AN ELECTRICALLY WEIGHED LYSIMETER FOR MEASURING EVAPORATION RATES
The water status of the leaves of a crop changes continuously, during the day and is determined by the relative difference between transpiration loss and water absorption. Although these two processes are interdependent, absorption is primarily controlled by root and soil factors and transpiration by meteorological factors, hence they often occur at different rates. If, in response to the evaporative demands of the atmosphere, the transpiration loss exceeds the rate of absorption, the leaves lose turgor and, in order to prevent them from drying out to a dangerous level, the stomata close, thereby increasing the resistance to diffusion of water molecules. Stomatal closure is likely to result in a temporary check to growth, as the diffusion of carbon dioxide molecules from the air into the sub-stomatal cavities of the leaves will also be restricted. Under any particular meteorological conditions, the maximum rate of water loss from the leaves of an actively growing crop, freely supplied with water, is termed the potential evaporation rate. Closure of the stomata for any length of time during the day will result in a lower daily water loss termed the actual evaporation rate. The conditions of the soil-plant-weather environment at which actual and potential evaporation rates diverge are therefore of considerable interest to agronomists and horticulturists.
Changes of weight can be used in the measurement of potential evaporation rates over intervals of 24 hours or less, and also of actual evaporation rates under decreasing soil water potentials. This requires a system that can detect small changes in a relatively large weight, in which the tare due to soil alone may account for nearly 75 per cent of the total weight. MORRIS (1959) developed a machine capable of providing a continuous record of small changes in a large weight (2600 kg) of soil with an accuracy of ± 5 g (equivalent to a depth of 0.0025 mm water), whilst McILROY and SUMNER (1961) have described a machine that is capable of automatically recording changes in weight of the order of 1:130000 in a total weight of 6500 kg. At Wageningen, BLOEMEN (1963) utilised a system of hydraulic pressure cells to support a container of soil and water weighing up