Onset and propagation of drought into soil moisture and vegetation responses during the 2012–2019 drought in Southern California

Abstract

Abstract. Despite clear signals of regional impacts of the recent severe drought in California within Central Valley groundwater storage and Sierra Nevada forests, our understanding of how this drought affected soil moisture and vegetation responses in lowland grasslands is limited. In order to better understand the resulting vulnerability of these landscapes to fire and ecosystem degradation, we aimed to generalize drought-induced changes in subsurface soil moisture and to explore its effects within grassland ecosystems of Southern California. We used a decadal in situ dataset of high-resolution climate and soil moisture from two grassland sites (coastal and inland), alongside greenness (NDVI) data from Landsat to explore drought dynamics in environments with similar precipitation but contrasting evaporative demand. Analysis of data from 2008 to 2019 showed that the negative impacts of prolonged net precipitation (netP) deficits on vegetation at the inlands site were buffered by fog and moderate temperatures at the coastal site. During the drought, the region experienced an early onset of the dry season, resulting in premature senescence of grasses by mid-April. We developed a parsimonious soil moisture balance model that captures dynamic vegetation–evapotranspiration feedbacks using netP–NDVI relationships as a leading indicator. We then analyzed the links between climate, soil moisture, and vegetation greenness over decadal timescales, exploring the impacts of plausible climate change scenarios that reflect changes to precipitation amounts, their seasonal distribution, and evaporative demand. We found that all scenarios generate early, extreme soil moisture deficits during drought below a vegetation stress threshold, further intensifying early dry season onset and vegetation die-off. These changes suggest potential increases in the risk of wildfires in this and similar regions under climate change, as well as increased grassland ecosystem vulnerability.

Global climate-driven trade-offs between the water retention and cooling benefits of urban greening

“…Soil moisture balance models (SMBMs) are standard tools in hydrology for simulating evapotranspiration and drainage, for example in the context of estimating crop irrigation water requirements 36 , evaluating moisture deficits in soils 39 , 40 , and estimating deep drainage (i.e. groundwater recharge) 25 .…”

Modeling seasonal vegetation phenology from hydroclimatic drivers for contrasting plant functional groups within drylands of the Southwestern USA

“…We chose to look at a lowered DTG independently of climate, as a representative of the potential effects of lowered precipitation and increased PET. Furthermore, we also explored the effects of temperature shifts and earlier warming, a trend which has been observed to affect vegetation phenology and advance the onset of green-up as well as senescence (Xin et al 2015, Munson and Long 2017, Warter et al 2020. We examined the impacts of such changes on vegetation responses of all plant functional groups, comparing mean growing season greenness distributions (March-September) between historic observations and model results through two-sample Kolmogorov-Smirnov testing and discuss the potential implications on vegetation and ecosystem functioning.…”

“…This has led to increased mortality and widespread vegetation die-off throughout riparian ecosystems (Stromberg and Tiller 1996, Asner et al 2016, Goulden and Bales 2019, Kibler et al 2021, Rohde et al 2021. Similarly, shifts in the seasonal availability of soil moisture and timing of phenological events, as a result of warming temperatures, have affected soil moisture dependent vegetation, leading to prolonged periods of senescence, widespread vegetation die-back and the spread of invasive species (Liu et al 2012, Gremer et al 2015, Wallace et al 2016, Munson and Long 2017, Warter et al 2020. The effects of warming on green-up and senescence, in conjunction with changes to seasonal water availability, have subsequently raised the occurrence of early browning and senescence throughout forest and grassland ecosystems in the past.…”

Assessing multi-year-drought vulnerability in dense Mediterranean-climate forests using water-balance-based indicators

“…Long-term satellite-based Normalized Difference Vegetation Index (NDVI) offers valuable information on vegetation response across the globe. This greenness index has been used as an early warning signal of forest mortality (Liu et al, 2019;Rogers et al, 2018), in mapping mortality (Furniss et al, 2020;Meddens et al, 2013), and in investigating vegetation responses to drought (Warter et al, 2020). Meanwhile, there exists a robust relation between Landsat NDVI and annual ET measured by flux towers (Goulden et al, 2012;Goulden and Bales, 2019;Ma et al, 2020;Maurer, 2021;Roche et al, 2018), providing a way to estimate ET at a resolution of 30 m based on vegetation condition.…”