Optimal regional irrigation requirements under changing climate in Bulgaria

Abstract

Net irrigation requirements (NIR, mm) that fully satisfy crop development and yield formation are basic in irrigation systems’ design and management. Traditionally, Bulgarian practice used to adopt the irrigation scheduling and demands developed by Zahariev et al. (1986) that were based on field experiments carried out in the period 1950-1980 and crop evapotranspiration computed by Delibaltov’s equation. The book consists of tables providing information on 31 crops and 97 irrigation regions (IR) used in design and exploitation of the national irrigation systems till 1990. Years having probability of occurrence of irrigation depth PI=10, PI=25 and PI=50% were considered. Detected climate change and drought aggravation however create uncertainties for irrigation management. To cope with them, simulations were performed for past (1950-1980) and present (1951-2004) weather conditions relative to unified agroclimatic AC regions. In former studies the irrigation scheduling simulation model WinISAREG was calibrated for maize using data from long-term experiments carried out in fields representing diverse soil, climate and management conditions for maize irrigation. Optimal АC regions were defined on the grounds of the average reference evapotranspiration totals for July-August EToJul-Aug relative to the period 1951-2004. Thus, average total EToJul-Aug was an indicator of regional NIR and used in “Zahariev” irrigation regions unification into agroclimatic (АC) regions (table 1). The impacts of soil properties on irrigation requirements were characterised by total available soil water TAW, mm m-1, being “small” if TAW=116 mm m-1, “average” if 136<TAW<157 and “large” when 173<TAW<180 mm m-1. Respective NIR, mm, relative to the period 1951-2004 were computed by model application to soil groups of small and large TAW in each АC region. Results indicate that when average total EToJul-Aug increase from 260 to 330 mm, net irrigation requirements NIR in “average” demand year (PI=50%) increase from 160 to 310 mm for soils of “small” TAW . Such substantial increase reflects the impact of climate diversity in Bulgarian plains on maize irrigation. Relative to past weather 1951-1980 (table 2), unified conventional irrigation depths were compared to those simulated. Results showed that “Zahariev” irrigation requirements were in the range of NIR derived by model application in most of the cases. It was concluded that simulated NIR covered a lager range than that of “Zahariev” estimates since the model took better into account the impact of climate change and different soil water holding capacity TAW. Compiled maps of computed NIR, Meteorological Stations, Irrigation and Macroclimatic regions illustrate the findings of the study. They make clear the position of “wet” and “dry” zones and respective irrigation requirements over the country territory during “the average demand” 1970, “the moderately dry” 1981 and “the very dry” 2000 in the 54-year period.