Joshua H. Viers’s research while affiliated with Climate Central and other places

Rising evaporative demand (ETo) with a warming climate contributes to diminished water availability in water-stressed agricultural regions globally. While increased ETo typically necessitates increased irrigation, we explore how crop phenological response can moderate this challenge. Focusing on five key agricultural crops in California’s San Joaquin Valley (SJV), we employ coupled water balance and phenology models to project crop water demands as a function of increased ETo and changing phenology. All crops exhibited accelerated growth from a shortened growing season with warming. The shortened crop maturation period partially to fully offset increased crop water demands due to rising ETo, with the largest phenological influence for annual crops such as tomato and corn. By contrast, models that do not account for phenological changes showed increased irrigation demands of approximately 3.5%–4.5% per °C of global warming primarily due to increased ETo. Integration with dynamic phenological models for the five key crops across the extent of agricultural land in the SJV showed a 1.6% decrease in irrigation needs under a 2 °C warming scenario. While phenological change alongside plant physiological responses to increased atmospheric CO2 may help buffer the impact of climate change on crop irrigation demand, decreased crop yields with a shorter growing season and continued reliance of groundwater reserves for agricultural water use and reduced spring snowpack will threaten coupled agricultural and water security in the region.

Increased market for specialty coffee and climate volatility in traditional coffee‐growing regions of the world has prompted interest in cultivating coffee outside of the tropics, including in California. While several small coffee farms have established in California over the past couple decade, no studies have identified and quantified climatically suitable regions for growing coffee. We developed a model of Coffea arabica suitability based on agronomic studies of thermal constraints to coffee cultivation, combining heat and cold intolerance with energy requirements for maturation. This model was applied to agricultural lands across California using high‐resolution climate datasets for both modern (1991–2020) and projected near‐term (2021–2050) conditions. We explored the potential for farm thermal management approaches—such as using agroforestry shade trees—to buffer temperature extremes and augment thermal suitability. Results indicate that, in the absence of thermal management approaches, nearly all agricultural lands in the state experience temperature extremes detrimental to coffee cultivation in modern climate. By contrast, we found that over 230 km² of agricultural land in coastal southern and central California is thermally suitable for coffee with management efforts. These suitable areas include most of the state's avocado cultivation—which may serve as a thermal buffer for coffee and favour the environmental and economic agricultural sustainability of this coupled crop system. We additionally show that projected near‐term climate coupled with management efforts leads to moderate increases in thermally suitable agricultural lands for coffee cultivation. Despite numerous economic and logistical challenges, that impede the growth of a burgeoning coffee region in coastal, southern and central California, we demonstrate that climate conditions in both today and in the future, combined with agronomic management efforts such as shading, provide an opportunity for a viable coffee production in California.

Environmental water allocation in California is a complex legal process involving various government agencies and stakeholders. E-flow requirements can be based on annual runoff typologies called water year types (WYTs), which dictate water volume, timing, and duration. In this study, we examined hydropower licensing documents of the major water and power projects in the Central Sierra Nevada to catalog e-flow requirements by WYT. In this study case, we identify how WYT classification systems and categories vary across and within different basins. Additionally, we assessed the impacts of climate change on hydrology, the frequency of WYTs identified, and the reliability and resilience of e-flows using future projections (2031–2060) of 10 Global Circulation Models (GCMs). We then propose a potential adaptation strategy using a 30 year moving percentiles approach to recalculate WYTs. We identified eight WYT classifications systems were identified, and their WYT distributions statistically significantly changes across all GCMs, even though most GCMs indicate no statistically significant change in hydrology. Disparities in future impacts are observed among and within hydropower projects, with some river reaches showing negative impacts on reliability and resilience. The adaptation strategy can generally boost resilience and improve reliability, but simply updating existing WYT thresholds without flexible regulatory frameworks reconsidering WYTs and e-flows thresholds, may not yield substantial improvements. Challenges in managing e-flows in California within regulatory and hydroclimatic contexts are intricate due to the lack of standardized approaches, leading to inconsistencies and potential conflicts among stakeholders, that will likely be exacerbated by climate change. Thus, we emphasize that targeted, site-specific, and adaptive management strategies are crucial, besides the need for a harmonized and consistent approach to defining and applying WYT categories and methods and/or e-flow assessments.

Inter‐annual precipitation in California is highly variable, and future projections indicate an increase in the intensity and frequency of hydroclimatic “whiplash.” Understanding the implications of these shocks on California's water system and its degree of resiliency is critical from a planning perspective. Therefore, we quantify the resilience of reservoir services provided by water and hydropower systems in four basins in the western Sierra Nevada. Using downscaled runoff from 10 climate model outputs, we generated 200 synthetic hydrologic whiplash sequences of alternating dry and wet years to represent a wide range of extremes and transitional conditions used as inputs to a water system simulation model. Sequences were derived from upper (wet) and lower (dry) quintiles of future streamflow projections (2030–2060). Results show that carryover storage was negatively affected in all basins, particularly in those with lower storage capacity. All basins experienced negative impacts on hydropower generation, with losses ranging from 5% to nearly 90%. Reservoir sizes and inflexible operating rules are a particular challenge for flood control, as in extremely wet years spillage averaged nearly the annual basins' total discharge. The reliability of environmental flows and agricultural deliveries varied depending on the basin, intensity, and duration of whiplash sequences. Overall, wet years temporarily rebound negative drought effects, and greater storage capacity results in higher reliability and resiliency, and lesser volatility in services. We highlight potential policy changes to improve flexibility, increase resilience, and better equip managers to face challenges posed by whiplash while meeting human and environmental needs.

