Establishing a water isotope framework for investigating ecosystem functioning and agricultural water use in central Kenya

Central Kenya is a region with a steep precipitation gradient, strong seasonality, and large inter-annual variability. Population and development pressures are raising questions about the sustainability of water supplies, necessitating a better basic understanding of regional hydrology. Here we present two years (2010-2012) of isotope data (δ²H, δ¹⁸O) for various water, soil and plant samples from Laikipia in Central Kenya. The samples were collected in support of evapotranspiration partitioning research at an eddy covariance flux tower located at Mpala Research Center. However, these results can be more widely useful in that they provide a framework for understanding short and long-term variability in the isotopic composition of various components of the hydrologic cycle. Through event-scale sampling of rain events, we have found a similar local meteoric water line, but with significantly more enriched precipitation compared with the long-term GNIP sites in Kenya. Boreholes, springs and streams were sampled periodically and reveal some distinct water sources in the region. Woody plant stem water isotopic composition tends to be more depleted than volume-averaged local rainfall, suggesting recharge in the root zone from highly depleted precipitation events. However, during the study period there was no significant correlation between isotopic composition and rain depth or rain rate.