The equa- tion was verified using independent material with cross- validation (Picard and Cook, 1984). Additionally, the amount of times the relative fore- cast error in the analysed period 1966-2005 amounted to | RFE |≤5% (a very good forecast), and 5%<| RFE |≤10% (a good forecast) was determined (Dobosz, 2001). Moreover, regression equations were verified using the relative forecast error calculated according to the formula: RFE y y p 100% (1) y and the average relative forecast error for all analysed COBORU stations and all considered years of the 1966- 2005 period, was calculated according to the formula: ARFE 1 n ¦ i n 1 RFE (2) where y -actual yield (t ha -1 ) y p -yield calculated accord- ing to the formula (t ha -1 ) n -number of years in a time se- ries (number of stations x number of years). The occurrence of autocorrelation of random components was checked with the Durbin-Watson test. The sig- nificance of regression coefficients was assessed with the t - Student test. efficient, was assessed with the F -Snedecor test. The data came from experimental stations located all over Poland, run by the Research Centre for Cultivar Testing (COBORU) in Słupia Wielka (Fig. This work used data concerning the total yield quan- tity and dates taken from the period between the end of emergence and beginning of leaf bending of medium-late cultivars of onion (Allium cepa L. The second goal was to determine the frequency of precipitation deficiency during the critical periods. The first goal of this study was to determine the po- tential reduction in onion yield caused by insufficient pre- cipitation in Poland. , 2007 Meranzova and Babrikov, 2002 Rumpel et al. The positive effects of irrigation mainly depend on the stage of plant growth, soil type, and the course of meteorological conditions (Enciso et al. , 2000 Patel and Rajput, 2008 Pelter et al. Irrigation of onion crops results in high production potential (Al-Jamal et al. In Poland, water requirements of the onion amount to about 350-400 mm in the growing sea- son (Dzieżyc and Dzieżycowa, 1986). (2008), water requirements of the onion mainly depend on the region of the world in which this plant is cultivated. Drought in the pe- riod of bulb formation causes a decrease in individual bulb weight (Pelter et al.
Drought in the periods of germi- nation and emergence cause a decrease in the number of plants per unit of surface area, and is also a cause of re- tarded, weak and uneven emergence. Water shortage in the soil causes a reduction in yield and deterioration in the quality of the onion (Tendaj, 2000). in the soil at every stage of growth, but especially in the periods of emergence, formation, and in- tensive bulb increase.
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