Use of stable isotopes of water to determine the origin of water used by the vegetation and to partition evapotranspiration. A case study from HAPEX-Sahel

At one of the ‘Supersites’ within the HAPEX-Sahel exercise, soil, vegetation and atmospheric water vapour were sampled for measurement of the concentration of stable isotopes of water. The sampling took place at the beginning of the wet season. The stable isotope compositions, in conjunction with other physical measurements, were used to determine the mixing of rainfall with soil water, the point of extraction of water from the soil profile by the vegetation, and the relative importance of transpiration and evaporation from the soil surface. In the sandy soils of the area, water infiltration from rainfall into the upper parts of the soil profile was rapid. The perennial vegetation of the area, Guiera senegalensis, extracted the water from shallow depths of the soil profile, consistent with its extensive superficial root system. The transpiration from these plants contributed only about 20% of the total evapotranspiration.