Satellite Altimetry and Hydrologic Modeling of Poorly-Gauged Upper Mahakam Sub-Watershed in Indonesia

Abstract:

This study presents results of hydrologic monitoring of a poorly gauged Upper Mahakam Sub Watershed in Kalimantan, Indonesia, using satellite radar altimetry data and a rainfall-runoff model. The study area is part of Mahakam Watershed that drains rugged and rolling terrain of 20,000 km² dominated by rain forest with patchy farmland with precipitation of about 2,000 mm/year. The Hydrologic Engineering Corps - Hydrologic Modeling System (HEC-HMS) is used to simulate discharges using parameters determined from various geospatial data, including soil type, land cover and digital elevation model. Due to the limited in situ meteorological, water level and discharge data, a modified Thiessen polygon method is used to spatially model the Tropical Rainfall Measuring Mission (TRMM) data to match the location of field meteorological stations. The challenge for employing ESA’s Environmental Satellite (Envisat) altimeter includes the limited spatial and temporal resolutions, e.g. the narrower river width compared to the satellite's ground footprint and the 35 days repeat period for the altimeter ground track . To mitigate the spatial limitation, or tracker biases causing the radar altimeter return waveforms to deviate from the expected waveform model, we selected Envisat altimetry water level data based on standard over-water waveform shapes for each of the 18 Hz averaged return signals. Results indicate that the use of Envisat altimetry is a viable approach for estimating water level of medium-sized river (200–800 m width). In addition, contrary to results from previous studies, the Ice-1 waveform retracker is not necessarily the best among the four standard radar waveform retrackers for Envisat altimetry for this study region. Further, although there is good comparison between HEC-HMS simulated and observed discharges, results indicate that satellite altimetry provided better estimates of water level than those inferred from HEC-HMS simulated discharges and rating curves.