Sediment yield from gullies, riparian mass wasting and bank erosion in the Upper Konto catchment, East Java, Indonesia

Upland watershed rehabilitation programmes in Indonesia have faced increased scrutiny for not delivering the desired reductions in downstream sedimentation rates. Partly this reflects the fact that conservation measures have not been widely adopted or maintained by upland farmers, mainly for socio-economic reasons. Another potential explanation is that sediment contributions by gullying, (riparian) mass wasting and bank erosion have been seriously underestimated or even ignored. This paper presents estimates of sediment contributions by gullies, riparian mass wasting and bank erosion in the upland volcanic Konto catchment, East Java. Runoff and sediment yield from gullies were studied in two areas with contrasting soils and land use. Gullies in the Maron area (few gullies, Andic Cambisols, maize and rice cultivation on stable broad-based terraces) were related to improper drainage of trails, roads and yards. In the Binangsri area (more widespread gullying, Eutric Cambisols, onion cultivation on forward-sloping terraces), gullying was further enhanced by the practice of downslope furrowing to promote field drainage. Estimated annual sediment yields from the two areas were strikingly different at 22–26 and 50–87 Mg ha− 1, respectively. Riparian mass wasting was estimated to contribute ca. 4% of total sediment yield at Maron and 8–19% in the main gully system at Binangsri, with the higher value in the latter case representing the effect of extreme rainfall in the latter half of the rainy season. Short-term wet season rates of gully wall retreat at Binangsri suggested a contribution by bank erosion of ca. 3% (8% including extreme events). As such, 11–27% of the annual sediment yield at Binangsri was estimated to have come from sources other than surface erosion. Substantial volumes of sediment (29–107 Mg km− 1 of river length) were also added to streams bordered by irrigated rice fields (sawah) in non-gullied areas, mainly through the collapse of the lowermost terraces due to undercutting.