Irrigation Needs in Ukraine According to Current Aridity Level

Global climate change has led to significant shifts in local climatic conditions of Ukraine with the trend to aridity aggravation. Sustainable crop production is at risk due to the aridity level increase. The study is aimed to evaluate aridity index in Ukraine (on the whole country and individual regions’ scales) and the needs in irrigation using hydro-meteorological data of the key regional stations for the periods of 1961–1990 and 2010–2020. The results of hydro-meteorological evaluation were supported by the data on the soil moisture regimes provided by Soil Explorer service. The general trend to aridity aggravation in Ukraine was proved. It was found out that modern cli mate in most territory of Ukraine is semi-arid, dry subhumid and humid zones are observed in the western regions only. Humid soil moisture regimes (ustic and udic) are observed in the West and the North of the country. As a result of combined analysis of both aridity index and soil moisture regimes the conclusion was drawn that 46.05% of Ukrainian croplands cannot provide sustainable crop production without irrigation, 42.65% need irrigation for the cultivation of crops with high water use, and just 11.30% of the arable land require little or no irrigation.

on agricultural production is undebatable fact. Most concerns relate to uncertainty in irrigation water demands for different crops in different regions, because while some crop production zones and some types of crops could ripen the benefits due to climate change [Woznicki et al., 2015], other regions could be doomed to failure if no sufficient water for irrigation would be available. For example, the projection for wheat and corn production in Turkey forecasts the increased demand for irrigation in wheat and the decreased one for corn [Yano et al., 2007]. However, most studies claim that irrigation demands are more likely to increase due to the climate change in many agricultural districts of the world [Elgaali et al., 2007;Fischer et al., 2007;Rehana & Mujumdar, 2013]. But the evidence is for that fact that humidification and irrigation water demand change differently in different areas of the world, so there is no universal pattern. Therefore, nowadays the issue of determining possible shifts in humidification regime in the certain crop production area is relevant because it is the question of food safety guarantee.
Ukraine is one of the leading agricultural countries in Europe [Chryniewicz et al., 2016]. Agriculture is a powerful sector of the economy in Ukraine, however, there is the evidence that recent climate change is challenging for national crop producers because it resulted in dramatical shift in the crop cultivation technology, and previously substantiated irrigation requirements for most crops are out of date now because of huge change in natural humidification conditions. The goal of our study is to determine the change in natural humidification in the territory of Ukraine and revise the demand for irrigation in the country in modern climate conditions.

MATERIALS AND METHODS
The study was carried out for the territory of Ukraine ( Figure 1) using historical meteorological data from the key stations of each oblast, which were averaged for the periods of January 1st -December 31st of 1961-1990 and 2010-2020 years.
Historical monthly meteorological data, including sunshine duration in hours, minimum, mean, and maximum air temperature, mean relative humidity, and wind speed were processed using ETo Calculator software by FAO to calculate potential evapotranspiration (PET) rates for each region of the country [Raes & Munoz, 2009]. Further, the obtained PET values were averaged for each of the studied period and the Aridity Index   (1) where: P is precipitation amount, mm; PET is potential evapotranspiration, mm.
The classification of climate by the values of AI was conducted using the guidelines by Colantoni et al. [2015] (Table 1).
Soil moisture regimes were studied using the information provided by Soil Explorer service, which is based on specialized interpretations of the USA's massive SSURGO database [Miller et al., 2018]. Humid soil moisture regimes are udic and ustic, while arid ones are xeric and aridic. The detailed map of soil moisture regimes was transformed into the simplified one by the prevailing soil moisture regime type in each oblast of Ukraine ( Figure 2). Classification of the territory by the demand for irrigation was performed using the developed methodology, explained in the Table 2, which involves the combined estimation both of soil moisture regime and climate type of each oblast.
To sum up the results of the study, the mapping of the AI and irrigation demands in the territory of Ukraine in the two studied time periods was performed to visualize the trend to aridity increase. The share of croplands, which need irrigation, was calculated based on the surveys of cropland areas

