Estimating soil wetness using satellite data

Abstract

Improved estimates of soil wetness were obtained using observations from both the NIMBUS-7 Scanning Multichannel Microwave Radiometer (SMMR) and the NOAA-7 Advanced Very High Resolution Radiometer (AVHRR). SMMR 6.6 GHz frequency, horizontal polarization, brightness temperature (T_BH) was first correlated with soil wetness, as computed using an Antecedent Precipitation Index (API) model, for a number of SMMR ground resolution areas involving a fairly wide range of vegetation densities. The API generally accounted for more than 70 per cent of the observed temporal variability in T_BH, with linear correlations being significant at the 1 per cent level. The regression slope of T_BH versus API correlated well, at the 1 per cent level, with a vegetation index derived from AVHRR visible and near-infrared observations. The regression intercept was found to correlate less satisfactorily, but was significant at the 5 per cent level. These linear regression results were used to develop a diagnostic model for soil wetness using SMMR and AVHRR data only. The model was found to be useful in describing four levels of soil wetness as compared to three levels when vegetation was not considered.     

A Review of: “Environmental Information Management and Analysis: Ecosystem 10 global scales”. Edited by W. K. MICHENER,J. W. BRUNT and S. G. STAFFORD (London: Taylor &: Francis Ltd, 1994). [Pp. 535.] Price £55,00.

Abstract

Satellite indices of vegetation from the Australian continent were calculated from May 1986 to April 1987 from NOAA-9 AVHRR (Advanced Very High Resolution Radiometer) and Nimbus-7 SMMR (Scanning Multichannel Microwave Radiometer) satellite data. The visible (VIS) and near infrared (N1R) reflectances and their combination, the Normalized Difference (ND) Vegetation Index were calculated from the AVHRR sensor. From the SMMR, the microwave Polarization Difference (PD) was calculated as the difference between the vertically and horizontally polarized brightness temperatures at 37 GHz. The AVHRR data were gridded to match the 25 km spatial resolution of the SMMR 37 GHz data and both data sets were analysed to provide a temporal resolution of one month. Using a one month lag, the ND, PD, VIS and NIR, indices were plotted against rainfall and water balance estimates of evaporation, calculated using the monthly rainfall data and long term averages of pan evaporation from 74 locations covering a range of vegetation types. The monthly plots had wide scatter. This scatter was reduced markedly by aggregating the data over twelve months, leading to the conclusion that direct satellite monitoring of annual evaporation across the Australian continent using PD or VIS is feasible for areas with evaporation less than 600 mm y^?1. The ND relationship was limited by scatter and the PD and VIS relationships by their saturation above 600 mm y^?1, which spanned about two-thirds of the continental range studied. Scatter was reduced and ND had a predictive range above 600 mm y^?1 if evaporation was normalized by evaporative demand. But prior knowledge of potential evaporation is needed in this approach. The NIR reflectance of forests were consistently lower than neighbouring areas of agriculture, thus ND may underpredict the evaporation of forests relative to agriculture. Temporal resolution of the satellite indices over periods of one month could not be evaluated due to spatial and temporal variability of climatic and biological factors not accounted for in the water balance estimates of evaporation.     

Interpretation of Nimbus-7 37 GHz microwave brightness temperature data in semi-arid southern Africa

Abstract

Monthly 37 GHz microwave polarization difference temperatures (MPDT) derived from the Nimbus-7 scanning multichannel microwave radiometer (SMMR) for southern Africa from 1979 to 1985 are compared with rainfall and Advanced Very High Resolution Radiometer (AVHRR) normalized difference vegetation index (NDVI) data. MPDT rose sharply during a drought episode which occurred within the period included in the data. The rise was seen not only in the growing season, but also in the dry season MPDT when no actively photosynthetic, water-containing leaves are present. The results suggest that scattering of the emitted microwave radiation by dead and living vegetation is a more important factor than has previously been recognized. The sensitivity of MPDT to small quantities of dry vegetation encourages the hope that standing dead vegetation and plant litter may be remotely sensed. In the absence of vegetation, rough terrain reduced the MPDT whereas a damp surface increased it.     

