Elephant trail runoff and sediment dynamics in northern Thailand

Although elephants may exert various impacts on the environment, no data are available on the effects of elephant trails on runoff, soil erosion, and sediment transport to streams during storms. We monitored water and sediment fluxes from an elephant trail in northern Thailand during seven monsoon storms representing a wide range of rainfall energies. Runoff varied from trivial amounts to 353 mm and increased rapidly in tandem with expanding contributing areas once a threshold of wetting occurred. Runoff coefficients during the two largest storms were much higher than could be generated from the trail itself, implying a 4.5- to 7.9-fold increase in the drainage areas contributing to storm runoff. Clockwise hysteresis patterns of suspended sediment observed during most storms was amplified by a "first flush" of sediment early on the hydrograph in which easily entrained sediment was transported. As runoff areas expanded during the latter part of large storms, discharge increased but sediment concentrations declined. Thus, sediment flux was better correlated to kinetic energy of rainfall on the falling limbs of most storm hydrographs compared to rising limbs. Based on a power function relationship between sediment flux and storm kinetic energy, the estimated annual sediment yield from the trail for 135 storms in 2005 was 308 to 375 Mg ha(-1) yr(-1), higher than from most disturbed land surfaces in the tropics. The eight largest storms (30% of total storm energy) in 2005 transported half of the total annual sediment. These measurements together with site investigations reveal that highly interconnected elephant trails, together with other source areas, directly link runoff and sediment to streams.