The Republic of Guinea has eight administrative regions, and this study focuses on the region of Faranah (Figure 1).

Figure 1. Map of the administrative region of Faranah (source: Galakpaye PIVI-2025).

The administrative region of Faranah is located in the centre of the country, between Fouta-Jallon, Forest Guinea and Upper Guinea. It is located between 8˚50 and 12˚ north latitude and 9˚15 and 11˚29 west longitude. It covers an area of 35.581 km².

It is bordered to the east by the administrative region of Kankan, to the north by the Republic of Mali, to the west by the administrative regions of Mamou and Labé, to the south-west by the Republic of Sierra Leone and to the south by the administrative region of Nzérékoré (Figure 1).

The general climate is typical of the Sudano-Guinean type, with two alternating seasons: a rainy season and a dry season. Thanks to its geographical location between Fouta-Djalon, Upper Guinea and Forest Guinea, the region is influenced by three types of microclimates: tropical mountain or “Foutanian” climate, tropical sub-Sudanese and sub-equatorial.

Average annual rainfall varies between 1,200 mm and 1,700 mm, reaching 2,000 to 2,500 mm in the south, on the border with the forest region. Temperatures are generally high, ranging on average between +25˚C and +30˚C. Average relative humidity varies between 69% and 85%, and the prevailing winds are the Harmattan and the monsoon.

The population of the Faranah region is mainly rural, and the functioning of economic systems (agriculture and livestock farming) is closely linked to weather and climate conditions.

The following meteorological data were used to conduct the research: air temperature characteristics and precipitation amounts for the Faranah region between 1994 and 2024. Data on air temperature, maximum and minimum temperatures, and average monthly and annual rainfall were obtained from NASA’s MERRA-2 project website.

To determine the variability of agroclimatic parameters, daily rainfall data recorded between 1994 and 2024 by the Faranah region meteorological observatory were used. FORTRAN, Excel, Word and Grapher software were used to process the data and present the results. QGIS was used to edit the maps.

The following indicators were used to assess agroclimatic resources: the sum of active temperatures (SAT) of the air during the active growing season of agricultural crops; Selyaninov’s hydrothermal coefficient (GTK) [15]; the Standardised Precipitation Index (SPI).

STA is the sum of daily average air temperatures above +10˚C during the active growing season (May to October). GTK is calculated using formula (1).

where ∑ P is the sum of precipitation during the active growing season (mm).

The classification of humidification zones based on Selyaninov’s hydrothermal coefficient (GTK) consists of several categories:

Regions with a GTK between 1.6 and 1.3 are considered humid;Regions with a GTK between 1.3 and 1.0 are considered slightly arid;Regions with a GTK between 1.0 and 0.7 are classified as arid;Regions with a GTK between 0.7 and 0.4 indicate very arid conditions;Regions with a GTK below 0.4 are classified as dry. The Standardised Precipitation Index (SPI),

where X i is the monthly rainfall value, X ¯ is the average rainfall, S is the standard deviation of rainfall over the last 30 years.

The characteristics of the humidification regime according to SPI indices are presented in Table 1.

Analysis of interannual variations in total precipitation in the Faranah region shows two periods of fluctuation relative to the normal value of 1,647 mm (Figure 2).

Table 1. Classification of droughts based on the Standardised Precipitation Index (SPI).

Overall, below-normal rainfall was observed between 2002 and 2011, while above-normal rainfall was recorded between 2012 and 2024, with a slight decrease in 2017. In other years, rainfall patterns varied from the climate norm, with the minimum recorded in 2008 and the maximum in 2021.

Figure 2. Inter-annual precipitation values between 1994 and 2024.

The above data show the instability of the interannual rainfall pattern in Faranah: the annual average is 1,630 mm, the minimum annual rainfall is 25 mm and the maximum annual rainfall is 2,800 mm (Figure 2). During the year, maximum precipitation falls in August, with approximately 428 mm, and minimum precipitation in December (Figure 3).

It can be claimed that there are two seasons based on rainfall patterns: the rainy season, from May to October, and the dry season, from November to April. The rainy season lasts 5 to 6 months. Maximum annual rainfall is observed in August (443.20 mm) with a temperature of +23.6˚C, and minimum rainfall in December (2.60 mm) with an average temperature of +22.5˚C.

The annual distribution of air temperature in Faranah between 1994 and 2024 shows that the maximum temperature is observed in April, at the end of the dry season (Figure 3). With the increase in rainfall, temperatures drop until August, and a second small peak in temperature occurs in October. The winter months are characterised by the lowest temperatures of the year.

Figure 3. Ombrothermic diagram of the Faranah region between 1994 and 2024.

In Faranah, seasonal rainfall varies between 3 mm and 465 mm. The lowest seasonal rainfall is recorded between December and February, and the highest between July and September. A comparison of the averages for the periods 1994-2003, 2004-2013 and 2014-2024 indicates a decrease in total rainfall from August to January, with the dry season beginning in November. From January onwards, rainfall gradually increases until July. The dry season ends in April and the rainy season begins in May (Figure 4).

