MOISTURE MEASUREMENTS IN VERY THIN SOIL LAYERS TO DETERMINE THE CONDENSATION AND EVAPORATION PROCESSES OF DEW
Apart from the contribution of dew to the moisture budget of the soil and the vegetative zone, the interest in measurements of the amount and duration of dew increased due to its role in the incidence and spread of plant diseases and its importance as a medium for optimum reaction of pesticides.
The formation of dew is essentially a nocturnal occurrence. The rate and duration of this process depends on the humidity, temperature and movement of the surface air layers, the sorption characteristics of the exposed surface, its radiation cooling and its heat supply from warmer soil layers. These features indicate that dew fall and consequently any dew measurement are critical procedures, as the material of the dew collecting surface and its way of placement have an essential influence on the measuring results.
A great variety of dew measuring devices has been developed and used in the past (Noffsinger). Dew duration and amounts are determined by volume, weight, electrical resistance or capacitance, by stretching of membranes or by classification of water droplets. The dew collecting surfaces are made of plastic or metal grids, ceramic, metal or plastic plates, wood or skin, and they are usually exposed some centimeters above the ground for measurement. However, measuring results necessarily deviate more or less from the actual dew formation or evaporation at the soil surface, as the probe materials and their placement alter the physical conditions of the process under consideration.
Therefore, a new dew meter has been developed featuring 3 basic improvements:
- The probe can be placed in the ground flush with the soil surface and in thermal contact with the underlying soil layers.
- The probe consists of local soil.
- The measuring sensitivity and accurracy are increased by applying the beta-absorption principle, which is used for measuring thicknesses of paper and foils, for example.
Fig. 1 shows a cutaway view of the Beta-Absorption Dew Meter. A T1-204 area source is placed in the ground under a 1-mm layer of soil. The radiation intensity is reduced according to the water content of the soil probe. It is measured by a surface barrier