Bilan hydrique comparé d'une savane et d'une plantation d'eucalyptus dans le bassin du Kouilou (République Populaire du Congo)

Soil-water balance of savannah and of Eucalyptus plantation in the Kouilou basin (D.R. of Congo) Since 1950 approximately, the savannah of the littoral Congolese is gradually planted of Eucalyptus in dense populations exploited by rotations of seven years. To appreciate the sustainability of these plantations of Eucalyptus, it is necessary to make compared study of energy, carbon, mineral and water balances of two ecosystems, the origin ecosystem, the savannah, and home-made ecosystem that have succeeded it, Eucalyptus plantations. The aim of this work is to study the water balance of the two ecosystems and more particularly their actual evapotranspiration (transpiration and evaporation). In a five years-old Eucalyptus plantation (Eucalyptus Article disponible sur le site https://climatology.edpsciences.org ou https://doi.org/10.4267/climatologie.806 Bilan hydrique comparé d’une savane et d’une plantation d’Eucalyptus dans le bassin du Kouilou (Congo) 100 PF1 and Eucalyptus 12ABL*saligna), mean height is 24.2 m, mean stem diameter at soil level is 0.17 m, stand density is 502 trees ha, total basal area is 11.0 m ha, leaf area index is 3.2, total wood production is 118.5 m ha) and in a savannah with Loudetia arundinacea, gross precipitations (Pi), throughfall (rain gauges) and soil water content (neutron and TDR probe) were measured weekly during the rainy season from November 1996 to April 1997, from November 1997 to April 1998, from November 1998 to April 1999 and during the dry season, from June to September 1997 and from June to September 1998. Actual evapotranspiration (Ea), net interception and transpiration were derived from the water balance equation method. Moreover, transpiration was measured using the radial flow meter. The soil water profiles (soil water content and potential plotted to soil depth) were established, giving an account of preferential water uptake zones and rooting depths of Eucalyptus plantation (5 m depth) and savannah species (3 m depth). Throughfall, net interception during the rainy seasons (1996-97, 1997-98 and 1998-99) were 867.1 mm and 112.1 mm (88.7 and 11.3% of Pi) for the Eucalyptus plantation and 878.1 mm and 101.1 mm (90.1% and 9.9% of Pi) for the savannah, respectively. The mean seasonal daily actual transpiration of the plantation and savannah was 3.6 and 2.6 mm day-1 during the rainy season (November to April) and 1.4 and 0.9 mm day-1 during the dry season (June to September); the mean seasonal daily actual evapotranspiration of the plantation and the savannah was 4.2 and 3.2 mm day-1 (rainy season) and 1.4 and 0.9 mm day-1 (dry season); the total seasonal actual evapotranspiration was 767.4 mm and 579.0 mm (rainy season) and 183.0 mm and 121.5 mm (dry season), with total annual respectively 1127.1 mm for a plantation (94.9% of Pi) and 820.7 mm for a savannah (69.1% of % of Pi). During the year transpiration/potential evapotranspiration ratio (T/Ep) is related to the soil-water depletion (T/Ep ratio was 0.79 and not reduced from field capacity (RFC=365 mm) until 65 % of RFC (255 mm), and then decreased quickly to near zero (0-0.2) at wilting point or 53.1% of RFC (RFC=328 mm). The drainage out of rooting depths of savannah has occured during the rainy season of 1997-1998 and 1998-1999, was of 827.3 mm, a total over 3 years (0, 389.7 and 437.6 mm); while of rooting depths of Eucalyptus plantation was of 470.3 mm (0, 207.1 and 263.2 mm), a difference in drainage between two ecosystems of 357.0 mm a total over these three years (0, 182.6 and 174.4 mm). The Eucalyptus plantation is the home-made ecosystem which takes up/transpires every day of year and uses all available water. The succession of several rain-deficient years will reduce the production from woods of the plantation, but knowing that between 1950 and 1998, there has been only an alone four episode successive rain-deficient years, and account held of the duration of rotation that are seven years, this « dry » episode does not compromise the surviving of the plantation, makes only reduce its wood production. The savannah has a cycle of vegetation such that at the end of the dry season (September), it remains the water in the rooting depths of savannah (15% of RAW), allowing to undergo three successive rain-deficient years without impact on its production.