Doug Alsdorf

Modeling large‐scale inundation of Amazonian seasonally flooded wetlands

This paper presents the first application and validation of a 2D hydrodynamic model of the Amazon at a large spatial scale. The simulation results suggest that a significantly higher proportion of total flow is routed through the floodplain than previously thought. We use the hydrodynamic model LISFLOOD-FP with topographic data from the Shuttle Radar Topography Mission to predict floodplain inundation for a 240 × 125 km section of the central Amazon floodplain in Brazil and compare our results to satellite-derived estimates of inundation extent, existing gauged data and satellite altimetry. We find that model accuracy is good at high water (72% spatial fit; 0.99 m root mean square error in water stage heights), while accuracy drops at low water (23%; 3.17 m) due to incomplete drainage of the floodplain resulting from errors in topographic data and omission of floodplain hydrologic processes from this initial model.

Water level changes in a large Amazon lake measured with spaceborne radar interferometry and altimetry

We demonstrate that interferometric processing of JERS-1 SAR data over an Amazon lake containing ∼1500 islands yields centimeter-scale changes in the height of the water surface from February 14 to March 30, 1997. For the method to work, we qualitatively find that inundation of about one or two leafless trees per 25 m² multi-look SAR pixel is sufficient to return the radar pulse to the side-looking antenna. Validation is provided by multi-temporal TOPEX-POSEIDON altimetry profiles, which directly measure surface heights relative to a fixed datum. Because SAR provides an image, the water height changes (∼12 cm) can be converted to a net volume measurement (280 million m³) over the 44 days separating the JERS-1 acquisitions. Compared to historical gauge records, removal of this volume from the lake required a ∼50% greater flow.

Preliminary Characterization of SWOT Hydrology Error Budget and Global Capabilities

River discharge and lake water storage are critical elements of land surface hydrology, but are poorly observed globally. The Surface Water and Ocean Topography (SWOT) satellite mission will provide high-resolution measurements of water surface elevations with global coverage. Feasibility studies have been undertaken to help define the orbit inclination and repeat period. Preliminary error budgets have been computed for estimating instantaneous and monthly river discharge from SWOT measurements (errors are assumed uncorrelated). Errors on monthly discharge due to SWOT temporal sampling were estimated using gauges and their observation times for two SWOT orbits with different inclinations (78° and 74°). These errors have then been extrapolated to rivers globally. The 78° and 74° orbital inclinations allow a good sampling frequency, avoid tidal aliasing and cover almost all the continental surface. For a 22-day repeat orbit, a single point at 72°N is sampled 11 and 16 times during one repeat period for the 78° and 74° inclination orbit, respectively. Errors in instantaneous discharge are below 25% for rivers wider than 50 m (48% of all rivers). Errors in monthly discharge are below 20% for rivers with drainage areas larger than 7000 km2 (34% of all rivers). A rough estimate of global lake storage change has been computed. Currently, available satellite nadir altimetry data can only monitor 15% of the global lake volume variation, whereas from 50% to more than 65% of this variation will be observed by SWOT, thus providing a significant increase in our knowledge of lake hydrology.

The need for global, satellite-based observations of terrestrial surface waters

River discharge as well as lake and wetland storage of water are critical terms in the surface water balance, yet they are poorly observed globally and the prospects for improvement from in-situ networks are bleak [e.g.,Shiklomanov et al., 2002; IAHS, 2001;Stokstad, 1999]. Indeed, given our basic need for fresh water, perhaps the most important hydrologic observations that can be made in a basin are of the temporal and spatial variations in discharge. Gauges measuring discharge rely on flow converging from the upstream catchment to a singular in-channel cross section. This approach has successfully monitored many of the worlds densely inhabited and typically heavily engineered basins for well over a century. However, much of the globally significant discharge occurs in sparsely gauged basins, many with vast wetlands that lack flow convergence (e.g., Figures 1 and 2); thus leading to poorly defined values of runoff at local, regional, and continental scales.

Tibetan satellite magnetic low: Evidence for widespread melt in the Tibetan crust?

The Tibetan plateau is associated with a pronounced satellite magnetic low. Forward modeling of this anomaly demonstrates that the crust underlying the Tibetan plateau is weakly magnetic compared to the crust on either side, and that the boundaries of the weakly magnetic region coincide closely with the topographic edges of the plateau (Himalaya, Kun Lun). Because there are no obvious changes in the bulk magnetization of the crust associated with the principal terrane boundaries within the plateau (Banggong and Yarlung-Zangbo sutures), the low likely manifests hot crust beneath Tibet. Forward modeling of the magnetic anomaly with simple assumptions suggests that the Curie isotherm is likely to reside in the upper crust across the Tibetan plateau (∼15 km depth), implying that granite minimum melt temperature (∼600–650 °C) is also achieved in the upper crust across the plateau (∼16–18 km depth). This inference is consistent with the shallow depth extent of earthquakes in Tibet, and with the recent suggestion that melt may be areally widespread within the Tibetan crust.

