Rainfall characteristics (δ¹⁸O, δ²H, ΔT and ΔH_r) in western Africa: Regional scale and influence of irrigated areas

Variation in stable isotope ratios of monthly rainfall in the Douala and Yaounde cities, Cameroon: local meteoric lines and relationship to regional precipitation cycle

“…5 is in agreement with the so-called amount effect, i.e., the greater the volume of rainfall, the lower the d 18 O or dD content (Dansgaard 1964;Coplen et al 2000). The amount effect, which is a characteristic of tropical low-latitude precipitation (Dansgaard 1964), have also been observed in Cameroon (Njitchoua et al 1999;Kuitcha et al 2012;Wirmvem et al 2014;Kamtchueng et al 2015), western Africa (Mbonu and Travi 1994;Taupin et al 2000;Risi et al 2008;Adomako et al 2015), East Africa (Kebede and Travi 2012), Costa Rica (Sánchez-Murillo et al 2013) and the Bhagirathi River Basin in India (Rai et al 2013). …”

“…4b) representing the typical two rainy and dry seasons during a hydrological year in Yaounde. The annual V-shaped isotopic patterns in precipitation during each hydrological year are typical of precipitation in Cameroon (Njitchoua et al 1999;Wirmvem et al 2014;Kamtchueng et al 2015) and across the entire western Africa (Taupin et al 2000). In this section, the isotopic patterns have been used to describe the annual evolution of the regional precipitation cycle.…”

“…4a, b) are associated with the higher vertical velocity of ascending air masses and smaller effect of exchange between atmospheric air and the falling raindrops (Rozanski et al 1993). Meanwhile, enriched isotopic values at the edges of the rainy season are likely due to large exchange with atmospheric vapour, and partial evaporation of the rain drops during precipitation (Rozanski et al 1993;Taupin et al 2000;Gat 2010;Rai et al 2013). An apparent inverse relationship between rainfall depths and wd 18 O from Fig.…”

“…Different d-intercepts in the local meteoric water lines (Eqs. 4, 5) may be attributed to changing conditions at the source of atmospheric moisture (Ingraham 1998;Gonfiantini et al 2001) and local climatic effects such as re-evaporation (Taupin et al 2000;Liu et al 2014). The observed slopes are similar to those reported across Cameroon (Njitchoua et al 1999;Gonfiantini et al 2001;Fantong et al 2010;Ketchemen-Tandia et al 2013;Wirmvem et al 2014;Kamtchueng et al 2015) and elsewhere in the tropics (Mbonu and Travi 1994;Gonfiantini et al 2001;Sánchez-Murillo et al 2013, 2016Rai et al 2013).…”

“…They are also useful in climatological studies including the study of atmospheric moisture circulation (Njitchoua et al 1999;Taupin et al 2000;Bony et al 2008;Risi et al 2008). Few studies in the region including the Douala and Yaounde urban cities have reported the significance of continental recycled moisture to precipitation and evidence of the amount effect (Njitchoua et al 1999;Taupin et al 2000;Risi et al 2008;Kuitcha et al 2012;Ketchemen-Tandia et al 2013;Wirmvem et al 2014). However, an in-depth analysis of the relationship between isotopic variations in rainfall and the known regional annual cycle of precipitation has not been appraised.…”