Grassland productivity estimates informed by soil moisture measurements: Statistical and mechanistic approaches

Abstract

Soil moisture is a fundamental determinant of plant growth, but soil moisture measurements are rarely assimilated into grassland productivity models, in part because methods of incorporating such data into statistical and mechanistic yield models have not been adequately investigated. Therefore, our objectives were to (a) quantify statistical relationships between in situ soil moisture measurements and biomass yield on grasslands in Oklahoma and (b) develop a simple, mechanistic biomass‐yield model for grasslands capable of assimilating in situ soil moisture data. Soil moisture measurements (as fraction of available water capacity, FAW) explained 60% of the variability in county‐level wild hay yield reported by the National Agricultural Statistics Service (NASS). We next evaluated the performance of mechanistic, evapotranspiration (ET)‐driven grassland productivity models with and without assimilation of measured FAW into the models’ water balance routines. Models were calibrated by comparing estimated ET with ET measured using eddy covariance, and calibration proved essential for accurate ET estimates. Models were validated by comparing NASS county‐level hay yields to the modeled yields, which were the product of normalized transpiration estimates (the ratio of transpiration to reference ET) and an empirically derived grassland water productivity (the ratio of accumulated biomass to normalized transpiration) estimate. The mechanistic model produced more accurate estimates of wild‐hay yields with soil moisture data assimilation (Nash–Sutcliffe efficiency [NSE] = 0.55) than without (NSE = 0.10). These results suggest that improved estimates of grassland productivity could be achieved using in situ soil moisture, which could benefit grazing management decisions, wildfire preparedness, and disaster assistance programs.

Using soil moisture information to better understand and predict wildfire danger: a review of recent developments and outstanding questions

“…In a different study that used in situ soil moisture data from the entire state of Oklahoma, Krueger et al (2015) www.publish.csiro.au/wf International Journal of Wildland Fire G showed that 90% of large growing season wildfires across all Oklahoma landscapes (forest, shrubland, and grassland) occurred for FAW < 0.40, which matches the threshold for transpiration reduction due to moisture stress in grassland vegetation (Krueger et al 2021). These soil moisture-wildfire relationships were further described using probabilistic models in a subsequent study (Krueger et al 2016).…”

“…Sharma et al (2021), using data from a grassland field study close to the MOISST site, reported that when soil moisture was plentiful (FAW values of at least 0.59), mixed fuel moisture was not related to soil moisture, but it declined as FAW decreased below this threshold. When FAW dropped below 0.40, the transpiration and growth rate of grassland live fuels declined, reflecting the intensification of drought stress (Krueger et al 2021). When FAW declined below 0.36, the greenness of the vegetation, as indicated by the normalised difference vegetation index (NDVI), began to decrease (Sharma et al 2021) (Fig.…”

Complete Issue

“…We also learned how soil moisture observations can provide an effective indicator of fuel moisture content and curing rate for grassland vegetation (Sharma et al, 2020) and dead fuel moisture content in Sierra Nevada forests (Rakhmatulina et al, 2021). In situ soil moisture measurements have also proven valuable for improving predictions of grassland fuel loads (Krueger et al, 2021), which is significant given the fact that grasslands account for a large portion of the area burned worldwide each year.…”

Using Soil Moisture Information to Better Understand and Predict Wildfire Danger

“…We also learned how soil moisture observations can provide an effective indicator of fuel moisture content and curing rate for grassland vegetation (Sharma et al, 2020) and dead fuel moisture content in Sierra Nevada forests (Rakhmatulina et al, 2021). In situ soil moisture measurements have also proven valuable for improving predictions of grassland fuel loads (Krueger et al, 2021), which is significant given the fact that grasslands account for a large portion of the area burned worldwide each year.…”