Irrigation Timing in Maize by Using the Crop Water Stress Index (CWSI)

The aim of the investigation was to determine if the CWSI could be used to determine when the irrigation water should be applied to maize under Hungarian changeable weather. Three water treatments were applied: "Ad libitum" watering in lysimeter's growing chambers, irrigated treatment (use of CWSI), and rainfed control. The theoretical approximation of crop water stress index was applied in our long-term investigation. Among the 17 consecutive seasons between 1989 and 2005 included in the study there were seven arid and four humid seasons. The remaining summers had semi-humid character. To represent the influence of drought on water stress index of maize we chose the year of 2000, the driest and warmest season from the beginning of weather observations of Keszthely. As a counterpoint stress index results of 1997 served as an example of humid seasons. The degree of the deviation in CWSI as a result of different weather may vary, but the tendency in arid or humid seasons remained the same. The basic parameter of CWSI, the canopy temperatures were measured using a hand-held infrared thermometer. Relevant meteorological and plant data were collected locally. Our final conclusion was that there is no problem in use of CWSI during arid summers, where radiation is undisturbed and the cloudiness does not disturb the canopy temperature sampling. In arid seasons increases in assimilatory surface size and yield of maize in irrigated treatments were between 20-35% and 10-30%, respectively. The extra production seems to be enough to cover the irrigation costs. There is also no problem in humid seasons, where the sky is often covered by clouds, but there is no need to irrigate maize in Hungary. In semi-humid weather not only the environmental circumstances may disturb the plant temperature measurements and calculation of CWSI (doubtful results caused by changeable radiation and wind), but the probable yield surplus - below 5% - is also negligible. The irrigation costs are higher than the production gain during semi-humid seasons. In Hungary about one third of the seasons have characteristics of an arid weather where sampling of canopy temperatures provides reliable basis to calculate CWSI and plan irrigation timing.