Determination of subsurface stormflow using tracer method

Abstract

The state of Kerala in India is a typical example of a region facing droughts of varying degrees despite receiving high annual rainfall. This is because a lion share of the precipitation received is going out as runoff without recharging the groundwater. The subsurface stormflow is considered as the fast moving component of subsurface runoff on which very little information is available due to its complex mechanism of movement through subsurface. Hence, this study has been taken up for gathering more knowledge on the phenomenon of subsurface stormflow with the help of monitoring of soil moisture movement and tracer method. For data collection, three experimental setups in two experimental plots have been setup in KCAET campus, Kerala, India. It is a lateritic terrain having sandy loam type of soil and a general slope. Through-flow trenches were constructed for all the three setups and studied the soil moisture variation on the trench face at three depths (0-40 cm, 40-80 cm and 80-120 cm) in order to study the subsurface stormflow through different depths. For the experimental setup 1, simulation of rainfall was done using a butterfly sprinkler as the input. Line application of water was done for the experimental setup 2. Natural rain was taken as the input for the experimental setup 3. The subsurface stormflow through the soil matrix is also studied by the salt tracer experiment for obtaining its velocities. From the study it was found that the subsurface stormflow discharge is greatly affected by the soil dry density and soil suction. The subsurface stormflow discharge was found to have negative correlations with both soil suction and soil dry density. The subsurface stormflow velocities through 0-40 cm, 40-80 cm and 80-120 cm depths obtained from the soil moisture variation studies were 23.74 cm day -1 , 36.23 cm day -1 and 17.41 cm day -1 respectively and the same obtained through the tracer studies were 27.27 cm day -1 , 30 cm day -1 and 26.67 cm day -1 respectively. It was also concluded that the prevailing subsurface stormflow in the experimental sites is due to matrix flow rather than the preferential flow. The results gave the conclusion that high value of rainfall along with low values of soil dry density and soil suction can induce the subsurface stormflow even in the area having gentle slope (<10 %).