Interannual variability of spring and summer monsoon growing season carbon exchange at a semiarid savanna over nearly two decades

•Large interannual variability of ecosystem carbon fluxes at a semiarid savanna site.•Flux variability governed by water availability metrics like precipitation and soil moisture.•Positive trend in 19-year soil moisture and carbon uptake record.•Models and satellites often capture less than half the variability in measured fluxes.