Irrigation Planning Taking into Account Climate Change and droughts intensity in the Steppes Zone of South Ukraine

As a result of research, climate changes on regional and local level in the Southern Steppe zone of Ukraine during the vegetation period has been established. Processes of increasing effects of extreme weather events such as droughts, dry winds etc. were observed. During the growing seasons in the period 1992 to 2017, the average daily air temperature increased by 0.8 °C and rainfall by 55 mm as compared to the long-term norm; in the years 2007 and 2012, the increase was even higher (2.5 ° C). During the observation period, the largest increase in precipitation was observed at the beginning and in the middle of the growing season (11-18 mm per month) with the exception of August. As a consequence of the heavy rainfalls (increase by 15%), the proportion surface run-off increased significantly as well. Besides, the duration of dry periods often extended by more than a month. The calculation of the coefficient of natural humidification showed that there is an expansion of the territory with high aridity; there the coefficient decreased by 0.08-0.02, which is 5-20% less than the figures at the end of the last century. The coefficient of natural humidification in the coastal zone of the Southern Steppe declined to 0.35 and below, indicating the threat of desertification. An additional impact is the destruction of forest belts leading to increased wind erosion. Taking all these facts into account, irrigation rates for major crops had to be re-calculated. Accordingly the irrigation rates were augmented to 5-28% in comparison with previous rates. If the current climate change trend persists over the next 20-50 years, water requirements can increase by another 10-20% compared with rational norms, when developing plans for upgrading irrigation systems. It is also important to take into account the peculiarities of the intra-season distribution of crop water consumption deficit and the irrigation module of water supply under typical crop rotation. In the context of global warming over the last 6 years, there have been strong and very severe droughts during the period of critical development and at the end of the crops growing time, which lasted 47 days. During these periods, plants are most vulnerable to high daytime temperatures (30-35 °С and more) and to a decrease in air humidity up to 30 %; the duration of dry periods in some years reached 75 days. Under conditions of extreme droughts, even in case of optimal irrigation, reduction of water consumption of agricultural crops and reduced biomass development took place. When planning irrigation regimes under such conditions, it is necessary to adjust the parameters of water consumption models based on data from field research and Remote Sensing. Dependencies between biomass growth and evapotranspiration have been obtained. These studies indicate a significant reduction in soy evapotranspiration in years of severe and very strong droughts. The obtained correlations allow correcting crop coefficients for drought conditions. This ensures an adequate determination of the required irrigation water volumes, considering the actual state of biomass and reduction of evapotranspiration. It has been established that influencing the microclimate of crops is important for optimal maintenance of water use and prevention of negative influence of drought. This is due to lowering the temperature and increasing the humidity of the air in the surface layer of the soil. This can be achieved by irrigation using small norms in dry periods being possible only by using of modern sprinkler technology. It is further necessary to implement additional measures that can provide a similar microclimatic effect as optimization of crops density, planting and saving of forest belts, application of minimal soil treatment, use of chemical agents etc.