**1. Introduction**

Agriculture, as a branch of our national economy, operates entirely under the open sky. For this reason, climate change and fluctuations and climate anomalies have a very significant impact on the growth, development and productivity of

#### *Agrometeorology*

agricultural crops. Climate change affects not only the quantitative but also the qualitative indicators of agricultural production. The development of agricultural practices and the application of precision and organic farming require in-depth knowledge of weather and climate, meteorological and climatic features and agro-climatic resources, in order to effectively and timely manage the processes to achieve maximum results.

The assessment of the influence of various factors shows that the most important for food production are the hydrothermal conditions or more precisely the balance between temperature and humidity conditions. In the agricultural zone of Bulgaria the conditions of humidification are limiting, and they are determined by the balance between temperatures and precipitation. The condition of the main meteorological elements determines the quantitative indicators of the hydrothermal conditions. The established trends of their change toward the end of the last century in Bulgaria showed an increase in the average daily air temperatures, an increase in the minimum temperatures [1–3] and a decrease in precipitation [1, 4].

In order to obtain high yields of high quality agricultural products, the role of soils and their fertility, varieties and hybrids of crops, their biological potential and resistance to extreme weather fluctuations - drought, frost, over wetting and heat waves, which are a prerequisite for stress and reduced productivity. Also, the resistance of cultivated plants to diseases and pests, because when environmental conditions are optimal for plants, they are optimal for the development of weeds, diseases and pests.

The knowledge and combination of the whole complex of biotic, abiotic, economic and technological factors [5, 6] against the background of the changing climate and climatic anomalies with increasing frequency are subject to research by the National Scientific Program - "Healthy foods for strong bio economy and quality of life", and the causal links between agro-climatic and soil conditions and the biological and genetic characteristics of the main types of crops for our conditions in the last 30 years are the subject of research by the project "Agricultural ecosystems adapted to climate change", which is part of the scientific program.

The aim of this study is to discover the changes in agro-climatic resources, as a result of changes in the main meteorological elements, at the end of the last century and the beginning of the present.

## **2. Experimental data**

For the preparation of this up-to-date development, which presents the state of the hydro-thermal conditions in the period 1986-2015 from an agro-meteorological point of view, daily data on the basic meteorological elements were used: minimum, maximum and average daily temperatures and 24-hour precipitation for the indicated period of 56 representative meteorological and agro-meteorological stations for the agricultural zone in the country, **Figure 1**.

The meteorological elements measured also include some agro-meteorological indices that more accurately define the meteorological conditions, namely: the dates of a steady transition of the average daytime air temperature at 5°C and 10°C in spring and autumn; the duration of period with temperatures above 5°C, also known as a potential vegetative season (PVP) and above 10°C - a real vegetative season (RVP); as well as the absolute minimum and maximum temperatures during the coldest and warmest months of the year - January and July, respectively, the absolute minimum temperatures in spring and autumn as an indicator of the occurrence of frost damage to agricultural production, as well as amounts of the active and effective temperatures and amounts of precipitation for the potential and real

**45**

*Changes in the Agro-Climatic Conditions in Bulgaria at the End of the 20th and the Beginning…*

growing season. In the course of this study, periods of dry spell (10-30 days without rainfall or rainfall less than 5 mm) and persistent drought (more than 30 days without rainfall or rainfall less than 5 mm) were identified. The analysis of the results obtained is a good basis to draw important conclusions, significant conclusions about the agro-climatic resources and agro-meteorological conditions in the NUTS2

*Map with location of the meteorological and agrometeorological stations in the agricultural zone of Bulgaria.*

The growth, development and productivity of agricultural crops are most significantly influenced by hydrothermal conditions during the growing season. For wintering cereals - wheat, barley and rapeseed - this is the period from October of the previous year to June of the next and for spring and heat-loving from March to September, and for orchards - until October. The analysis of the average monthly longterm values of temperatures during the study period shows that they have increased, in almost all regions and months, compared to the reference period 1961-1990. In precipitation, the trends are not clearly expressed in all months and regions.

**3.1 Change in basic meteorological elements during the period 1986-2015** 

The analysis of the deviations of the average multi-annual monthly temperature by months and seasons for the research period (1986-2015) was made by administrative-territorial planning zones, **Figure 2**. In all agricultural areas the temperature deviations from the reference values in December are predominantly negative. Exceptions are some of the stations located in Northwestern and North-Central Bulgaria, where the average monthly temperature for 1986-2015 is higher than the

**compared to the reference period 1961-1990**

*3.1.1 Deviation of the average monthly temperature from the norm*

*DOI: http://dx.doi.org/10.5772/intechopen.93808*

administrative territorial planning zones.

**3. Results and discussion**

**Figure 1.**

*Changes in the Agro-Climatic Conditions in Bulgaria at the End of the 20th and the Beginning… DOI: http://dx.doi.org/10.5772/intechopen.93808*

**Figure 1.** *Map with location of the meteorological and agrometeorological stations in the agricultural zone of Bulgaria.*

growing season. In the course of this study, periods of dry spell (10-30 days without rainfall or rainfall less than 5 mm) and persistent drought (more than 30 days without rainfall or rainfall less than 5 mm) were identified. The analysis of the results obtained is a good basis to draw important conclusions, significant conclusions about the agro-climatic resources and agro-meteorological conditions in the NUTS2 administrative territorial planning zones.
