A biophysical index to assess forest ecosystem moisture status

The assessment of ecosystem and fuel moisture status on a large spatial scale requires several observations and estimates and is often time consuming and costly due to labor and transportation expenses. In addition, most models are based on empirical functions of weather variables. An ecosystem and fuel dryness index (Fd) based on biophysical principles associated with energy exchange was recently presented and applied to monitor moisture content for annual grassland ecosystem. An advantage from using this method is that it gives information on both the short time variation and the seasonal trend of moisture content and it integrates contributions of all surface layers, so calibration for different surfaces seems unnecessary. Fd is based on the surface energy balance, where available energy (Rn - G) is partitioned into sensible and latent heat exchanges (H + LE). When soil water is not limiting, then H is typically small relative to LE and (Rn - G) is a measure of the potential or maximum possible LE. When the surface is dry and soil water is limited, evaporation from the surface is reduced, LE decreases relative to (Rn - G) and H increases. Therefore, the fuel dryness index can be calculated as Fd = 1- (LE/Rn-G) = H/Rn-G). Fd approaches 0 when ecosystem moisture values are high, and it approaches 1 as the moisture is low and the surface is dry. When both the surface litter and the vegetation show little evapotranspiration, this implies local drought conditions are severe. In this study, the Fd index was used to investigate the potential for characterization of ecosystem and fuel dryness in shrublands and forests where the canopy volume is larger and the surface includes living vegetation and dead material. Micrometeorological methods for estimating H and LE in conjunction with Rn and G measurements can provide accurate values of Fd. In this paper, hourly and daily values of Fd were determined as the ratio of sensible heat flux density to the available energy using data from the CARBOEUROPE network. The data were collected in several European forest ecosystems over a number of years. The energy balance closure from half-hour eddy covariance data was used to assess if the data were acceptable for the Fd computation. The Fd values and trends were compared with three well-known slow response Keetch–Byram drought index, two modified versions of the drought factor in the McArthur forest fire-danger meter, and the fast response fine fuel moisture code (FFMC) of the Canadian fire weather index. Moreover, Fd index was compared with the McArthur forest fire-danger meter. In general, the Fd index provided good predictions of the ecosystem moisture status over a wide range of climate and vegetation conditions. The index was more responsive to daily changes than most of the other indices, providing accurate information on dryness condition. In addition, it can potentially eliminate the need for calibrated empirical weather models and fuel stick measurements. Since the Fd index quantifies surface dryness, it could be used with remotely sensed data on biomass availability to improve fire-danger models and as ground truth to develop correlations with vegetation moisture values estimated from satellite data.