Estimation of Fuel Water Content in the Forest Ecotone of Guangzhou Based on the PROSAIL Model

Abstract

Fuel moisture content (FMC), which is the ratio of equivalent water thickness (EWT) to dry matter content (DMC), plays a crucial role in the estimation of vegetation ignition probability and the fire propagation rate. The PROSAIL model can adequately simulate the canopy reflectance of vegetation, with the input of field-measured data into the model ensuring conformity with the ecological rules. If the EWT and DMC can be estimated by an empirical statistical method according to the reflectance spectrum, the universality of the physical model and the efficiency of the empirical statistical method can be considered. In this study, a fast and versatile method is established for estimating FMC based on the EWT, DMC, leaf area index measured data, and the PROSAIL model. The Normalized Difference Infrared Index (NDII) and Normalized Dry Matter Index (NDMI) were obtained from the spectral curves, with the results showing an obvious linear relationship between the NDII and EWT, NDMI, and DMC. Therefore, EWT and DMC can be estimated using the NDII and NDMI. The accuracy of the estimation results is verified to be high. The estimation model can be extended to Landsat 8 data to estimate FMC. The estimated FMC data verified by the measured data showed that R² was 0.743 and the RMSE was 34.2%. The model accuracy was reliable owing to large dynamic changes in the FMC. However, the estimated value of the FMC shifted significantly to the left during this study. The reasons for this are as follows: 1) The field-measured points are not sufficient to support the analysis according to different vegetation types, and the physical and chemical properties of different types are varied, leading to altered simulated spectral curves; 2) The vegetation spectrum is sensitive to the DMC at 1,650 nm, 1,720 nm, and 2,270 nm bands, and the sensitivity near the 1,650 nm and 1,720 nm bands is greater than that at 2,270 nm. However, because the Landsat 8 image does not have a 1,720 nm band, the 2,270 nm band was selected to calculate the NDMI, making the value of the estimated DMC too large, resulting in a small value of the estimated FMC and a significant shift to the left; 3) 1,650 nm and 2,270 nm are not in the central wavelength of the two bands of Landsat 8; therefore, the estimated DMC and FMC are shifted. In addition, the fast and versatile method, which is established based on the EWT, DMC, leaf area index measured data and the PROSAIL model, can realize large-scale and high-precision monitoring of the FMC, providing a scientific reference for forest fire prevention