Quantifying spatial and temporal variabilities of microwave brightness temperature over the U.S. Southern Great Plains

Abstract

Abstract. Spatial and temporal variabilities of microwave brightness temperature over the U.S. Southern Great Plains are quantified in terms of vegetation and soil wetness. The brightness temperatures (T_Brpar; are the daytime observations from April to October for 5 years (1979 to 1983) by NIMBUS-7 Scanning Multichannel Microwave Radiometer (SMMR) at 6-6GHz frequency, horizontal polarization. The spatial and temporal variabilities of vegetation are assessed using visible and near-infrared observations by NOAA-7 Advanced Very High Resolution Radiometer (AVHRR), while an Antecedent Precipitation Index (API) model is used for soil wetness. The API model was able to account for more than 50 per cent of the observed variability in T_B although linear correlations between TB and API were generally significant at the I per cent level. The slope of the linear regression between TE and API is found to correlate linearly with an index for vegetation density derived from AVHRR data.     

Monitoring arid lands using AVHRR-observed visible reflectance and SMMR-37 GHz polarization difference

Abstract

Previous studies have shown the usefulness of visible reflectance observed by varied space-borne sensors for monitoring arid and semi-arid regions of the world, with particular reference to desertification. Visible reflectance along a transect through the Sahel and Sudan zones of Africa has been derived from observations by the advanced very high resolution radiometer (AVHRR) on board the NOAA-7 and NOAA-9 satellites and compared with concurrent observations of the 37 GHz polarization difference by the scanning multichannel microwave radiometer (SMMR) on board the Nimbus-7 satellite. The study period was January 1982 to December 1986, which included an unprecedented drought during 1984 over the Sahel zone. While spatial and temporal patterns of these two data sets are found to be highly correlated, there are also quantitative differences which need to be understood.     

Comparing SMMR and AVHRR data for drought monitoring

Abstract

Coincident Scanning Microwave Multi-channel Radiometer 37 GHz and Advanced Very High Resolution Radiometer normalized difference vegetation index satellite data have been compared from drought-affected regions of sub-Saharan Africa and northeastern Brazil for the time period of 1980–1985. Although the two satellite data types can be highly correlated, differences between them were found for the Sahel zone in 1985 and for northeastern Brazil from 1984–1985. These findings suggest that scattering or surface roughness contributions may be greater than previously assumed for the 37 GHz microwave data. A programme of field measurements should be undertaken to increase our understanding of natural vegetation at 37 GHz and higher microwave frequencies.     

Merging AVHRR and SMMR data for remote sensing of ice and cloud in polar regions

Abstract

Multispectral data from the Advanced Very High Resolution Radiometer (AVHRR) were digitally processed and merged with Scanning Multichannel Microwave Radiometer (SMMR) imagery. Five channels of AVHRR data, four channels of SMMR brightness temperatures and SMMR-derived ice concentration and ice type were co-registered to a polar stereographic grid. The merged data sets are currently being used in combination with meteorological information for integrated studies of clouds and sea ice.     

Observation of hydrological features with Nimbus-7 37 GHz data,applied to South America

Abstract

The microwave polarization difference temperature (MPDT) prepared from 37 GHz Scanning Multifrequency Microwave Radiometer (SMMR) data from the Nimbus-7 satellite allows a unique vision of hydrological features, especially of humid areas. From the series of monthly images from 1979 to 1985 the major hydrological features of four major river basins of South America are briefly examined. These include rivers of the Amazon, La Plata, Orinoco and Sao Francisco basins, wetlands in the La Plata and Amazon basins and floods in the La Plata basin. Many hydrological features appear much as they do on conventional maps. An interesting perspective can be derived from the ways in which they differ from conventional maps.     

Satellite observed seasonal and inter-annual variation of vegetation over the Kalahari,The Great Victoria Desert,and The Great Sandy Desert: 1979–1984

We present time-series observations by two spaceborne sensors over three desert regions, the Kalahari (in southern Africa), and the Great Victoria Desert and the Great Sandy Desert (in western Australia). The observations are by the Advanced Very High Resolution Radiometer on board the NOAA-7 satellite from April 1982 to December 1984, and by the Scanning Multichannel Microwave Radiometer on board the Nimbus-7 satellite from January 1979 to February 1985. The objective was to compare and contrast seasonal and interannual variation of vegetation over these three deserts using the normalized difference vegetation index and the 37 GHz brightness temperature. The seasonal variation from both sensors was found to be most pronounced over the Kalahari, followed by the Great Sandy Desert and the Great Victoria Desert. The normalized difference vegetation index was roughly identical over the two Australian deserts and was significantly higher for the Kalahari. Additionally, there was no consistent change from both sensors over the two Australian deserts, but a consistent decrease from 1979 to 1984 over the Kalahari was found in the 37 GHz microwave data.     

