Guangcheng Hu

Monitoring of Evapotranspiration in a Semi-Arid Inland River Basin by Combining Microwave and Optical Remote Sensing Observations

As a typical inland river basin, Heihe River basin has been experiencing severe water resource competition between different land cover types, especially in the middle stream and downstream areas. Terrestrial actual evapotranspiration (ETa), including evaporation from soil and water surfaces, evaporation of rainfall interception, transpiration of vegetation canopy and sublimation of snow and glaciers, is an important component of the water cycle in the Heihe River basin. We developed a hybrid remotely sensed ETa estimation model named ETMonitor to estimate the daily actual evapotranspiration of the Heihe River basin for the years 2009–2011 at a spatial resolution of 1 km. The model was forced by a variety of biophysical parameters derived from microwave and optical remote sensing observations. The estimated ETa was evaluated using eddy covariance (EC) flux observations at local scale and compared with the annual precipitation and the MODIS ETa product (MOD16) at regional scale. The spatial distribution and the seasonal variation of the estimated ETa were analyzed. The results indicate that the estimated ETa shows reasonable spatial and temporal patterns with respect to the diverse cold and arid landscapes in the upstream, middle stream and downstream regions, and is useful for various applications to improve the rational allocation of water resources in the Heihe River basin.

Characterizing the Footprint of Eddy Covariance System and Large Aperture Scintillometer Measurements to Validate Satellite-Based Surface Fluxes

To validate satellite-based surface fluxes by ground measurements properly, several numerical simulations were carried out at a homogeneous alpine meadow site and mixed cropland site, considering various atmospheric conditions and different land cover distribution types. By comparing various pixel selection methods, the results showed that footprint was significant in insuring a consistent spatial scale between ground measurements and satellite-based surface fluxes, particularly for heterogeneous surface and high-resolution remote sensing data. Because large aperture scintillometer measurements cover larger areas than eddy covariance (EC) system measurements, the spatial heterogeneity at a subpixel scale in complicated surface should be further considered in validating coarse satellite data. Thus, more accurate validation data and scaling methods must be developed, such as measuring surface fluxes at the satellite pixel scale by a flux measurement matrix or airborne EC measurements.

Mapping of Interception Loss of Vegetation in the Heihe River Basin of China Using Remote Sensing Observations

Interception loss is an important component of the regional water balance for the Heihe River Basin which is an inland basin with limited precipitation. We used a modified Gash analytical model by combining remote sensing observations to estimate the interception loss of several vegetation types, e.g., grass, crop, forest and shrub for the years 2003-2012 in the Heihe River Basin. The estimated monthly interception ratio (in percent) was compared with field measurements made in Dayekou and Pailugou forest hydrology experimental sites and the results showed reasonable accuracy with RMSE of 5.0% and 4.3% at the two sites, respectively. The regional distribution of the interception loss showed strong spatial and temporal variability at monthly scale. At annual scale, the interception ratio could be treated as a stable indicator for long-term water balance research. The annual average interception loss is about 7.2% of gross rainfall for the vegetation covered area in the Heihe River Basin.

Estimation of Growing Season Daily ET in the Middle Stream and Downstream Areas of the Heihe River Basin Using HJ-1 Data

Spatial mapping of evapotranspiration (ET) is specifically critical for the semi-arid inland river basin with great heterogeneity in land-cover types. This letter estimates the spatial distribution of daily ET over the middle stream and downstream areas of the Heihe River Basin during the growing season of 2012 by using the Surface Energy Balance System algorithm with land surface temperature at high spatial resolution (300 m) derived from observations by the Chinese satellite HJ-1. The results demonstrate that ET estimates are consistent with ground-based measurements collected during the Heihe Watershed Allied Telemetry Experimental Research (HiWATER) with acceptable accuracy. The magnitude of daily ET in the downstream area is obviously lower than that in the middle stream area. Further analysis based on classification maps shows that there is significant temporal-spatial heterogeneity of daily ET over different land-cover surfaces and also within the same vegetation type. The temporal variation of ET in the middle stream area has clear seasonality with an obvious peak in July, whereas it is flat in the downstream area due to the dominating arid-region vegetation species and low soil water content in growing season. In addition, because of the abundant irrigation in the maize and irrigated orchard fields, the daily ET values of them are higher than that of wetland and even comparable with that of water surface in the middle stream area of the Heihe River Basin.

Evaluation of Harmonic Analysis of Time Series (HANTS): impact of gaps on time series reconstruction

In recent decades, researchers have developed methods and models to reconstruct time series of irregularly spaced observations from satellite remote sensing, among which the widely used Harmonic Analysis of Time Series (HANTS) method. Many studies based on time series reconstructed with HANTS documented the excellent performance of this method. While some limitations of HANTS have been noticed in these applications, there is no dedicated study on a systematic evaluation on the performance of the HANTS method. In this study, we evaluated the impact of gaps on the time series reconstruction of NDVI by HANTS. For global representativeness, a simulated NDVI time series dataset was constructed for four generic patterns and was applied as a reference dataset. Then random gaps were introduced into the reference series and both the reference and gapped series were reconstructed by harmonic analysis. The deviations between the two reconstructed results were used to evaluate statistically the accuracy of harmonic analysis under different gap conditions. The size of maximum gap (MGS), the number of loss (NL) and the number of gaps (NG) were selected to parameterize the gap distribution. The results showed that MGS, NL and NG were significant factors in the process of reconstruction and the two terminals and the peak of the series are crucial positions. MGS and NL should not be too large in the time series for all seasonal or non-seasonal case; otherwise the reconstructed series is not reliable. These conclusions can be taken as a reference to indicate the reliability of HANTS for particular cases towards the definition of a quality indicator of any time series.

