C

Fig. 4.1 Evaporimeters: (a) UK standard tank or Symons tank; (b) US Class A pan; (c) Russian GGI-3000 tank.

is an attraction to birds and animals and is usually covered with a wire mesh. Such a screen of 25 mm chicken wire mesh, while preventing the bulk removal of water, gave an average reduction of 14 per cent in measurements of monthly mean evaporation over 2 years of measurements in Kenya, owing to reduction in radiation. In using evaporation measurements from a US Class A pan, careful note must be made of its siting and installation. Another instrument that has been accepted by many countries

Table 4.2 Example

calculation of the depth of evaporation

Date

Observations made at 0900 h GMT (mm)

Daily records (mm) (from 0900 h)

Rain

Hook gauge

Evaporation

Rainfall

June 1

0.3

21.1

1.0

-

2

-

20.1

1.6

8.9

3

8.9

27.4

0.7

12.7

4

12.7

39.4

1.8

-

5

-

37.6

2.3

-

6

-

35.3

2.0

-

7

-

33.3

-

-

is the Russian tank (Fig. 4.1c). The Russian GGI-3000 tank has a smaller surface area (0.3 m2, 0.618 m diameter) than the other instruments, but has the depth of the British tank (0.60-0.685 m). It is cylindrical with a conical base and is made of galvanized iron. The tank is installed in the ground with the rim about 75 mm above the surface. A comparison between the GGI-3000 tank and the Class A pan was made at Valday (USSR) over 11 summer seasons and an average ratio (tank/pan) for the seasonal evaporation totals was 0.78. Thus like the Symons tank, the GGI-3000 tank gives a measure of Eo of the correct order of magnitude, but a measure of heat storage effects is required before the reservoir loss can be evaluated. Because of the length of records available from networks of evaporation pans, their use remains valuable for identifying long-term trends in evaporation, as within the recent studies published within Science and the Journal of Geophysical Research (Roderick and Farquhar, 2002; Liu et al., 2004).

4.2.3 Atmometers

These are devices that can give direct measurement of evaporation. A water supply is connected to a porous surface and the amount of evaporation over a designated time period is given by a measure of the change in water stored. Thus Eo = AS. This evaporation mechanism has been likened to transpiration from leaves, but as the biological control is not simulated, atmometer data are considered as measures of Eo. It is essential to have a constant instrument exposure to ensure consistent observations and it has been found satisfactory to have atmometers set in a well-ventilated screen as is used for exposing thermometers to register air temperature. Atmometers are simple, inexpensive and easy to operate, but care must be taken to see that the porous surfaces from which the evaporation takes place are kept clean. Two types are described here.

The Piche evaporimeter consists of a glass tube 14 mm in diameter and 225 mm long with one end closed. A circular disc of 32-mm diameter absorbent blotting paper is held against the open end by a small circular metal disc with a spring collar. The evaporating surface area is 1300 mm2 and this is fed constantly by the water in the tube hung up by its closed end. The tube is graduated to give a direct reading of evaporation (Eo) over a chosen time period, usually a day. The measurement in millimetres is related to the evaporating surface of both sides of the paper. The tube holds an equivalent of 20 mm of evaporation; the water is replenished when necessary. When the Piche evaporimeter is exposed in a standard temperature screen, the annual values have been found to be approximately equivalent to the open water evaporation from a US Class A pan. This type of instrument is used widely within Africa and the Near East.

In the Bellani atmometer, the porous surface is provided by a thin ceramic disc, 85 mm in diameter. This is attached to a graduated burette holding the water supply. As with the Piche evaporimeter, the difference in burette readings over a specified time gives the measure of evaporation. Bellani atmometers remain in use by research scientists (Gavilán and Castillo-Llanque, 2009) and are particularly attractive when fitted with electronic sensors (Giambelluca et al., 1992)

0 0

Post a comment