Estimating water fluxes in the critical zone using water stable isotope approaches in the Groundnut and Ferlo basins of Senegal

Abstract

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 (E a ), transpiration (T a ), 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 E a ); (ii) water and isotope mass balances (to partition E a and T a for groundnut and pasture); and (iii) the piston displacement method (for estimating R). E a losses derived from the isothermal evaporation model corresponded primarily to Stage II evaporation, and ranged from 0.02 to 0.09 mm d À1 in the Groundnut basin, versus 0.02-0.11 mm d À1 in Ferlo. At the groundnut site, E a rates ranged from 0.01 to 0.69 mm d À1 ; T a was in the range 0.55-2.29 mm d À1 ; and the T a /ET a ratio was 74%-90%. At the pasture site, the ranges were 0.02-0.39 mm d À1 for E a ; 0.9-1.69 mm d À1 for T a ; and 62-90% for T a /ET a . The ET a 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 previous validated HYDRUS-1D model. However, the HYDRUS-1D model gave a lower T a /ET a ratio (23.2%). The computed groundwater recharge for the groundnut site amounted to less than 2% of the local annual precipitation. Recommendations are made regarding protocols for preventing changes to isotopic compositions of water in samples that are collected in remote arid regions, but must be analysed days later. The article ends with suggestions for studies to follow up on evidence that local aquifers are being recharged via preferential pathways.

Toward a common methodological framework for the sampling, extraction, and isotopic analysis of water in the Critical Zone to study vegetation water use

“…One vertical profile may perhaps suffice in a flat grassland with a homogeneous soil and vegetation cover, but to characterize a steep, heterogeneous forest, multiple profiles should be collected from several locations. In many studies, samples are taken from at least 3–5 locations (e.g., under and in between trees, in uphill and downhill locations, or in wet and dry areas in the riparian zone; e.g., Diongue et al, 2023). In heterogeneous landscapes, the variability among soil profiles will be driven by factors such as the preferential entry of isotopically enriched stemflow around superficial roots, tree cover, aspect, microtopography, distance to a stream or a road (e.g., Goldsmith et al, 2018; Moreno‐Gutiérrez et al, 2012; Palacio et al, 2017; Pinos et al, 2022; Querejeta et al, 2021).…”