Water fluxes partitioning in the critical zone based-stable isotope in Senegal’s Groundnut basin and Ferlo valley

Sustainable water management in semi-arid agriculture practices requires quantitative knowledge of water fluxes within the soil-vegetation-atmosphere system. Therefore, we used stable-isotope approaches to evaluate evaporation (Eₐ), transpiration (Tₐ), and groundwater recharge (R) at sites in Senegal's Groundnut basin and Ferlo Valley pasture region during the pre-monsoon, monsoon, and post-monsoon seasons of 2021. The approaches were based upon (i) the isothermal evaporation model (for quantifying Eₐ); (ii) water and isotope mass balances (to partition Eₐ and Tₐ for groundnut and pasture); and (iii) the piston displacement method (for estimating R). Eₐ losses derived from the isothermal evaporation model corresponded primarily to Stage II evaporation and ranged from 0.02–0.09 mm d^-¹ in the Groundnut basin, versus 0.02–0.11 mm d^-¹ in Ferlo. At the groundnut site, Eₐ rates ranged from 0.01 to 0.69 mm d¹ and Tₐ was in the range of 0.55–2.29 mm d^-¹. At the pasture site, the ranges were 0.02–0.39 mm d^-¹ for Eₐ and 0.9–1.69 mm d^-¹ for Tₐ. The ETₐ value derived for the groundnut site via the isotope approach was similar to those from eddy covariance measurements, and also to the results from the previously validated HYDRUS-1D model. However, the HYDRUS-1D model gave a lower Tₐ/ETₐ ratio (23.2%). The computed groundwater recharge for the groundnut site amounted to less than 2% of the local annual precipitation. The results are discussed, and suggestions are made for studies to follow up on evidence that local aquifers are being recharged via preferential pathways.

The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)