THE CLIMATIC AND AGROCLIMATIC CHARACTERIZATION OF THE AUTUMN SEASON

ON THE TERRITORY OF THE REPUBLIC OF MOLDOVA

From calendar point of view, the spring starts on 1^st March, but from meteorological point of view the beginning of spring conventionally is considered along with the stable transition of the average daily temperature through 0ºС towards its increase. In the Southern part this transition takes place generally at the   end of February, on the rest of the territory – in the first decade of March.

Depending on the particularities of the meteorological processes, in some years the spring start date may significantly divert. Since the beginning of observations the earliest spring was recorded on the biggest part of the territory of the country on 10 January  (1989),  and the latest on 26 March (1963).

In late March – early April mainly there takes place the transition of the average daily temperature through 5ºС, between 16-23 April – through 10ºС (beginning of crops active grow and development), and between 7 and 19 May the transition through 15ºC is recorded (start of the meteorological summer).

The transition of the atmospheric circulation from the winter conditions to the summer one leads to a strong weather variability: sudden change of warm and cold periods, of rainy and dry one.

The average air temperature in spring oscillates between +8ºC and +10ºC. The coldest spring was registered in 1987, with the average air temperature of +5..+6ºC, that is 3-4ºC lower than the norm. The warmest spring was registered in 1983 and 2007. The average air temperature during the mentioned seasons constituted +11..+13ºC, exceeding the long term average by 2-3ºC.

Since the beginning of observations the lowest spring temperature was recorded on 4 March 1955 –  -26ºC (Bravicea), and the highest constituted +37ºC (Cahul, 17 May 1969  and  Ştefan  Vodă, 20 May 1996).

Frosts occur often in spring. In the air they continue until 5-21 April, on the soil surface – until 22-30 April. Nevertheless, in the last years, the frosts may occur even in May. The latest date of frost stop in the air in the Northern and Central parts is  21-24 May (1980), and  in  the  Southern - 1-10 May (1990). On the soil surface the frosts may occur even on 1 June (1955). The snow layer usually  completely  melts  between  15-27  March, and the latest date of its melting was between 20-27 April (1997). The snow may be noticed until 27 April-20 May (1952).

In spring, unlike in winter, the heavy rains fall more often and sometimes they are accompanies by thunderstorms. During spring there may fall in average 105-150 mm, or circa 24% of the total annual precipitations. The most arid was the spring of 2003. During that season the quantity of fallen precipitations was twice lower than the norm and constituted­ 30-60 mm. During the 1991 spring, on contrary, the total fallen precipitations (200-280 mm) exceeded twice the norm. The maximum daily quantity of precipitations constituted 30-100 mm.

The fog (in average 6 days) and thunderstorm (in average 8 days) are frequent phenomena in spring.  The frosts and the strong winds are the most dangerous meteorological phenomena in April. They are registered in average once in 3 and 5 years.

The beginning of the spring agricultural works depends on the dates of snow melt and soil thaw. The soil thaw up to 30 cm depth, is usually recorded between 10-17 March, the total thaw –11-20 March. The soil dry up is usually noticed on 18-28 March and it indicates the optimal dates for the beginning of tillage before the sowing works.

The restart of the vegetation period of the autumn crops takes place between 16-25 March. During the early springs, the vegetation period may restart in late February, during the late one – in the middle of April. The productive soil humidity reserves in 1 meter depth at the beginning of the restart of  autumn crops vegetation period are generally fine (140-160 mm), the Southern region – satisfactory (125-135 mm). Such humidity reserves boost the autumn crops growth. During the very arid springs, the productive humidity reserves are insufficient and constitute 60-70 mm.

The favorable conditions for the beginning of early crops sowing (barley and oats) appear when the average daily temperature (on the date of sowing) constitutes +5°C and higher, the soil has a plastic and soft texture and gets warm up to +5..+6°C. Such conditions occur almost yearly (75% of years) in late March – early April. During this period of time, the long term average productive humidity reserves in the plowed soil layer in the Southern part constitutes 25-30 mm, in the rest of the territory – 35-40 mm. In early springs, the beginning of the spring crops sowing starts in late February–early March, and in late springs – in the second decade of April.

Usually, the multiannual plantations on the territory of the country restart their vegetation period between 23 March and 4 April (along with the stable transition of the average temperature through +5°C). Usually, the tree bud swelling starts in the second half of March, the sap circulation in vineyards starts in late March. In early springs, the beginning of bud swelling and the flowering of apricot tree takes place between 15-28 March.

The usual period for flowering of apricot tree is between 15-23 April. The bud swelling for plums, apples, pears is noticed in the first decade of April. The vineyards bud swelling is noticed between 5-22 April, when the stable average daily air temperature transits through +10°C, towards its increase.

The sowing dates for the sugar beet are determined by the soil warming up to +7..+8°C and  disappearance of frosts that may cause damage to raised plants. During the warmest springs the sugar beet sowing is carried out usually at the end of March – beginning of April, and in the coolest springs – at the end of April – beginning of May. During the sugar beet sowing period in the Northern part of the republic the humidity reserves in the plowed layer of the soil usually constitute 30-40 mm.

The most appropriate terms for sunflower sowing are between 7-18 April (the soil warms up to +10°C). The productive humidity reserves in the plowed layer of the soil for this period are more than 20 mm everywhere.

The most optimal dates for maize sowing are in the third decade of April. The productive humidity reserves in the plowed layer of the soil for this period are usually fine and constitute 25-40 mm.

In the last pentad of April – beginning of May the sunflower rises up, the sugar beet rise up in mass, the autumn wheat straw gets longer and the flowering of apples and pears begins.

May is characterized by the beginning of the active growth and development of thermophilic crops. In the first decade of May, the twinning phase spring barley starts everywhere. The mass rise of the maize plants is recorded between 13-17 May and for the potato - on 12-22 May. The flowering of the lilac is noticed between 5-9 May and of the white acacia around 30 May.

According to the long term data the crops seedlings are planted in opened soil in the first half of May.

The mass autumn wheat spike forms in the period 24 May-2 June. The productive humidity reserves in the 1 meter depth soil on the fields with autumn wheat  during the spike formation stage constitutes, basically,  85-115 mm, in some South and Central regions – 65-75 mm.

The risk climatic phenomena that represent a threat for crops in May are droughts and hot dry winds. The unfavorable spring conditions have a negative impact especially on autumn crops, they partially reduce the weeding crops and trees yield. In May the hot dry winds are recorded in average in 3-7 days. The spring drought is recorded in April – May. Its frequency is 15% of year.

Also a big threat for crops in spring represents the frosts that may occur in May. They are recorded in average once in 10 years and lead to damaging and even dying of flower buttons, flowers, trees, buds, grapes and walnuts inflorescences, maize, beans, potatoes, peas, tobacco, vegetables and other thermophilic plants.

Chief MCMC                   T. Bugaeva 

 Chief AFC                      T. Mironova