Droughts and Climate Change in Bulgaria:
Assessing Maize Crop Risk and Irrigation Requirements in Relation to Soil and Climate Region

1 Agricultural Academy, “N. Poushkarov” Institute of Soil Science, Agrotechnology and Plant Protection, BG - 1080 Sofia, Bulgaria
2 Technical University of Lisbon, Institute of Agronomy, Tapada de Ajuda CEER-Biosystems Engineering, 1349-017 Lisboa, Portugal
3 Bulgarian Academy of Sciences, National Institute of Meteorology and Hydrology, BG - 1784 Sofia, Bulgaria


POPOVA, Z., M. IVANOVA, L. PEREIRA , V. ALEXANDROV, M. KERCHEVA, K. DONEVA and D. MARTINS, 2015. Droughts and climate change in Bulgaria: assessing maize crop risk and irrigation requirements in relation to soil and climate region. Bulg. J. Agric. Sci., 21: 35–53

This study aims at assessing maize cropping risk due to observed trends for drought aggravation for the maize crop season which is associated with possible climate change trends relative to precipitation, temperature and reference evapotranspiration (ETo) at selected weather stations in Bulgaria. Water balance and relative yield computations were performed with the model WinISAREG after its calibration and validation for maize at various locations of Bulgaria and using long-term experimental data. Rainfed maize is associated with a great yield variability (29 < Cv < 72%) due to inter-annual and spatial climatic variability during the maize season. The largest yields variability were found for Sandanski and Plovdiv when a soil with low total available water (TAW = 116 mm m-1) is considered. The least variable yields are those for Sofia when TAW = 180 mm m-1. Basing upon economic considerations, relative yield decreases (RYD) were computed with the threshold of 60 and 48% of the potential maize productivity in Plovdiv and Sofia. Maize production is risky in 32% of years in Plovdiv when TAW is large, which is the double of risk in Sofia. If TAW is medium the risky years double and reach 50% of years in Varna. A relationship between the SPI-2 index computed for “July-Aug” with the simulated RYD of rainfed maize was found. It is quite significant in Plovdiv where R2 > 91% was found. Results are less good for Sandanski and Sofia (73 < R2 < 83%).
Results indicate that when rainfed maize is grown on soils of large TAW, maize development is less affected by the water stress. Economical losses are produced when high peak season (July-Aug) SPI-2 is less than +0.2 in Sandanski, -0.50 in Plovdiv and -0.90 in Sofia. In North Bulgaria the respective SPI threshold ranges between -0.75 at Lom and -1.5 at Pleven. When irrigation is considered, a relationship was also found relating the net irrigation requirements (NIRs) with SPI-2 “July-Aug”. Consequently, the corresponding economic thresholds relative to the yield losses were computed for all locations this allowed to estimate the NIR thresholds that may lead to favourable cropping returns. The derived reliable relationships and related
SPI-2 thresholds relative to “July-Aug”, under which soil moisture deficits lead to severe impact of drought on rainfed maize yield for the studied climate regions and soil groups were used for mapping the risk of rainfed maize yield decreases as a function of drought intensity, which combined with maps relative to NIR to overcome drought effects on maize production.

Key words: Drought Intensity-Yield Relationships, Isareg Model, Net Irrigation Requirements, Rainfed Maize Vulnerability, Relative Yield Decrease, SPI-index

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