Drought onset and propagation into soil moisture and grassland vegetation responses during the 2012–2019 major drought in Southern California

Abstract

Abstract. Despite clear signals of regional impacts of the recent severe drought in California, e.g., within Californian 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 high-resolution in situ dataset of climate and soil moisture from two grassland sites (coastal and inland), alongside greenness (Normalized Difference Vegetation Index) data from Landsat imagery, to explore drought dynamics in environments with similar precipitation but contrasting evaporative demand over the period 2008–2019. We show that negative impacts of prolonged precipitation deficits on vegetation at the coastal site were buffered by fog and moderate temperatures. During the drought, the Santa Barbara region experienced an early onset of the dry season in mid-March instead of April, resulting in premature senescence of grasses by mid-April. We developed a parsimonious soil moisture balance model that captures dynamic vegetation–evapotranspiration feedbacks and analyzed the links between climate, soil moisture, and vegetation greenness over several years of simulated drought conditions, exploring the impacts of plausible climate change scenarios that reflect changes to precipitation amounts, their seasonal distribution, and evaporative demand. The redistribution of precipitation over a shortened rainy season highlighted a strong coupling of evapotranspiration to incoming precipitation at the coastal site, while the lower water-holding capacity of soils at the inland site resulted in additional drainage occurring under this scenario. The loss of spring rains due to a shortening of the rainy season also revealed a greater impact on the inland site, suggesting less resilience to low moisture at a time when plant development is about to start. The results also suggest that the coastal site would suffer disproportionally from extended dry periods, effectively driving these areas into more extreme drought than previously seen. These sensitivities suggest potential future increases in the risk of wildfires under climate change, as well as increased grassland ecosystem vulnerability.

Spatial Distribution of Optimal Plant Cover and Its Influencing Factors for Populus simonii Carr. on the Bashang Plateau, China

“…In zone III, with a MAP > 420 mm, in addition to MAP, the AET was negatively correlated with sand content. Sand content affected the AET by affecting the soil water holding capacity [52]. There was no correlation between the AET and MAT because the elevation in this region is relatively higher and no obvious differences exist.…”

stoPET v1.0: a stochastic potential evapotranspiration generator for simulation of climate change impacts

“…Water availability to plants is not only one of the limiting constraints for crop production and food security (Funk et al, 2008;Funk and Brown, 2009;Kang et al, 2009;Ayyad and Khalifa, 2021) but also for the health and functioning of the vegetative ecosystem in natural settings (Mayes et al, 2020;Sabathier et al, 2021;Warter et al, 2021). Forecasts of crop water requirement and irrigation demand for major crops like maize, barley, and wheat (Ewaid and Abded, 2019) are paramount for preparing advisory reports related to the timing of planting, crop choice, and irrigation scheduling, especially in arid and semi-arid regions, where high atmospheric evaporative demand and erratic rainfall make farming a risky economic activity (Nyakudya and Stroosnijder, 2011).…”

Local groundwater decline exacerbates response of dryland riparian woodlands to climatic drought

“…Specifically, we asked: (1) What were the principal drivers and indicators of drought stress for cottonwoods, and (2) how did these drought effects and their recovery vary temporally and spatially? Our results provide insights into the resistance and resilience of dryland riparian woodlands to recent trends and forecasted increasing temperatures and precipitation deficits (Bradford et al, 2020), which are expected to result in higher frequency of multiyear soil moisture droughts (Warter et al, 2021).…”

“…From 2012 to 2019, California experienced its most extreme drought on record (Robeson, 2015 ), which coincided with the hottest 3‐year period (2012–2014) for the region in the last century (Luo et al, 2017 ; Mann & Gleick, 2015 ). The interaction of drought and high temperatures resulted in severe soil moisture depletion (Warter et al, 2021 ) and water table decline during the peak hydrological drought period from 2013 to 2016 (Thomas et al, 2017 ; Xiao et al, 2017 ), when the study area was considered to be in severe, extreme, or exceptional drought conditions (U.S. Drought Monitor, 2021 ).…”

“…In this study, we used dendroecological methods to analyze the response of riparian phreatophytes to California's unprecedented 2012–2019 drought along a gradient of sites that varied in impact severity. Our study system is the Santa Clara River (SCR), the last free‐flowing river in Southern California whose lower reach flows through a heavily agricultural region, where severe drought has impacted riparian trees and upland grasslands (Kibler et al, 2021 ; Warter et al, 2021 ). Indeed, remote sensing analyses demonstrated that riparian woodlands along the SCR were highly sensitive to water table declines observed during the drought (Kibler et al, 2021 ).…”