Phenology-Based Biomass Estimation to Support Rangeland Management in Semi-Arid Environments

Livestock plays an important economic role in Niger, especially in the semi-arid regions, while being highly vulnerable as a result of the large inter-annual variability of precipitation and, hence, rangeland production. This study aims to support effective rangeland management by developing an approach for mapping rangeland biomass production. The observed spatiotemporal variability of biomass production is utilised to build a model based on ground and remote sensing data for the period 2001 to 2015. Once established, the model can also be used to estimate herbaceous biomass for the current year at the end of the season without the need for new ground data. The phenology-based seasonal cumulative Normalised Difference Vegetation Index (cNDVI), computed from 10-day image composites of the Moderate-resolution Imaging Spectroradiometer (MODIS) NDVI data, was used as proxy for biomass production. A linear regression model was fitted with multi-annual field measurements of herbaceous biomass at the end of the growing season. In addition to a general model utilising all available sites for calibration, different aggregation schemes (i.e., grouping of sites into calibration units) of the study area with a varying number of calibration units and different biophysical meaning were tested. The sampling sites belonging to a specific calibration unit of a selected scheme were aggregated to compute the regression. The different aggregation schemes were evaluated with respect to their predictive power. The results gathered at the different aggregation levels were subjected to cross-validation (cv), applying a jackknife technique (leaving out one year at a time). In general, the model performance increased with increasing model parameterization, indicating the importance of additional unobserved and spatially heterogeneous agro-ecological effects (which might relate to grazing, species composition, optical soil properties, etc.) in modifying the relationship between cNDVI and herbaceous biomass at the end of the season. The biophysical aggregation scheme, the calibration units for which were derived from an unsupervised ISODATA classification utilising 10-day NDVI images taken between January 2001 and December 2015, showed the best performance in respect to the predictive power (R2cv = 0.47) and the cross-validated root-mean-square error (398 kg·ha−1) values, although it was not the model with the highest number of calibration units. The proposed approach can be applied for the timely production of maps of estimated biomass at the end of the growing season before field measurements are made available. These maps can be used for the improved management of rangeland resources, for decisions on fire prevention and aid allocation, and for the planning of more in-depth field missions

Phenology-based biomass estimation to support rangeland management in semi-arid environments

Livestock plays an important economic role in Niger – especially in the semi-arid regions – while being highly vulnerable due to the large inter-annual variability of precipitation and hence rangeland production. This study aims to support effective rangeland management by developing an approach for mapping rangeland biomass production. The observed spatiotemporal variability of biomass production is utilised to build a predictive model based on ground and remote sensing data for the period 2001 to 2015. The phenology-tuned seasonal cumulative Normalised Difference Vegetation Index (cNDVI), computed from 10-day image composites of the Moderate-resolution Imaging Spectroradiometer (MODIS) NDVI data, was used as proxy of biomass production. A linear regression model was tuned with multi-annual field measurements of herbaceous biomass at the end of the growing season. Besides a general model utilizing all available sites for the calibration, different aggregation schemes of the study area with a varying number of calibration units and different biophysical meaning were tested. Sampling sites belonging to a certain calibration unit of a selected scheme were aggregated to compute the regression. The different aggregation schemes were evaluated with respect to their predictive power. Results gathered at the different aggregation levels were subjected to a cross-validation (cv) applying a jackknife technique (leaving one year out at a time). In general, the model performance increased with increasing model parameterization indicating the importance of additional unobserved and spatially heterogeneous agro-ecological effects (which might relate to grazing, species composition, optical soil properties, etc.) in modifying the relationship between cNDVI and herbaceous biomass at the end of the season. The biophysical aggregation scheme, whose calibration units were derived from an ISODATA classification utilizing 10-day NDVI images from January 2001 to December 2015, showed the best performance in respect of the predictive power (R2cv = 0.47) and the RMSEcv (398 kg ha-1) although not being the model with the highest number of calibration units. The proposed approach can be applied for the timely production of maps of estimated biomass at the end of the growing season before field measurements are made available.JRC.D.5-Food Securit

Rangeland Inventory and Monitoring With Unmanned Aerial System Imagery

Publicly managed rangelands today are seeing higher demand from society for the goods and services they can provide, including livestock production, wildlife habitat, myriad forms of recreation, and ecosystem services. Adaptively managed multiple-use lands could benefit from more objective and synoptic data to evaluate ecosystem function and to carry out and defend land health assessments that allow or exclude certain land use activities. Field methods to measure critical soil and vegetation indicators are well-established and becoming standardized across jurisdictions. However, field methods have two main limitations: 1) most can only observe small portions of the landscape, which may produce an incomplete picture of the status and trend of rangeland health; and 2) field methods cannot measure some indicators very well or not at all. This research focused on developing methods to measure soil and vegetation characteristics from unmanned aerial system (commonly known as drones) imagery, which can observe significantly more land than their field counterparts. I demonstrated the measurement of one soil (erosion/deposition) and four vegetation (forage utilization, fractional cover, vegetation height, canopy gaps) indicators using drone imagery and compared each with established field methods. The results show that drone imagery methods can serve as a complement to field methods or even a replacement in some cases. I found that drone imagery methods can precisely map topographic change and forage utilization across extents not previously possible. Imagery methods can outperform field methods for vegetation heights and canopy gaps in some vegetation communities. Drone-imagery indicators have matured to the point where they can start being integrated into adaptive land management. An online space dedicated to sharing imagery workflows amongst the range community could quicken the pace of identifying best practices to facilitate the transition toward this technology. Adopting drone-based inventory and monitoring data, however, will not replace field skills in plant identification, knowledge of vegetation phenology and succession, and logical interpretation of the data for land health assessments