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The sole factors of major importance to Bulgaria’s agrarian output are temperature, and water probability. Between these two factors it is the component relating to soil moisture that proves more limiting. Probabilities of occurrence of water and of given temperatures are estimated by reference to summed temperatures and total rainfall, but also in relation to certain more specific indicators. Heat conditions and heat resources can be summed up by reference to the continuity of the vegetation period or growing season, which are limited for each type of plant, with thresholds for each crossed over the spring-autumn period in relation to biological minima. In the case of agricultural crops in Bulgaria, the biological minima are taken to be: 5°C - for wheat, barley, oats, peas, lentils and sunflowers; 10°C for corn, haricot beans and soybeans, and 15°C for cotton, vegetables and other spring cultures. The durations of cold and warm periods are of course interrelated characteristics. In the first period a key further issue is to determine the number of days with snow fall and with snow cover, these being fundamental to the shaping of soil-moisture reserves after the spring snow melt. Defining regions with thermal stress during the vegetation season is a further priority in describing agroclimatic conditions. Values indicative of limitations on crop growth would be one or more periods of at least 10 consecutive days with maximal air temperature over 35°C. Temperatures over 28°C are considered stresses that slow growth, and may even destroy plants if heat stress develops. The components most limiting the growth, development and formation of yields from agricultural crops are conditions as regards moisture represented in relation to atmospheric and soil moisture. The most apparent indicator is the annual total rainfall, or else the rainfall total in periods with average daily temperatures over 5 or over 10°C. A cross correlation matrix between the meteorological elements upon which evapotranspiration depends, i.e. air temperature, relative air humidity, wind speed and vapour pressure deficit, is discussed. One of the ways of assessing the actual necessity for water is to consider the difference between rainfall totals and potential water use, i.e. evapotranspiration. The difference between these two variables presents the balance of atmospheric moisture (BAM). Values for the relationship between real and potential evapotranspiration were calculated for two potential vegetation sub periods: March-June (the period in which the yields from winter crops are shaped) and July-August (the period in which the yields for spring crops are shaped).
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