Figure 4. Cumulative seasonal rainfall for three periods in the Faranah region, 1994-2024.

The local temperature is actually the result of global radiation (solar and infrared) and the heat balance of land areas.

In 2007, the Intergovernmental Panel on Climate Change (IPCC) predicted that warming in the 21st century would be strongest over land and in the highest northern latitudes. Over the next two decades, warming of approximately 0.2˚C per decade is expected.

However, temperatures in the Faranah region are slightly moderated by vegetation and high humidity. The maximum temperature, above +33˚C, is higher than in the coastal area. The average annual temperature is around +38˚C. Minimum temperatures rarely fall below +12˚C, accompanied by heavy rainfall in July, August, September and December-January, due to the Harmattan wind [7]. The highest temperatures are recorded at the end of the dry season. The average maximum is +33.6˚C in January and rarely exceeds this value.

Statistical analysis of air temperature characterises the frequency of certain threshold values necessary for evaluating the thermal regime.

Maximum temperatures vary between +29.8˚C and +38.8˚C. The maximum temperature is higher during the dry season (November-February) and lower during the wet season (July-August) (Figure 5). Annual minimum temperatures between 1994 and 2024 in the Faranah region vary between +12.0˚C and +21.3˚C. They are lowest in January and highest in May (Figure 5). The temperature range is variable in winter (December, January and February), while in summer it varies very little (Figure 6).

Figure 5. Annual changes in extreme temperatures for the period 1994-2024 in the Faranah region.

Figure 6. Air temperature range from 1994 to 2024 in the Faranah region.

Data from the ECMWF intermediate reanalysis (ERA) show that 2010 ranks second among the warmest years on record, with the difference between it and 2005 falling within the margin of uncertainty.

The decade 2001-2010 was also the warmest on record. This result is confirmed by examples from the northern hemisphere and Africa, where the regional temperature in 2010 was the warmest on record [16].

An important agroclimatic indicator is the sum of active temperatures during the peak growing season, from May to October. The results of the SAT calculation from 1994 to 2024 are shown in Figure 7.

Figure 7. Multi-year evolution of the SAT.

According to the multi-year curve of the sum of active air temperatures (SAT), the maximum values were observed between 2002 and 2007, when they exceeded normal climatic values. After 2007 and until 2024, air temperature values were below normal climatic values, except in 2019.

In accordance with (formula 1), Selyaninov’s hydrothermal coefficient (GTK) is calculated from the SAT values obtained and the cumulative precipitation during the growing season. The results obtained (Figure 8) indicate a trend

Figure 8. GTK values in the Faranah region over time.

towards an increase in multi-year GTK coefficients. Overall, the period from 1994 to 2024 is characterised by relatively wet values, but the period from 2001 to 2007 was slightly dry.

As indicated above, the SPI index is also analysed to characterise humidity. Analysis of the Standardised Precipitation Index (SPI) for the Faranah region (Figure 9) over 30 years (1994-2024) shows that the main positive precipitation anomalies range from 1.02 to 2.26.

Figure 9. Evolution of the SPI value for the Faranah region.

The most significant positive anomalies in terms of intensity were those of 1994, 2012, 2013 and 2018, with respective indices of 1.88, 2.26, 1.02 and 1.34.

The most significant negative anomalies in terms of intensity were those of 2002, 2003, 2006 and 2008, with respective indices of −1.32, −1.20, −1.86 and −2.07. The first decade in the Faranah region was characterised by increasingly abundant rainfall, followed by a dry period that coincided almost exactly with high STA values (see Figure 7 and Figure 9). The dry period was followed by a slow increase in rainfall over the last decade, a result similar to that obtained in [17]. The platform and the associated environmental report (such as those from the WWF or IUCN) specify that this drought is mainly linked to anthropogenic factors such as agricultural expansion, wood energy production (charcoal) and bush fires often linked to land clearing [18].

During the analysis of interannual values for agro-meteorological characteristics from 1994 to 2024, we were able to establish a link between these parameters. To confirm the existence or absence of links between the parameters studied, we will use the correlation and regression analysis method.

According to the results of the correlation and regression analysis, the influence of STA on the SPI index is characterised by an inverse relationship. The polynomial function with a correlation coefficient of −0.56 should be chosen as the regression function (Figure 10). An increase in STA leads to a decrease in SPI, and therefore to an increase in the frequency of drought events.

Analysis of the correlation between the SPI and GTK indices shows that they are correlated with a coefficient of 0.58 and that the theoretical correlation line is characterised by the regression equation of a direct linear dependence (Figure 11). This value of dependence between GTK and SPI justifies the use of Selyaninov’s GTK to analyse the humidification regime in arid regions, and not only in temperate latitudes.

Figure 10. Correlation between SPI value and STA in the Faranah region.

Figure 11. Correlation between SPI value and GTK in the Faranah region.