Das Oasenklima und sein ökologischer Stellenwert

Abstract

The term oasis climate serves to describe the climate within areas of vegetation, which differs remarkably not only from that of the surrounding desert but also from that of its settlement because of the particular reciprocal effects within the mostly irrigated tree cultures. In order to illustrate the characteristic qualities of this type of climate, measurements were taken in a grove of mature old palm trees in the oasis of Tozeur in southern Tunisia, the location of which is characterized by a multiple shading of the ground by foliage, and those from a younger one where the trees are widely spaced. These measurements were contrasted with the conditions of the climate outside the oasis. In the oasis gardens the diurnal temperature fluctuations are significantly smaller than in the open; within the tree cultures, however, there are considerable additional differences between the air-layer immediately above the ground and that in the region of the upper trunk. The thermal differences correspond with the various values of the relative humidity of the atmosphere, which in turn depend upon the irrigation and the actual evapotranspiration. Thanks to reduced net radiation and surface heating, together with poorer ventilation, potential evapotranspiration in a well-irrigated palm grove is considerably lower than in the open. A comparison of the mean annual value for open land, young oasis and old oasis shows a relation of 100:61:42. The resulting consequences for the amount of irrigation required are emphasized by the two water balances. All irrigated soils are subject to the danger of salination. This problem varies with the density of vegetation in the oasis gardens. It is discussed on the basis of some soil analyses, and a link is established between the quantity and timing of irrigation, evapo transpiration potential and the heating-up of the atmosphere. In addition, the amount of humus formation and the composition of ground flora are derived from the micro-climatic and hydrological peculiarities of the oasis climate. On the basis of the ecological conditions described above, inferences are drawn for the cultivation of the palm groves, the density of vegetation and the intensity of irrigation.