MEASUREMENT OF EVAPOTRANSPIRATION BY LYSIMETER

CONTENT:-

Introduction Potential evapotranspiration Construction of lysimeter Installation Daily routine Calculations Record of observations References

INTRODUCTION:-

Evapotranspiration can be measured, at a site which also measures rainfall, by using a lysimeter which gives additional information on soil water balance.

The lysimeter, measuring potential evapotranspiration, is an apparatus which can be constructed from readily available do-it-yourself (DIY) materials for less than Rs. 2500

POTENTIAL EVAPOTRANSPIRATION:-

What would be evaporated from a surface if water was never a limiting factor.

PE can be calculated using empirical formulae, from the simple needing only temperature.

PE = R + A – P

R:- RainfallA:- Additional waterP:- Percolated water

CONSTRUCTION OF LYSIMETER:- The tank was a black plastic builders' bucket

(30 cm diameter and 25 cm deep) The receiving vessel was a discarded plastic

25 litre emulsion paint container (30 cm x 42 cm)

Sufficient space must be available in the bottom to allow collection of the percolated water.

Another discarded container (5.5 litre capacity), holding about 80 mm rainfall, forms a large collecting vessel

ASSEMBLY:-

INSTALLATION:-

Decide on the location (choose a level piece of ground) near but not too close to the rain gauge.

Cut and remove the turf and put Tank & receiver

There should be a slight fall between tank and receiver to allow drainage.

Use the cut off top of the bucket as a template to shape the piece of turf to be used for the lysimeter

Pack the tank above the gravel and below the turf with soil

In areas subject to high and intense rainfall the soil level should be proportionally lower.

The receiver is best placed with its top about 5 cm above ground so that surface water cannot enter.

Measurements can begin in a few days.

DAILY ROUTINE:-

Measure the water percolated P in mls at the same time as rainfall.

An amount of water A should be sprinkled on to the lysimeter such that there is some percolation (50 to 100 mls) each day.

Record the condition of the grass surrounding the lysimeter.

CALCULATIONS:-

Rainfall observations are now returned as mm of rain, previously inches were used. The calculations below are based on the 5 in standard British gauge and the modern mm measurement.

To find the dividing factor to convert mls to mm calculate the area of the tank in cm2 and divide by 10. Thus for a 30 cm diameter tank of radius (r) of 15 cmFactor = (r2 x pi) /10 = (15 x 15 x 3.142) / 10 = 706.95/10 = 70.7

To convert standard 5 in raingauge measures (mm) to read percolate from a 30 cm diameter lysimeter (mm) calculate the dividing factor as area (cm2) of lysimeter/ area of raingauge

Factor = 706.95/126.69 = 5.6

Example extract from spreadsheet showing the daily lysimeter readings for part of April 1977 at Llansadwrn, Anglesey

Water added + Rain = Total Percolate PE *(mls) (mm) (mm) (mm) *(mls) (mm) (mm) 300 4.2 4.2 8.4 290 4.1 4.3 290 4.1 10.7 14.8 680 9.6 5.2 0 0.0 1.5 1.5 130 1.8 -0.3 130 1.8 4.3 6.1 370 5.2 0.9 200 2.8 7.8 10.6 408 5.8 4.9 __________________________________________ * For a 30 cm diameter tank volume (mls) is divided by 70.7 to obtain mm

RECORD OF OBSERVATIONS:-

REFERENCES:-

1: PERKINS, D. F. (1996).The lysimeter: A robust alternative apparatus to the Piche evaporimeter. Bulletin, Climatological Observers Link., 317, 42-45.

2: PENMAN, H. L. (1948). Natural evaporation from open water, bare soil and grass. Proc. Roy. Soc. (London) A., 193, 120-145.

3: THORNTHWAITE, C. W. (1948). Geograph. Rev., 38, 56.

4: GREEN, F. H. W. (1960). A technique for measuring potential evaporation and some of its applications. Water & Water Eng., December, 3-8.

5: PERKINS, D. F. (2001). Lysimeter PE. Bulletin, Climatological Observers Link., 371, 41.

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