Assessing the sensitivity of two new indicators of vegetation response to water availability for drought monitoring

Two new drought indicators based on satellite observations of vegetation index and land surface temperature, i.e. the Normalized Temperature Anomaly Index (NTAI) and the Normalized Vegetation Anomaly Index (NVAI) were applied to monitor drought events in different regions in China and India. We carried out this analysis for drought events with distinct duration, intensity and surface condition in 2006 in Sichuan-Chongqing, in 2009 in Inner-Mongolia (China) and in the Ganga basin (India) using the MODIS LST and NDVI data products and TRMM rainfall data for the period 2001 – 2010. Two newly proposed drought indicators NVAI and NTAI were evaluated against widely accepted indicators such as Precipitation Anomaly Percentage (PAP), Vegetation Condition Index (VCI) and Temperature Condition Index (TCI). The results show that NTAI and NVAI responded consistently to climate forcing. Long lasting rainfall anomalies led to severe drought and anomalies in rainfall, anomalies in NTAI appeared almost simultaneously and followed by negative anomaly in NVAI. The two new drought indicators NTAI and NVAI can distinguish the stages of drought evolution. The sensitivity of the indicators and of their anomalies to drought conditions and severity was also evaluated against drought assessments by operational drought monitoring services, documented how well the indicators meet expectations on the timely and reliable detection of environmental change.

Global evapotranspiration derived by ETMonitor model based on earth observations

Evapotranspiration (ET) is an important ecohydrological process especially in arid and semi-arid regions. In current study, a process-based model named ETMonitor was developed to estimate the ET, based mainly on the biophysical and hydrological parameters retrieved from satellite earth observations. And global daily ET from 2008 to 2012 with a spatial resolution of 1 km was estimated based on multi-source earth observations datasets. The estimated ET agreed well with the in situ observations at field scale, with R2 = 0.74, Bias = -0.05 mm d-1, RMSE = 0.87 mm d-1. The spatial patterns of estimated ET also agree well with the current available global ET products such as MOD16 and GLEAM. The ET products provide critical information on global terrestrial water and energy cycles and environmental change.

A new method to estimate changes in glacier surface elevation based on polynomial fitting of sparse ICESat—GLAS footprints

We present in this paper a polynomial fitting method applicable to segments of footprints measured by the Geoscience Laser Altimeter System (GLAS) to estimate glacier thickness change. Our modification makes the method applicable to complex topography, such as a large mountain glacier. After a full analysis of the planar fitting method to characterize errors of estimates due to complex topography, we developed an improved fitting method by adjusting a binary polynomial surface to local topography. The improved method and the planar fitting method were tested on the accumulation areas of the Naimona’nyi glacier and Yanong glacier on along-track facets with lengths of 1000 m, 1500 m, 2000 m, and 2500 m, respectively. The results show that the improved method gives more reliable estimates of changes in elevation than planar fitting. The improved method was also tested on Guliya glacier with a large and relatively flat area and the Chasku Muba glacier with very complex topography. The results in these test sites demonstrate that the improved method can give estimates of glacier thickness change on glaciers with a large area and a complex topography. Additionally, the improved method based on GLAS Data and Shuttle Radar Topography Mission-Digital Elevation Model (SRTM-DEM) can give estimates of glacier thickness change from 2000 to 2008/2009, since it takes the 2000 SRTM-DEM as a reference, which is a longer period than 2004 to 2008/2009, when using the GLAS data only and the planar fitting method.

Assessment of Water Use in Pan-Eurasian and African Continents by ETMonitor with Multi-Source Satellite Data

The Pan-Eurasian and African Continents are characterized by large ranges of climates varying from humid, semi-humid, semi-arid and arid regions, and great challenges exist in water allocation for different sectors that related to water resource and food security, which depends strongly on the water use information. Quantitative information on water use is also important to understand the effectiveness of water allocation and further to prevent from water stress resulted by drought in water-scarce regions. Explosive development of satellite remote sensing observations provide great chance to provide useful spatiotemporal information for quantifying the water use at regional to global scales. In this paper, a process-based model ETMonitor was used in combination with biophysical and hydrological parameters retrieved from earth observations to estimate the actual evapotranspiration, i.e. the agricultural and ecological water use. The total water use is also partitioned into beneficial part, e.g. plant transpiration, and non-beneficial part, e.g. soil evaporation and canopy rainfall interception, according to the water accounting framework. The estimated water use show good agreements with the ground observation, indicating the ability of ETMonitor for global and continental scale water use estimation. The spatial and temporal patterns of the water use in the Pan-Eurasian and African Continents were further analysed, while large spatial variation of water use was convinced. Current study also highlights the great capability of satellite observations in studying the regional water resource and continental water cycle.

Terrestrial water cycle in South and East Asia: Hydrospheric and cryospheric data products

The state of the land surface and the water cycle over the South and East Asia can be determined by space observation. New or significantly improved algorithms have been developed and evaluated against ground measurements. Variables retrieved include land surface properties, i.e. NDVI, LAI, FPAR, albedo, soil moisture, glacier and lake levels. Based on these biophysical parameters derived from microwave and optical remote sensing observations, a hybrid remotely sensed evapotranspiration (ET) estimation model named ETMonitor was developed and applied to estimate the daily actual ET of the Southeast Asia at a spatial resolution of 1 km. The changes in glaciers and lakes on the Tibetan Plateau, and the drainage links between glaciers and lakes are determined in this climate-sensitive region.