Multi-year sea ice mapping by thermal infrared radiometry

Marine operations in polar and subpolar regions rely on accurate sea ice information for operational planning purposes. Before venturing into operations, however, mapping of the prevailing sea ice conditions are important to feasibility analyses and planning of the operation. Multi-year sea ice information is often derived from passive microwave radiometers such as the Special Sensor Microwave Imager (SSM/I) on board the U.S. Defense Meteorological Satellite Program (DMSP) satellites, or the Scanning Multichannel Microwave Radiometer (SMMR) on board the Nimbus-7 satellite. These sources provide wide aerial coverage and all weather capability, but offer only low spatial resolution, 30 km. In contrast, the thermal infrared channel of the Advanced Very High Resolution Radiometer (AVHRR) on board the NOAA satellites, provides a 1 1km spatial resolution at nadir with a reasonable cost. A technique for extraction of multi-year sea ice information from thermal infrared AVHRR data was thus created. It relies on surface temperature differences between first-year and multi-year sea ice. Adjustments of the absolute concentration levels were made based on a regression relation between AVHRR and SMMR based multi-year sea ice concentrations. The technique is inapplicable during periods of dark, cloud cover, or melting conditions.     

Estimates of primary productivity over the Thar Desert based upon Nimbus-7 37 GHz data: 1979-1985

Abstract

An empirical relationship is developed between the difference of vertically and horizontally polarized brightness temperatures (ΔT) observed at 37 GHz frequency of the Scanning Multichannel Microwave Radiometer (SMMR) on board the Nimbus-7 satellite and primary productivity over hot arid and semi-arid regions of Africa and Australia. The primary productivity values for this empirical relationship are calculated according to Leith (1975). The empirical relationship is then used, together with the observed ΔT, to estimate primary productivity over a roughly 60000 km² area in the Thar Desert from 1979 to 1985. The highest areally averaged productivity of 0.368kgm^?2 yr^?1 is estimated for 1979, while the lowest 0.222kgm^?2 yr^?1 for 1985; the average for the 7 years being 0.271 kgm^?2 yr^?1. The spatial variability of the productivity values within the study area is found to be quite substantial; the standard deviation about the mean being about 008kgm^?2yr^?1.     

Atmospheric effects on SMMR and SSM/1 37 GHz polarization difference over the Sahel

Abstract

The difference of vertically and horizontally polarized brightness temperatures (referred to here as the polarization difference, δT) observed at 37GHz frequency of the scanning multi-channel microwave radiometer (SMMR) on board the Nimbus-7 satellite and special sensor microwave imager (SSM/I) on board the DMSP-F8 satellite could provide useful information about land surface change within the span of these global observations, November 1978 to August 1987 for SMMR and July 1987 to present for SSM/I. The atmospheric effects on the δT are studied over two 2-5° by 2-5° regions within the Sahel and Sudan zones or Africa from January 1985 to December 1986 through radiative transfer analysis using surface temperature, atmospheric water vapour and cloud optical thickness developed under the International Satellite Cloud Climatology Project (ISCCP). The atmospheric effects are also studied using surface observations of air temperature and vapour pressure at Niamey (13-5° N, 2-2° E) for the period January 1979 to December 1990. It is found that atmospheric effects alone cannot explain the observed temporal variation of δT, although the atmosphere introduces important modulations on the observed seasonal variations of δT due to rather significant seasonal variation of precipitable water vapour. Therefore, these δT data should be corrected for atmospheric effects before any quantitative analysis of land surface change over the Sahel and Sudan zones. The entire global data set from December 1978 to December 1990 has been archived for unrestricted distribution and use.     

