Soil moisture as an indicator of growing-season herbaceous fuel moisture and curing rate in grasslands

Abstract

Soil moisture depletion during the growing season can induce plant water stress, thereby driving declines in grassland fuel moisture and accelerating curing. These drying and curing dynamics and their dependencies on soil moisture are inadequately represented in fire danger models. To elucidate these relationships, grassland fuelbed characteristics and soil moisture were monitored in nine patches of tallgrass prairie under patch-burn management in Oklahoma, USA, during two growing seasons. This study period included a severe drought (in 2012), which resulted in a large wildfire outbreak near the study site. Fuel moisture of the mixed live and dead herbaceous fuels (MFM) clearly tracked soil moisture, expressed as fraction of available water capacity (FAW). MFM decreased with decreasing soil moisture below an FAW threshold of 0.59 and fell below 30% only when FAW fell below 0.30. Likewise, the curing rate increased linearly as FAW declined below 0.30, while Normalized Difference Vegetation Index (NDVI) readings failed to adequately respond to rapid drying and curing of the fuelbed. Incorporating soil moisture observations into grassland fuelbed models could result in more accurate fuel moisture and curing estimates, contributing to improved wildfire danger assessments and reduced losses of life and property due to wildfire outbreaks.