Inroads of remote sensing into hydrologic science during the WRR era

Author(s): Lettenmaier, DP; Alsdorf, D; Dozier, J; Huffman, GJ; Pan, M; Wood, EF | Abstract:

Global Distribution of Outbreaks of Water-Associated Infectious Diseases

Background Water plays an important role in the transmission of many infectious diseases, which pose a great burden on global public health. However, the global distribution of these water-associated infectious diseases and underlying factors remain largely unexplored. Methods and Findings Based on the Global Infectious Disease and Epidemiology Network (GIDEON), a global database including water-associated pathogens and diseases was developed. In this study, reported outbreak events associated with corresponding water-associated infectious diseases from 1991 to 2008 were extracted from the database. The location of each reported outbreak event was identified and geocoded into a GIS database. Also collected in the GIS database included geo-referenced socio-environmental information including population density (2000), annual accumulated temperature, surface water area, and average annual precipitation. Poisson models with Bayesian inference were developed to explore the association between these socio-environmental factors and distribution of the reported outbreak events. Based on model predictions a global relative risk map was generated. A total of 1,428 reported outbreak events were retrieved from the database. The analysis suggested that outbreaks of water-associated diseases are significantly correlated with socio-environmental factors. Population density is a significant risk factor for all categories of reported outbreaks of water-associated diseases; water-related diseases (e.g., vector-borne diseases) are associated with accumulated temperature; water-washed diseases (e.g., conjunctivitis) are inversely related to surface water area; both water-borne and water-related diseases are inversely related to average annual rainfall. Based on the model predictions, “hotspots” of risks for all categories of water-associated diseases were explored. Conclusions At the global scale, water-associated infectious diseases are significantly correlated with socio-environmental factors, impacting all regions which are affected disproportionately by different categories of water-associated infectious diseases.

Characterization of surface water storage changes in Arctic lakes using simulated SWOT measurements

The planned Surface Water and Ocean Topography (SWOT) satellite mission will measure freshwater storage changes in global lakes. Herein, the anticipated SWOT storage change accuracy is evaluated for the lakes in the Peace-Athabasca Delta, Northern Alaska and Western Siberia. Because of the significant lack of Arctic lake measurements, we simulated realistic daily to seasonal changes in water elevations in the study region using a combination of data from lake gauges, satellite radar altimeter, and satellite imagery. This ‘truth’ dataset is sampled with several candidate SWOT orbits and then corrupted with expected instrument errors to simulate SWOT observed storage changes. The number of revisits increases with increasing or decreasing latitude for a given repeat cycle (e.g. four to eight revisits for a 22-day cycle), allowing us to investigate storage change errors at monthly sampling. SWOT storage change accuracy is primarily controlled by lake size. Lakes larger than 1 km2 have relative errors generally less than 5% whereas one-hectare size lakes are about 20%. We concluded that the storage change accuracy is insensitive to the orbital inclination or repeat periods, but is sensitive to lake shapes.

Repeat-pass multi-temporal interferometric SAR coherence variations with Amazon floodplain and lake habitats

We have analysed interferometric coherence variations in Japanese Earth Resources Satellite (JERS-1) L-band synthetic aperture radar (SAR) data at three central Amazon sites: Lake Balbina, Cabaliana and Solimões-Purús. Because radar pulse interactions with inundated vegetation typically follow a double-bounce travel path that returns energy to the antenna, coherence will vary with vegetation type as well as with physical and temporal baselines. Lake Balbina consists mostly of upland forests and inundated trunks of dead, leafless trees whereas Cabaliana and Solimões-Purús are dominated by flooded forests. Balbina has higher coherence values than either Cabaliana or Solimões-Purús probably because the dead, leafless trees support strong double-bounce returns. The mean coherences of flooded woodland are 0.28 in Balbina and 0.11 in both Cabaliana and Solimões-Purús. With increasing temporal baselines, flooded and nonflooded wetland habitats show a steadily decreasing trend in coherence values whereas terra-firme and especially open-water habitats have little variation and remain lower in value. Flooded and nonflooded wetland coherence varies with the season whereas terra-firme and open water do not have similarly evident seasonal variations. For example, flooded habitats in all three study regions show annual peaks in coherence values that are typically 0.02 greater than coherence values from temporal baselines 180 days later, yet open water shows no variation with time. Our findings suggest that, despite overall low coherence values, repeat-pass interferometric coherence of flooded habitats is capable of showing the annual periodicity of the Amazon flood wave.

Capability of SRTM C- and X-band DEM Data to Measure Water Elevations in Ohio and the Amazon

We analyze Shuttle Radar Topography Mission (SRTM) water surface elevation data to assess the capacity of interferometric radar for future surface water missions. Elevations from three Ohio reservoirs and several Amazon floodplain lakes have standard deviations, interpreted as errors, that are smaller in C-band compared to X-band and are smaller in Ohio than in the Amazon. These trends are also evident when comparing water surface elevations from the Muskingum River in Ohio with those of the Amazon River. Differences are attributed to increased averaging in C-band compared to X-band, greater sensitivity to surface water motion in X-band, and generally larger off-nadir look angles in X-band. Absolute water surface elevations are greater in the C-band DEM for much of the two study areas and yield expected slope values. Height and slope differences are attributed to differing usage of geoids and ellipsoids. These SRTM measurements suggest the great possibility for space-based, laterally-spatial (2D) measurements of water surface elevations.