IMPACT OF CLIMATE UNCERTAINTIES AND SOIL CHARACTERISTICS ON IRRIGATION SEASON DURATION | ВЛИЯНИЕ НА КОЛЕБАНИЯТА НА КЛИМАТА И СВОЙСТВАТА НА ПОЧВАТА ВЪРХУ ПРОДЪЛЖИТЕЛНОСТТА НА ПОЛИВНИЯ СЕЗОН

Variability of climate and soil characteristics produces uncertainties of irrigation scheduling and demands in Bulgaria. In former studies the irrigation scheduling simulation model ISAREG was calibrated for two maize hybrids of different water stress resistance in a vertisol and a chromic cambisol soils in the Thrace plain and then used to build irrigation scheduling alternatives in agreement imposed by the irrigation methods. The objective of this study is to assess the impacts of several irrigation scheduling strategies for reduces irrigation demand upon number and irrigation season duration ISD by application of the formerly validated ISAREG model to the period 1970-2005. Aiming at improved water use under furrow irrigation the following scheduling strategies are studied in the vertisol total available water TAW=173 mm m-1: 1 refilling the soil reservoir adopting managementallowed depletion fraction MAD of 0.60; 2 refilling the soil reservoir adopting MAD=0.47; 3 refilling the soil reservoir adopting MAD=0.33 and 4 partially refilling the soil reservoir adopting MAD=0.47. The following irrigation scheduling alternatives for surge furrow and sprinkler irrigation are simulated in the chromic cambisol soil TAW=136 mm m-1: 2 refilling the soil reservoir adopting MAD = 0.60; 3 refilling the soil reservoir adopting MAD = 0.40 and 4 partially refilling soil reservoir adopting MAD=0.60. Statistical analyses of results relative to the vertisol indicate that when economically justified water saving is aimed at the number of irrigation events ranges from 1 to 3 alternative 1 and from 2 to 4 alternative 2 and reaches 2 - 6 alternatives 3 and 4. Comparing with the vertisol, an additional irrigation is required in soils of moderate water holding capacity chromic cambisol over the high and average irrigation demand years. Thus the larger are soil water reservoir TAW and management-allowed depletion fraction MADthe shorter is the irrigation season. When water is depleted from the deeper soil layers alternatives 2 and 4 average irrigation season starts at the beginning of July and ISD is 26 - 29 days in the vertisol and 37 - 38 days in the chromic cambisol. When MAD0.40 alternative 3the irrigation season predominantly begins in June and it is longer by ten dais on the average when compared with ISD relative to alternative 2.