations in the mercury column. These separations must be reunited by sharply slinging the thermometer numerous time until all separations are removed. This is a frustrating process at best. Also, deposits from the water often make the temperature graduations difficult to read. The thermometer must be cleaned regularly and graduations reblacked with a black grease pencil. Deposits accumulated on the thermometer tube are very difficult, if not impossible, to remove. If these deposits cannot be removed, the unit should be replaced.
Be very careful with the sixes thermometer. The material inside the glass tubing is considered hazardous material. If the tube breaks, a very expensive clean up process will be needed.
The NWS medium-sized Cotton Region Shelter (CRS) houses the maximum and minimum thermometers. In some locations there is also hygrothermograph in the shelter.
The CRS is mounted on a support with 4 legs that are buried in the ground approximately 15 inches. The depth of burial is such that the thermometers are at a height of approximately 5 to 5 1/2 feet above the ground. The shelter must be painted white to avoid radiational heating. The door must open to the North to eliminate direct sunlight on the thermometers.
Routine maintenance of the CRS includes a visual inspection of louvers, slats and the roof. Rotten or broken sections must be repaired. Also, the thermometer mounting board inside the CRS should be checked to ensure it is not loose. If it is, a 6 penny nail in the corners will tighten it up. The Townsend Support that holds the thermometers must also be cleaned and lubricated to ensure smooth operation. The maximum thermometer should spin freely and lock in place horizontally. Occasionally, the CRS should be painted with a fresh coat of white paint. It is often advisable to add a fungicide to the paint to deter mold and algae growth.
At the time of observation, add or remove water from the pan until the tip of the fixed point coincides with the surface of the water in the stilling well. As the water approaches the tip of the point, pour slowly to prevent overfilling as several seconds are required for the added water to move into the well.
When water must be added, fill the transparent tube to zero mark before pouring water into the pan.
When removing water, care must be taken to determine the amount. For instance, if enough water is removed to reach the 10 marking on the plastic tube, the actual amount removed would be measured from the bottom of the tube and would be 15 minus 10 or 5, which is equivalent to .05 inch.
Record the change in depth to the nearest hundredth of an inch in the appropriate columns of Form B-92. For entry and computation of evaporation, water added to the pan is positive (+), and water removed from the pan is negative (-).
Evaporation may be computed as follows:
At the time of observation, place the hook gage on the stilling well and, if necessary, lower the hook in the well until the point is below the surface of the water. Slowly turn the adjusting nut clockwise until the point just pierces the water surface. Remove the gage from the well and read the gage.
The figures on the stem represent whole inches, and the intermediate graduations represent tenths of an inch. The figures on the circular scale represent hundredths of an inch and the intermediate graduations represent thousandths.
Whenever an observation is taken, the scale on the stem is read to tenths of an inch, as indicated by the first graduation at or above the top of the knurled adjusting nut. The circular scale on the adjusting nut is read to the nearest hundredth of an inch at the index.
Both scale readings are added, and the sum represents the gage reading.
A scale reading of 32 on the stem indicates 3.2 inches; a scale reading between 85 and 86 on the circular scale indicated a value between 0.085 and 0.086 inch, or 0.09 inch as determined to the nearest hundredth. The gage reading in therefore 3.2+0.09, or 3.29 inches.
Enter the readings obtained as above in the appropriate spaces on Form B-92. When no rain had occurred, the actual amount of evaporation is the difference between the preceding gage reading and the current reading. The amount should be entered in the column for the actual amount of evaporation.
When rain has occurred, add the amount of rainfall to the preceding hook-gage reading, and from the sum, subtract the present reading.
|March 10, 8 am||Hook Gage Reading||2.71 inch|
|March 11, 8 am||24 Hour Precipitation||0.53 inch|
|Sum of Above||3.24 inch|
|March 11, 8 am||Hook Gage Reading||3.12 inch|
|March 11, 8 am||24 Hour Evaporation||0.12 inch|
In the table above, the depth of the water in the evaporation pan at 8 am on March 10th was 2.71 inch. During the next 24 hours, 0.53 inch of rain fell. This additional water would increase the depth of the pan water to 3.24 inch, assuming no evaporation. However, the water depth in the pan at 8 am on March 11th was 3.12 inch. Thus, (3.24 - 3.12) inch or 0.12 inch of water evaporated from the pan during the 24 hour period.
Owing to small, unavoidable inaccuracies in measurements, and to condensation of water into the pan, the water lever sometimes may be higher than that of the previous day. In such cases, the value of evaporation should be computed as above and entered in the evaporation column with a minus sign, as -0.01 inch.