RESULTS AND DISCUSSION
In the result of the Aridity Index computations, the climate peculiarities for every oblast of Ukraine were obtained, which were visualized on the map of the country to provide better understanding of the dynamics in climate patterns change (Table 3, Figure 3).
As the study results testify, the difference in the average AI for the whole territory of Ukraine between the studied periods is -0.30, that is the territory of the country gradually becomes significantly more arid than it used to be 30 years ago ( Table 4). The greatest level of aridity increase is observed in the western regions of the country, Kyivs'ka and Vinnyts'ka oblasts -the difference for these regions is within the range of -0.37-0.47 (the greatest for Ternopils'ka oblast). At the same time, initially in the country dated 2019 (with accordance to State Statistical Service Yearbook) [Verner, 2019].
It should be mentioned that due to the course of political change, Luhans'ka and Donets'ka oblasts are no more under control of Ukraine since April 27th, 2014. Besides, the autonomic republic of Crimea is under the control of Russian Federation since March 21st, 2014, and it is not the zone of Ukraine's economic activity at present. Therefore, the evaluation of climate indices for Luhans'ka and Donets'ka oblasts for the period of 2010-2020 was impossible, and they are marked gray in the maps. The data for Crimea was obtained, however, this peninsula cannot be considered as a part of the territory of Ukraine anymore. Therefore, the calculation of the share of croplands, which need irrigation, was performed for both studied periods excluding the abovementioned regions from the accounted area. semi-arid southern regions of Ukraine demonstrated the least reaction to the climate change (the difference within -0.07-0.13), and even some improvement of natural humidification is observed for Odes'ka oblast (+0.03).
By the results of the Aridity Index calculations and soil moisture regimes surveys the demands for irrigation by every oblast of Ukraine was determined and mapped according to the developed methodology of classification (Figure 4).
The trend to the aridity increase resulted in significant enlarge in the territory requiring irrigation for sustainable crop production. The calculation of the share of the territories with different demands for irrigation testifies that nearly 90% of the territory of Ukraine are currently needing irrigation to grow the full specter of crops, while in the period of 1961-1990 this area shared just about 55% or was nearly twice less (Table 5). Nowadays, just a single region of the country (Lvivs'ka oblast) can cope with crop production under the rainfed conditions. The most vulnerable regions are located in the South of Ukraine (so-called coastal Black Sea area including Khersons'ka, Mykolaivs'ka, Odes'ka and Zaporiz'ka oblasts) and in the center of the country (Dnipropetrovs'ka, Kirovohrads'ka oblasts). Sufficient natural humidification and favorable water balance according to the results of the study is observed only in five oblasts, which are concentrated in North-Western part of the country, while in 1961-1990 period there were fifteen (thrice more) regions, which were characterized with favorable water supply.
The results of our study are supported by the results obtained for the previous (1991-2016) period by Romashchenko et al. [2017], who first pointed out the problem of growing increase in the aridity in the territory of Ukraine, testifying that the area of hyper humid and humid zones of the country decreased by 10.0%, and the territory of insufficiently humid, dry, and extremely dry areas increased by 13.0% in comparison to . But now (for the period 2010-2020) it is evident that the aridity level of the Ukraine's territory is on the pace to greater increase in the severity of aridity in the country even compared to the period of 1991-2016. The work by Vozhegova & Kokovikhin [2018] also claims that to provide for competitiveness and stability of Ukrainian crop production irrigation development is urgent, and steps for the reconstruction of present and building new irrigation systems is required to satisfy the demands for water in agricultural sector of the economy. Such ideas were also proposed by another domestic Ukrainian scientists, but they mainly focused their attention on the zone of the South of Ukraine [Maliarchuk et al., 2018], [Vozhehova et al., 2020]. But not only domestic scientists are calling for steps to diminish negative impacts of climate change on agriculture. It is highlighted that water balance modeling testifies that the trend to an increase in water demands for crop production is obvious reality for all the countries of Mediterranean basin [Gorguner & Kavvas, 2020]. Therefore, it is urgent not only to ensure the satisfaction of crops' demands for irrigation water in the regions where irrigation systems are readily available, but also it is needed to find the ways of irrigation water supply in the territories where irrigation was not previously present, e.g., the West and the North of Ukraine. Besides, transfer from old (furrow, surface, sprinkler) ways of irrigation to modern water-saving ones (drip,

CONCLUSIONS
The study on the climate change and consequent change in irrigation demand in Ukraine discovered the main regularities and trends in this process. Current climate change in Ukraine shows an obvious trend to aridity increase on the whole territory, excluding just coastal region of Odes'ka oblast. Increased aridity of the climate requires revision of irrigation demand in the country, which embraces the change in the areas needing irrigation for sustainable crop production. As for the period of 2010-2020, 46.05% of Ukrainian croplands require mandatory irrigation, 51.29% require irrigation for some crops, and only 2.66% of the croplands might remain rainfed. Coping with current climate change challenge requires extension of irrigation systems in Ukraine and transferring to water-saving cultivation and irrigation techniques to guarantee satisfactory level of provision of crop production with water.