Soil moisture controls on canopy-scale water and carbon fluxes in an African savanna

Tower-based measurements of mass and energy exchanges at the end of the growing season in central Botswana were used to evaluate functional relationships commonly applied to predict water and carbon fluxes between savanna landscapes and the atmosphere. Following a large rainfall event, daily evapotranspiration (ETdaily) exhibited an exponential decay consistent with a derived analytical expression based on critical and wilting-point soil moisture limits for savanna vegetation native to the study region. A piecewise linear soil moisture limitation function provided good estimates of ETdaily as a function of potential evapotranspiration and soil moisture (R2 = 0.92). Comparison of a soil moisture mass balance with measured ETdaily indicated deeper root water uptake at a site with more woody vegetation compared with a grass-dominated site. Linear correlation (R2 = 0.90) of daytime CO2 flux and evapotranspiration supported a constant water use efficiency to estimate carbon fluxes from water fluxes. Daytime and nighttime CO2 fluxes responded similarly to soil drying, enabling estimation of total daily CO2 flux from ETdaiiy. These experimental results support a simple model of savanna land-atmosphere exchange over interstorm periods