Understanding the spatial patterns of plant diversity across vernal pool complexes remains challenging, as plant communities change rapidly in time and concurrent collection of relevant data for modeling remain logistically elusive. In the absence of coupled ecohydrological data, we demonstrate that the application of drone-mounted light detection and ranging (LiDAR) systems to vernal pools enables estimation of species richness using hydrological proxies and spatial modeling. Parameters related to hydrologic connectivity, soil moisture, and hydroperiod describe substantial variation in species richness patterns (r2 = 0.28 ± 0.03) across vernal pool complexes. Converging factors, such as proximity to areas of hydrologic connectivity with low surface roughness, tend to promote forb richness but describe less of the variation in grasses. Estimates of species richness are accurate to within 2-3 species using models derived from UAV-LiDAR, providing an approximation of potential biodiversity hotspots in lieu of in-situ surveys.

Understanding the spatial patterns of plant diversity across vernal pool complexes remains challenging, as plant communities change rapidly in time and concurrent collection of relevant data for modeling remain logistically elusive. In the absence of coupled ecohydrological data, we demonstrate that the application of drone-mounted light detection and ranging (LiDAR) systems to vernal pools enables estimation of species richness using hydrological proxies and spatial modeling. Parameters related to hydrologic connectivity, soil moisture, and hydroperiod describe substantial variation in species richness patterns (r2 = 0.28 ± 0.03) across vernal pool complexes. Converging factors, such as proximity to areas of hydrologic connectivity with low surface roughness, tend to promote forb richness but describe less of the variation in grasses. Estimates of species richness are accurate to within 2-3 species using models derived from UAV-LiDAR, providing an approximation of potential biodiversity hotspots in lieu of in-situ surveys.

Societies globally are struggling to meet freshwater demands while agencies attempt to address water access inequities under a rapidly changing climate and growing population. An understanding of dynamic interactions between people and water, known as sociohydrology, regionally could provide approaches to addressing local water mismanagement and water access inequity. In semi-arid California, local water agencies, primarily agricultural irrigation districts, are at the intersection of rethinking approaches to balance freshwater demands. More than 150 years of complex water governance and management have defined San Joaquin Valley irrigation districts and the region’s water access inequities and sociohydrologic instability. Older irrigation districts have higher surface water allocations and less groundwater dependence. About 60% of irrigation districts with pre-1914 water rights have twice the crop water demand in surface water allocations. In contrast, 86% of irrigation districts depend on groundwater, of which 12% rely exclusively on groundwater to supply irrigation demands. This study found that disadvantaged communities within irrigation districts do not have increased water access or better environmental conditions than those outside irrigation district boundaries, which underscores the need for inclusive water management structures to address the multifaceted water and environmental inequities. Groundwater overdependence across irrigation districts shows that imbalanced surface water allocations and inflexible crops could imperil agriculture and impact agricultural disadvantaged communities, especially under California’s SGMA and prolonged drought events. It is imperative that underserved communities are prioritized communities in achieving equitable water rebalance in California in addition to developing and implementing essential infrastructure and policy changes.

Article published at Water Resources Research: https://doi.org/10.1029/2023WR035966 Inter-annual precipitation in California is highly variable, and future projections indicate an increase in the intensity and frequency of hydroclimatic “whiplash.” Understanding the implications of these shocks on California's water system and its degree of resiliency is critical from a planning perspective. Therefore, we quantify the resilience of reservoir services provided by water and hydropower systems in four basins in the western Sierra Nevada. Using downscaled runoff from 10 climate model outputs, we generated 200 synthetic hydrologic whiplash sequences of alternating dry and wet years to represent a wide range of extremes and transitional conditions used as inputs to a water system simulation model. Sequences were derived from upper (wet) and lower (dry) quintiles of future streamflow projections (2030–2060). Results show that carryover storage was negatively affected in all basins, particularly in those with lower storage capacity. All basins experienced negative impacts on hydropower generation, with losses ranging from 5% to nearly 90%. Reservoir sizes and inflexible operating rules are a particular challenge for flood control, as in extremely wet years spillage averaged nearly the annual basins' total discharge. The reliability of environmental flows and agricultural deliveries varied depending on the basin, intensity, and duration of whiplash sequences. Overall, wet years temporarily rebound negative drought effects, and greater storage capacity results in higher reliability and resiliency, and lesser volatility in services. We highlight potential policy changes to improve flexibility, increase resilience, and better equip managers to face challenges posed by whiplash while meeting human and environmental needs.