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The study is analyzing 20-year results of the effect of irrigation and irrigated crop rotation on the sugar beet quality and yield. The Dobrovicka A variety (polished seed) was cultivated on the plots (over the years 1967 to 1987) of the Experimental Irrigation Base of the Department of General Plant Production and Agrometeorology of the Faculty of Agronomy of University of Agriculture in Prague.

The trials were conducted on humous, sorptively saturated loam Luvisol soils, in warm and arid climatic zone, in the region of large-scale irrigation system Vltava-V. Sugar beet was included in a five-plot crop rotation (lucerne, lucerne, winter wheat, sugar beet, spring barley with underseeding). Two essential experimental variants were cultivated permanently on 1 hectare area per each crop. An irrigated variant with optimum moisture regime and the variant without supplementary irrigations. A unified cultivation technology, including nutrition and stand protection, was finding on the plots with mentioned essential variants. Except 40 t.ha-1 of manure, 260 kg of pure nutrients NPK in the ratio 1 : 0.4 : 1.5 was applied on average annually. Average amount of irrigation during 20 growing seasons was 165.8 mm (Tab. 1).

The root yields were finding as average of harvest data of five times replicated plots of an area 25 m2. Digestion and alfa-aminonitrogen were determined in beet root gruel colometrically. The yield of refined sugar was found by calculation according to empiric formula.

Tab. I gives average air temperatures and sums of precipitation over the growing ’ season. A certain image of the character of different years are given by calculated values of hydrothermic coefficient (HTK - HTC). Furthermore, Tab. I shows the total amount of irrigation water supplied to sugar beet ranging from 47 cm (1984) to 312 mm (1979 .

It is evident in the data of 20-years persisting time sequence that supplementary irrigations increased the yields of the crop under study in individual years from 6.7 % (1987) to 257.4 % (1976), by 51.7 % on average for 20 year. Favourable is also an increase in the yield stability (Tab. II). There were the same trends in the effects of irrigation on the refined sugar production, as shown in Fig. 1. The only exception was the moist growing season of 1981 in which the positive effects of irrigations were eliminated by abundant later precipitation.

As one can see from Tab. III, the yield and quality data are differentiating pursuant to the pattern of weather conditions. The highest increase of root yields due to irrigation (128.9 %) was achieved in more arid years (evaluation according to HTC) with the decrease of the sugar content relatively by 12.4 %, through with the decrease of alpha-aminonitrogen relatively by 8.8 %. An increase of refined sugar production in irrigated crop rotation in more arid years was 94 %, in medium moist one 30 %, and in moister year this was 20 %.

The above trend can be seen in Fig. 2, illustrating the dependence of the refined sugar yield on the HTC value. The refined sugar yield was increasing in the set of 20 years data achieved in irrigated and non-irrigated crop rotation with increasing HTC value.