Jianbin Duan

Helmand River Hydrologic Studies Using ALOS PALSAR InSAR and ENVISAT Altimetry

The Helmand River wetland represents the only fresh-water resource in southern Afghanistan and one of the least mapped water basins in the world. The relatively narrow wetland consists of mostly marshes surrounded by dry lands. In this study, we demonstrate the use of the Advanced Land Observing Satellite (ALOS) Phased Array type L-band Synthetic Aperture Radar (PALSAR) Interferometric SAR (InSAR) to detect the changes of the Helmand River wetland water level. InSAR images are combined with the geocentric water level measurements from the retracked high-rate (18-Hz) Environmental Satellite (Envisat) radar altimetry to construct absolute water level changes over the marshes. It is demonstrated that the integration of the altimeter and InSAR can provide spatio-temporal measurements of water level variation over the Helmand River marshes where in situ measurements are absent.

Inter-annual Water Storage Changes in Asia from GRACE Data

Time-varying gravity field solutions from the Gravity Recovery and Climate Experiment (GRACE) satellite mission have been used to investigate the inter-annual changes of hydrologic water storage (∆ S) within Asia, focusing on the India–China–South Asia region. Instead of computing GRACE monthly ∆ S from geopotential coefficients, we choose to compute the annually averaged ∆ S before data smoothing, which improved the accuracy for the resulting inter-annual water storage changes. We then applied a novel method of decorrelation, filtering and land signal leakage reduction to the data, which yielded more accurate and higher spatial resolution (200 km or longer, half-wavelength) GRACE storage change observables over the study region. The technique provides a tool for future more in-depth studies of terrestrial hydrology in this region or globally. GRACE inter-annual variations in water storage change (2002–2007) exhibit large extremes over the region: droughts in Eastern China in 2004 and Indo-China and Bangladesh in 2005; and flooding in Indo-China and India in 2006. In general, GRACE inter-annual ∆ S variations have significantly larger amplitudes (increase or decrease of water storage change) than the values predicted by the Global Land Data Assimilation System (GLDAS) hydrologic model, and than the total precipitation observed by the Topical Rainfall Measurement Mission (TRMM).