Monitoring the phenology of Tunisian grazing lands

Abstract

Normalized difference vegetation index (NDVI) data obtained from the Advanced Very High Resolution Radiometer (AVHRR) on board NOAA-9 have been analysed to assess their utility for monitoring the vegetation of Tunisian grazing lands. Preliminary analysis shows that the NDVI provides a sensitive indicator of monthly variations in biomass which correlate with spatial and temporal changes in growing conditions. Investigations suggest that the percentage contribution of the soil background to total recorded reflectance, provides an important limiting factor to the sensitivity of the NDVI, creating a threshold beyond which the accuracy of this index becomes less reliable.     

Evapotranspiration estimates using NOAA AVHRR imagery in the Pampa region of Argentina

We used multiple regression analysis to relate evapotranspiration (ET), computed from a water balance technique, to both thermal infrared and normalized difference vegetation index data obtained from the Advanced Very High Resolution Radiometer (AVHRR) sensor on board on the National Oceanic and Atmospheric Administration (NOAA) satellite. This approach, based on only remotely sensed data, provided a reliable estimate of ET over the Pampas, the main agricultural region of Argentina. The relationship between spectral data and ET was more sensitive to the dates than to the sites used to generate the models.     

Attribution of divergent northern vegetation growth responses to lengthening non-frozen seasons using satellite optical-NIR and microwave remote sensing

The non-frozen (NF) season duration strongly influences the northern carbon cycle where frozen (FR) temperatures are a major constraint to biological processes. The landscape freeze-thaw (FT) signal from satellite microwave remote sensing provides a surrogate measure of FR temperature constraints to ecosystem productivity, trace gas exchange, and surface water mobility. We analysed a new global satellite data record of daily landscape FT dynamics derived from temporal classification of overlapping SMMR and SSM/I 37 GHz frequency brightness temperatures (T_b). The FT record was used to quantify regional patterns, annual variability, and trends in the NF season over northern (≥45°N) vegetated land areas. The ecological significance of these changes was evaluated against satellite normalized difference vegetation index (NDVI) anomalies, estimated moisture and temperature constraints to productivity determined from meteorological reanalysis, and atmospheric CO₂ records. The FT record shows a lengthening (2.4 days decade^?1; p < 0.005) mean annual NF season trend (1979–2010) for the high northern latitudes that is 26% larger than the Northern Hemisphere trend. The NDVI summer growth response to these changes is spatially complex and coincides with local dominance of cold temperature or moisture constraints to productivity. Longer NF seasons are predominantly enhancing productivity in cold temperature-constrained areas, whereas these effects are reduced or reversed in more moisture-constrained areas. Longer NF seasons also increase the atmospheric CO₂ seasonal amplitude by enhancing both regional carbon uptake and emissions. We find that cold temperature constraints to northern growing seasons are relaxing, whereas potential benefits for productivity and carbon sink activity are becoming more dependent on the terrestrial water balance and supply of plant-available moisture needed to meet additional water use demands under a warming climate.     

Evaluation of MODIS and NOAA AVHRR vegetation indices with in situ measurements in a semi‐arid environment

Much effort has been made in recent years to improve the spectral and spatial resolution of satellite sensors to develop improved vegetation indices reflecting surface conditions. In this study satellite vegetation indices from the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Advanced Very High Resolution Radiometer (AVHRR) are evaluated against two years of in situ measurements of vegetation indices in Senegal. The in situ measurements are obtained using four masts equipped with self‐registrating multispectral radiometers designed for the same wavelengths as the satellite sensor channels. In situ measurements of the MODIS Normalized Difference Vegetation Index (NDVI) and AVHRR NDVI are equally sensitive to vegetation; however, the MODIS NDVI is consistently higher than the AVHRR NDVI. The MODIS Enhanced Vegetation Index (EVI) proved more sensitive to dense vegetation than both AVHRR NDVI and MODIS NDVI. EVI and NDVI based on the MODIS 16‐day constrained view angle maximum value composite (CV‐MVC) product captured the seasonal dynamics of the field observations satisfactorily but a standard 16‐day MVC product estimated from the daily MODIS surface reflectance data without view angle constraints yielded higher correlations between the satellite indices and field measurements (R ² values ranging from 0.74 to 0.98). The standard MVC regressions furthermore approach a 1?:?1 line with in situ measured values compared to the CV‐MVC regressions. The 16‐day MVC AVHRR data did not satisfactorily reflect the variation in the in situ data. Seasonal variation in the in situ measurements is captured reasonably with R ² values of 0.75 in 2001 and 0.64 in 2002, but the dynamic range of the AVHRR satellite data is very low—about a third to a half of the values from in situ measurements. Consequently the in situ vegetation indices were emulated much better by the MODIS indices than by the AVHRR NDVI.     

