International Journal of Applied Earth Observations and Geoinformation (Aug 2024)
Assessing the responsiveness of multiple microwave remote sensing vegetation optical depth indices to drought on crops in Midwest US
Abstract
Agricultural drought is a major natural disaster affecting biomass accumulation and causing food loss, exacerbated by the increasing frequency of flash droughts and compounded drought-heatwave events. Traditional optical remote sensing indices cannot directly represent the water content of vegetation, resulting in a limited understanding of crop response to drought. To address this gap, we investigated the responsiveness of microwave Vegetation Optical Depth (VOD) with four bands (L-, C-, X-, KU-) and four emerging VOD-derived products to drought conditions in crops in the Midwest US. These products include the normalized Difference Between Night and Day VOD (nVOD), Slope of the Regression of Day and Night VOD (σ), Standardized VOD Index (SVODI), and VOD to estimate Gross Primary Productivity (VOD2GPP). They employ different theoretical modeling approaches to crop growth and water use strategies. We comprehensively analyzed the trend, seasonality, and residual of VODs, using Leaf Area Index (LAI) for comparison, and further assessed the lagged and cumulative effects, quantified drought sensitivity, and captured responsiveness to cumulative drought using thresholds. The results showed a time lag in the response of VOD series to drought as indicated by the Standardized Precipitation Evapotranspiration Index (SPEI). VODs achieved faster responses and higher correlations compared to LAI. Among them, VOD_L exhibited the most statistically significant pixels (39.84%) and positive Rmax-lag with 96.81% of all pixels. For cumulative effects, VOD_L, VOD_C, VOD_X, VOD_KU, and SVODI were highly correlated in the early stages of droughts. We also found that crops in Iowa exhibited medium to high drought sensitivity (average values of 0.55 to 0.74), with the highest drought sensitivity calculated using the isohydricity indicator, σ. Based on threshold comparison, σ showed a timely response in the first month of drought (average of −0.62), whereas VOD_L and VOD_C performed best in the second month (both averaging −1.85), and VOD2GPP (−2.94) was the most responsive in the third month. Due to water use strategy, maize responded more quickly to the onset of drought compared to soybeans. Overall, the results demonstrated that VOD is promising for crop phenology and drought research. This study provides the first comprehensive investigation of the diverse capabilities of multiple VOD-based indices in drought monitoring across various timescales and croplands.
