oa Agrochemophysica - Die effek van verskillende grondwaterregimes op grondtemperatuur in 'n appelboord*

Volume 13, Issue 4
  • ISSN : 0302-7112



A medium-long term irrigation project with Malus communis cv. Granny Smith was conductedat the Welgevallen Experimental Farm of the University of Stellenbosch. The treatments, which were replicated four times in a randomised block design, comprised the application of irrigation when prechosen total available soil water (TAW ) levels were attained. Four TAW levels, viz. 85%, 65%, 45% and 25%, were implemented. The soils o f the experimental area consisted mainly of the Dundee and Westleigh forms and varied texturally from loamy sands to sandy loams in the 0-300 nun soil layer. Plant available soil water ranged from 42 mm to 45 mm per 300 nun o f soil, and bulk densities from 1 530 to 1 590 kg m-3. For the growing season lasting from October to May the mean rainfall was 353 nun and pan evaporation 1 241 mm. Mean daily maximum and minimum air temperatures were 24,4 and 11,7 �C, respectively. The effects o f the various soil water regimes on soil temperature, measured at three depths, were investigated during the course o f three growing seasons. The results showed that, at a shallow depth, soil temperature (Ts) was primarily a function of ambient air temperature (Ta). The former not only followed the normal diurnal course o f Ta but also the seasonal trends, with the former course superimposed upon the seasonal trend. The introduction of the concept of accumulated soil temperatures to detect treatment effects proved very successful: an 85% TAW level gave the lowest heat sums and, consequently, induced the lowest soil thermal regime. On the other hand, under the influence o f a 25% TAW level, the highest soil thermal regime was established because o f the highest heat sums. Soil thermal regimes for the 65 % and 45 % TAW levels were in between, with the 65 % TAW level the lower o f the two. Soil thermal diffusivity, K, was increased with an increase in TAW level from 25% to 85%, when comparing these TAW levels at the same soil depth. The higher the K value was at a specific depth, the better was the heat conduction of that soil, resulting in an effective spatial distribution of heat and, consequently, an enhanced cooling off, with time, of the soil. This explains why, at a particular depth, a soil at a 85 % T AW level was cooler than one, say at a 25 % TAW level.

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