Aboveground Biomass Estimation of Highland Barley in Qinghai–Tibet Plateau—Exploring the Advantages of Time Series Data and Terrain Effects

The timely and precise estimation of crop aboveground biomass (AGB) is crucial for evaluating crop development and forecasting yields. The objective is to examine the differences, advantages, and limitations between time series parameters and single-time-phase indicators derived from various vegetation indices in AGB estimation. Moreover, we aim to quantitatively investigate and elucidate the impact of the topographic and geographic conditions of the study region on the estimation of highland barley AGB. Results indicate that AGB simulations utilizing time series parameters from vegetation index time series (VI-TS) curves yield satisfactory results for all three VIs, with the exception of the Normalized Difference Vegetation Index (NDVI), which encounters saturation issues. The performance metrics are as follows: the Enhanced Vegetation Index (EVI) (R2 = 0.73, RMSE = 20.24 g/m2), the Soil-Adjusted Vegetation Index (SAVI) (R2 = 0.67, RMSE = 20.97 g/m2), and the Normalized Difference Mountain Vegetation Index (NDMVI) (R2 = 0.54, RMSE = 24.92 g/m2). The inclusion of our quantitative terrain factor improves the simulation accuracies of NDVI, SAVI, and NDMVI. Overall, the terrain factor has a beneficial impact on the highland barley AGB simulation outcomes. This study establishes a foundational framework for the timely and precise estimation of highland barley biomass, crucial for monitoring agricultural production in plateau mountainous regions.