Satellite and ground-based pasture production assessment in Niger: 1986-1988

Abstract

The standing crop of herbaceous biomass produced during the 2-4?month summer rainy season by the annual grasses in the Sahel zone provides an indication of resource availability for livestock for the following 9-month dry season. Combined use of NOAA advanced very high resolution radiometer (AVHRR) local area coverage (LAC) satellite data and biomass data, obtained through vegetation sampling of 25-100 km₂ areas, allowed the development of a method for biomass assessment in Niger. Vegetation sampling involved both visual estimates and clipped plots (double sampling). The relationship between time-integrated normalized difference vegetation index (NDVI) statistics derived from NOAA AVHRR LAC data (dependent variable) and total herbaceous biomass (independent variable) was obtained through regression analysis. An inverse prediction was used to estimate biomass from the satellite data. Biomass maps and statistics of the grasslands were produced for the end of each rainy season: 1986, 1987 and 1988. This information is being used for planning purposes by the pastoral resource managers of the Government of Niger.     

Assessment of ecological conditions associated with the 1980/81 desert locust plague upsurge in West Africa using environmental satellite data

National Oceanic and Atmospheric Administration (NOAA) satellite data from the Advanced Very High Resolution Radiometer (AVHRR) sensor were analysed to document the vegetation biomass dynamics associated with the regional desert-locust upsurge in West Africa during 1980/81, which affected an area of some 600 000 km² in Mali, Niger and Algeria. Comparisons were made among locust population survey reports, rainfall records from eighteen stations in the same area, and the satellite data in vegetation index format. The satellite-recorded temporal and spatial distributions of desert vegetation biomass were closely correlated with both the locust population surveys and the available rainfall data. An attempt was made to develop a quantitative relationship between a satellite-derived potential breeding activity factor (PBAF) and the observed desert locust populations. Analysis of the multitemporal satellite data set indicates that, had the NOAA/AVHRR vegetation index data been operationally available in June 1980, effective preventive control measures would have only been necessary for an area of 600 km².     

Estimating soil moisture from 6·6 GHz dual polarization,and/or satellite derived vegetation index

Eight and a half years (January 1979 to August 1987) of Scanning Multichannel Microwave Radiometer (SMMR) data taken at a frequency of 6·6 GHz for both day and night observations at both polarizations were processed, documented and used to study the relationship between brightness temperature (T_B) and antecedent precipitation index (API) in a wide range of vegetation index (normalized difference vegetation index (NDVI) varies from 0·2 to 0·6) in the mid-west and southern United States. In general, this study validates the model structure for soil wetness developed by Choudhury and Golus. For NDVI greater than 0·45 the resultant microwave signal is substantially affected by the vegetation. The night-time observations by both polarizations gave a better correlation between T_B and API. The horizontal polarization is more sensitive to vegetation. For the least and greatest vegetated areas, nighttime observations by vertical polarization showed less scatter in the T_B versus API relation. A non-linear model was developed for soil wetness using horizontal and vertical plarization and their difference. The estimate of error for this model is better than previous models, and can be used to obtain six levels of soil moisture.     

On the relationship between monthly mean and maximum-value composite normalized vegetation indices

Abstract

A relationship between the maximum-value composite and monthly mean normalized difference vegetation index (NDVI) is derived statistically using data over the U.S. Great Plains during 1986. The monthly mean NDVI is obtained using a simple nine-day compositing technique based on the specifics of the scan patterns of the NOAA-9 Advanced Very High Resolution Radiometer (AVHRR). The results indicate that these two quantities are closely related over grassland and forest during the growing season. It is suggested that in such areas a monthly mean NDVI can be roughly approximated by 80 per cent of the monthly maximum NDVI, the latter being a standard satellite data product. The derived relationship was validated using data for the growing season of 1987.