SHORT-TERM TEMPERATURE, WIND AND CURRENT EFFECTS IN AND AROUND LAKE HARTBEESPOORT

SUMMARY

Lake Hartbeespoort is subject to an unusual combination of physical conditions, in that it lies in a summer rainfall zone where the weather and climate vary dramatically over time-scales stretching from hours to millennia, the land morphometry is irregular and scum affects radiation absorption in the lake.

A study of relationships between winds, temperatures and currents, carried out largely under drought conditions, indicates that air moves over the Hartbeespoort hill/valley-complex in several meso-scale modes of circulation. Wind over the dam-wall is generated by valley-plain processes and intensified by a constriction in the notthern end of the gorge. In spite of the small area affected, it tends to dominate the winter water temperature profile, when winds over the main basin, generated by slope processes, are much weaker. However, it seems to exert little seasonal influence on the summer water temperature profile over a monthly time-scale, when there is less difference between the wind-strengths.

Nevertheless, wind-impulses in the gorge on many summer nights are strong enough to generate internal seiches, which decay after a single cycle due possibly to absorption both on the rocky bottom and in the irregularly shaped main basin surrounded by reed beds. In response to gorge-winds, a curved path of wind and surface-current was detected in the gorge in winter, leading to a helical water current and a strong inversion, which could be of interest to biologists.

Water flows out of the lake roughly at the level of the seasonal thermocline, which would have had the effect of intensifying it. However, some of the inflow also enters around this level, counteracting the outflow effect. This effect, in addition to erosion of the thermocline on the rocky bottom during seiche-action, may account for the rather weak stability that was found in contrast to those of other lakes in the hemisphere.