The necessity for monitoring the δ18O - δ2H relationship during tracer experiments carried out with deuterium under monsoon conditions in lands under flood irrigation

If environmental isotopes (18O or 2H) are used as artificial tracers in infiltration studies their natural variation can be established as an input function during the duration of the experiment. In on other study Stichler et al. proposed a mathematical formula to calculate the weighted input function of a mean constant infiltrating amount, which can be applied considerably well in temperate climates. Exemplarily will be shown that in areas with a monsoonal climate which underlay strong irrigation the establishment of such an isotope input function is not practicable. Here, the determination of the input function depends mainly on the mixing ratio of precipitation and irrigation water and on the permanently changing infiltration rates due to the irrigation. The investigation was carried out in Nepal at an experimental site that is characterised by a monsoon climate with a mean precipitation amount of 1250 mm/year, hereof 85% falling from May to September. The rain stable isotopic content falls, generalized, continuously from positive to negative values during the rainy season. The water for ponding and furrow irrigation is taken from a nearby river that shows a seasonal isotope variation with maximum in the end of May and minimum in November. The tracing experiment that aimed on the determination of the percolation flux for a transport study was carried out with 1 L 85% D2O on a 10m2 test field. The presented study shows that if deuterium is used under these conditions as an artificial tracer the results drawn from the tracer experiments may be misleading unless the deuterium excess is used as the parameter of interpretation instead of the δ2H-value. (author)