Integration of Optical and X-Band Radar Data for Pasture Biomass Estimation in an Open Savannah Woodland

Michael Schmidt,Grant Stone,John Carter,Peter O’Reagain

doi:10.3390/rs8120989

Abstract

Pasture biomass is an important quantity globally in livestock industries, carbon balances, and bushfire management. Quantitative estimates of pasture biomass or total standing dry matter (TSDM) at the field scale are much desired by land managers for land-resource management, forage budgeting, and conservation purposes. Estimates from optical satellite imagery alone tend to saturate in the cover-to-mass relationship and fail to differentiate standing dry matter from litter. X-band radar imagery was added to complement optical imagery with a structural component to improve TSDM estimates in rangelands. High quality paddock-scale field data from a northeastern Australian cattle grazing trial were used to establish a statistical TSDM model by integrating optical satellite image data from the Landsat sensor with observations from the TerraSAR-X (TSX) radar satellite. Data from the dry season of 2014 and the wet season of 2015 resulted in models with adjusted r2 of 0.81 in the dry season and 0.74 in the wet season. The respective models had a mean standard error of 332 kg/ha and 240 kg/ha. The wet and dry season conditions were different, largely due to changed overstorey vegetation conditions, but not greatly in a pasture ‘growth’ sense. A more robust combined-season model was established with an adjusted r2 of 0.76 and a mean standard error of 358 kg/ha. A clear improvement in the model performance could be demonstrated when integrating HH polarised TSX imagery with optical satellite image products.

Highlights

Savannahs cover approximately 20% of the Earth’s land surface and are characterised as a grassland ecosystem, with trees being sufficiently widely spaced so that the canopy does not close [1].The understorey herbaceous layer consists primarily of grasses [1] which are a major contributor to the carbon balance
In our approach we have focused on the question of whether satellite imagery can be used to establish a pasture biomass model; and if TSX X-band data with 3.1 cm wavelength have sufficient interaction with grass species to add to pasture biomass estimation in comparison with biophysical image products from optical imagery alone
In the form of total standing dry matter (TSDM), at the Wambiana grazing trial were spatially averaged to paddock scale (100 ha) and parcels of land type subdivisions

Summary

Introduction

Savannahs cover approximately 20% of the Earth’s land surface and are characterised as a grassland ecosystem, with trees being sufficiently widely spaced so that the canopy does not close [1].The understorey herbaceous layer consists primarily of grasses [1] which are a major contributor to the carbon balance. To a large extent these areas are extensively grazed by native, domestic, and feral herbivores—supporting conservation, tourism, and pastoral activities [2]. Pastures play an important role in rangeland ecology, ecosystem services, and livestock-related industries [2]. Physical sampling of pasture biomass over large areas is not generally considered feasible in rangeland and savannah systems; it is not possible to collect and collate sufficient field data to adequately inform land managers and provide sufficient input for pasture biomass modelling [3]. A major issue is the estimation of pasture biomass for livestock forage budgeting and conservation purposes [4]. Explicit seasonal pasture biomass estimates could assist land managers and a host of other stakeholders to make assessments relating to livestock production and land resource management

Methods

Results

Discussion

Conclusion