Rainfall Measurement
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Rainfall at Barcombe, East Sussex in
June 2008 |
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| In July 2006 SOCS became an official rainfall recorder for the Environment Agency. |
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| Flooded fields at Barcombe in 2000 |
What is Rain?
Rain is a form of precipitation, other forms of which include snow, sleet, hail, and dew. Rain forms when separate drops of water fall to the Earth's surface from clouds. Rain plays a major role in the hydrologic cycle in which moisture from the oceans evaporates, condenses into clouds, precipitates back to earth, and eventually returns to the ocean via streams and rivers to repeat the cycle again.
Why do we measure Rainfall?
Since 1995 one of our S.O.C.S. team has been measuring rainfall at his home in Barcombe, East Sussex . When analysed, the data provides some interesting comparisons over past years and gives an indication that maybe climate change is contributing to the water shortages we are now experiencing . The data also gives a local understanding of how low rainfall is affecting the flow of the river and streams we care about. All forms of precipitation are measured.
Rainfall Analysis - June 2008
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| Total rainfall recorded for June in each of the past 10 years plotted against the long term average (since 1995) |
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Total rainfall measured each year for the last 10 years
(Updated - January 2008) |
The rainfall in June totalled 30.2mm which was less than the long term average and less then half that recorded in June last year. Locally it has been a fairly dry month with some heavy periods of rainfall making up a bulk of the total recorded.
Local observations of the rivers and streams around the Barcombe area, seem to indicate that after the spates have subsided, the flows return to the low levels seen during the summer months. Particularly the Bevern Stream, whose headwaters are heavily abstracted.
Whereabouts do we measure the Rainfall?
The measuring site is located at Scobells Farm, Barcombe in the heart of the Sussex Ouse catchment (TQ42731659) . Although representative of the river catchment the site is naturally influenced by the local topography and buildings which may make results vary from other similar locations nearby.
Why rainfall is essential to the health of our rivers & streams
Without rainfall there wouldn't be any rivers and the amount of rain has a direct effect on the health of the river. Rain that falls eventually soaks through the ground and collects in underground aquifers from which our rivers and streams are fed. Regular rainfall keeps the aquifers topped up thus providing water at the source of these watercourses. Runoff from the surrounding land also adds to the flow.
Periods of low rainfall combined with abstraction by water companies cause aquifer levels to fall. This in turn decreases the flow rate of streams, having a detrimental impact on these sensitive habitats.
Low flow has the following effects:
- Reduces the amount of dissolved oxygen in the water that is essential for aquatic life.
- Allows sediment to settle in gravel substrates where salmonids spawn. The resultant oxygen depletion can destroy the developing ova.
- Reduces the dilution of discharges from sewage treatment works, which include ammonia and phosphates. Ammonia in high concentrations is harmful to life. High levels of phosphate promote rapid growth in aquatic vegetation especially algae .
- Promotes the growth of undesirable aquatic vegetation like algae. These smother macrophytes and compromise their growth. Massive algal proliferation, (blooms), associated with low flows and high nutrient levels ultimately deplete the dissolved oxygen when they decompose.
- Some macropyhtes such as Ranunculus spp. (water crowfoot) depend on vigorous flows to thrive. If the flow is not robust they become choked with algal deposits.
Periods of high flow or spate conditions are beneficial and allow migratory fish like sea trout to ascend river structures and reach the upper tributary streams where they spawn.
How we measure and record rainfall
As from July 2006 the simple calibrated rain gauge used for many years has been replaced by a Met Office specification one. This new rain gauge was sited and installed by the Environment Agency, for whom we now record officially.
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| Assembled rain gauge |
The gauge is sunk into the ground so that the top of the funnel is 30 cm above the ground and away from the shadow of surrounding trees and buildings.
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| Met Office spec rain gauge - exploded view |
The exploded picture on the right shows the seperate components, comprising the guage.
It shows a copper bucket, which, during assembly is put in first. This is to catch any rain if the bottle should overflow. The bottle follows the bucket and the funnel slides onto the base.
The Method
- Rain is measured at 0900 hr every morning to record the previous days rainfall.
- The water in the bottle is emptied into the measuring cylinder, which is calibrated to read the rainfall in millimetres. The diameter of the funnel is 12.5 cm, which is larger than the old gauge that I had been using. It is now possible to measure to 0.1 mm whereas the old one measured to 1 mm although you could estimate to 0.5 mm if the amount collected was small.
- The sum of all the daily measurements are then entered on a spreadsheet which is used to calculate averages and produce the rainfall graphs seen on our website.
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