Environmental controls on stable isotope precipitation values over Mali and Niger, West Africa

This study evaluated the effects of the environmental factors precipitation amount (P), temperature (T), and vapor pressure (V (p)) on the stable isotope composition (delta) of precipitation and the excess-deuterium (d) parameter in Mali and Niger, West Africa. At both locations, delta values were greatly affected by seasonal climatic conditions and a statistically significant negative correlation was observed between delta and P. In Mali, a statistically significant negative correlation was also observed between delta and V (p). There was no statistically significant correlation between delta and T. Mean-weighting of delta values may be masking both dry-season and individual storm event precipitation phenomena, which are otherwise observed in arithmetically averaged delta values. Mean monthly d values for peak monsoonal months indicate that precipitation is first-fraction, gaining little recycled water on its journey inland from the Atlantic Ocean near the equator. Agreement between rainy-season precipitation and groundwater delta values, specifically peak monsoonal months, indicates that groundwater is being recharged by infiltration of current monsoonal rains and is not paleo-groundwater recharged under past climatic conditions. Similarity between rainy-season precipitation and groundwater d values provide another indication that monsoonal precipitation under current climatic conditions is the source of groundwater recharge. The knowledge that groundwater is recently recharged monsoonal rains, rather than paleo-groundwater, is important for evaluating groundwater sustainability and the effects that climate change will have on water resources. Understanding and predicting changes in precipitation amounts and intensities, which ultimately affect timing and amount of groundwater recharge, is critical for water resource management.