Biomass quantification of Andean wetland forages using ERS satellite SAR data for optimizing livestock management

doi:10.1016/S0034-4257(02)00111-6

Remote Sensing of Environment

Volume 84, Issue 4, 10 April 2003, Pages 477-492

https://doi.org/10.1016/S0034-4257(02)00111-6 Get rights and content

Abstract

Spatio-temporal information on the biomass of totora reeds and bofedal water-saturated Andean grasslands, which are a critical forage resource for smallholders in Bolivia's Altiplano, is needed to promote their protection and improve livestock management. Satellite radar data appear well adapted to map biomass and to monitor biomass changes in this environment for two reasons: (a) the C-band (5.3 GHz) radar data is particularly sensitive to vegetation biomass when the canopy is over an underlying water surface or a water-saturated soil; this is through the dominant scattering mechanisms involving vegetation–water surface interaction; (b) the cloud cover during the growing period which corresponds to the rainy season. This paper assesses the potential of ERS satellite radar data for retrieving biomass information, which is spatially highly variable owing to the numerous small, nonuniform areas of totora harvesting and bofedal grazing. Ground data, including vegetation humid and dry biomass, were collected over 18 months during satellite descending passes at 12 sites located between the Eastern Cordillera and Titicaca Lake, representing three vegetation units: shoreline and inland totoras, and Puna bofedales.

ERS-SAR data were analysed as a function of plant biomass at homogeneous totora and bofedal areas. Because of the small size of these areas (typically 20×30 m), the SAR data need to be processed using an advanced multitemporal filter which improves radiometric resolution without significant reduction of the spatial resolution. The radar backscattering coefficient (σ° in dB) measured by ERS was found to be sensitive at both per site and per vegetation unit levels to humid and dry biomass of totora reeds and bofedal grasslands. The sensitivity of the signal to biomass variation is high for dry biomass ranges less than 1 kg/m² for totora, and less than 2 kg/m² for bofedal. The corresponding biomass maps provided by inversion of SAR data are valuable information for livestock management for three critical periods: after the calving season (October–November), when animal pressure is most significant; toward the end of the rainy season (March–April), as an indicator of coming trends to promote the adoption of measures aimed at preventing shortages during the winter season; in the middle of the winter dry season (June–July), to adjust animal charge.

Introduction

In the vast Andean area which stretches from Venezuela to Chile, biomass-rich wetland pastures are a vital native forage resource for the extensive livestock production, since the herbaceous vegetation of dry areas in this region has a very low productivity. However, these fragile native grasslands are being threatened by overgrazing pressure while livestock living on them are in many cases underfed. It is thus critically important, for the protection of these ecosystems and for a more efficient livestock management, to develop appropriate methods of spatial assessment and temporal monitoring of the vegetation aboveground biomass, allowing to evaluate their carrying capacity and to detect overgrazing.

This research focuses on the use of radar remote sensing (ERS-SAR data) to extract biomass information on Bolivian Altiplano wet grasslands, namely: totoras, aquatic plants of the Cyperaceae family, similar to reeds, up to 2 m tall and bofedales, high-coverage, dense carpets of short grasses and dwarf reeds with tufts of gramineae, on a permanently water-saturated soil. These are the only native forage resources available around the year, including during the dry winter season.

Section snippets

Conventional field methods

Overall biomass estimations have been reported by Alzérreca (1992), Alzérreca, Prieto, Laura, Luna, and Laguna (2001), Laguna (1987), and Loza de la Cruz, Moreau, Liberman, Lizeca, and Gasc (2000) for bofedales, and by Collot, Koriyama, and Garcia (1983) for totoras. There exists no systematic studies giving detailed monthly values, except for one performed by Collot et al. (1983) who measured above- and below-water biomass of totoras on one single site and only during the growing season

Site and vegetation characteristics

The test site is located between the eastern shore of the Lesser Titicaca Lake (Wiñay Marka) and the Eastern Cordillera de Los Andes (16°–16°15′ and 68°–68°40′), within an area of approximately 30×30 km. Ten representative sampling sites of about 1 km² each were selected in the flat Puna ecological floor, at altitudes between 3,800 and 4,100 m, namely:

•
to the West, five totora sites (two in inland areas subject to seasonal flooding—Cohana 1 and 2—and three of shoreline reeds—Chililaya 1 and 2

Field sampling

Periodical field measurements of humid and dry total biomass standing aboveground or above-water (HTB and DTB, respectively)—depending on water presence—were carried out during the descending passes of ERS satellite (every 35 days), between January 1997 and April 1998, and in some sites in October and December 1996, using the nondestructive double-sampling-scheme method of Haydock and Shaw (1975) also described by Bonham (1989), developed for natural herbaceous vegetation. This method was found

Physical background

In general, the intensity backscattered from a vegetation canopy is governed by the following major scattering mechanisms: the direct scattering from the canopy volume, the reflection of the boundary surface followed by scattering from the volume scatterers and the opposite volume-surface scattering, the direct scattering from the boundary surface through gaps in the canopy, and the scattering from the boundary surface including the two-way attenuation by the canopy. The relative importance of

Conclusion: applications, limitations and prospects

The feasibility of inverting ERS data into plant biomass shows the relevance of using C-band VV polarized data for the detailed spatial estimation of biomass of values up to 1 kg/m² of dry biomass for totoras and 2 kg/m² of dry biomass for bofedales, at any specific date, with acceptable accuracy for rangeland management purposes (RMSE of 0.2 and 0.3 kg/m², respectively). The retrieved information could be used to promote the adoption of measures aimed at preventing forage shortages and

Acknowledgements

The 12 ERS scenes for this research were provided by the European Space Agency (ESA) in an Announcement of Opportunity project. Support for the fieldwork was given by the Dutch International Service for National Agricultural Research (ISNAR) in the framework of the DME-SUR project. The Confederación Sindical Unica de Trabajadores Campesinos de Bolivia (CSUTCB) helped establish the link with smallholders to perform field sampling on their lands and understand their specific needs, whereas the

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Biomass quantification of Andean wetland forages using ERS satellite SAR data for optimizing livestock management

Abstract

Introduction

Section snippets

Conventional field methods

Site and vegetation characteristics

Field sampling

Physical background

Conclusion: applications, limitations and prospects

Acknowledgements

Agricultural and Forest Meteorology

Remote Sensing and Environment

Caracterı́sticas y distribución de los bofedales en el ámbito boliviano del sistema T.D.P.S.

Measurements for terrestrial vegetation

Multi-variate optimal speckle reduction in SAR imagery

International Journal of Remote Sensing

Répartitions, biomasses et productions des macrophytes du Lac Titicaca

Revue d'Hydrobiologie Tropicale

Los bofedales de la cuenca alta del valle de La Paz

The comparative yield method for estimating dry matter yield of pasture

Australian Journal of Experimental Agriculture and Animal Husbandry

Determinacion de la capacidad de carga para la epoca invernal en praderas nativas de Huarina, Batallas y Penas