MEASURING, UNDERSTANDING AND MODELING ECOHYDROLOGICAL SEPARATION

My dissertation sought to answer some of the fundamental questions on how subsurface water may be partitioned between root water uptake and streamflow. I explored a phenomenon called ecohydrological separation – plants using water of a character different from the mobile water found in soils, groundwater and streams. The generality of ecohydrological separation, however, remained wanting; and, possible controls in both space and time was elusive. I began with testing the generality of ecohydrological separation, first at two sites in the tropics with contrasting moisture conditions, and then at the global scale. Using a global database of water stable isotopes, I then quantified the degree of groundwater use by vegetation. Finally, I unscrambled the possible process controls behind the partitioning of subsurface water between root water uptake, groundwater recharge, and streamflow generation by conducting controlled drought-rewetting experiments in a tropical mesocosm. Key results of these research efforts were: (1) ecohydrological separation was widespread across biomes of the world, providing clues to fundamental controls; (2) groundwater use by vegetation globally was not as widespread as increasingly assumed in the literature; and, (3) transpiration flux was older than groundwater recharge flux, supporting a perceptual model whereby transpiration and groundwater recharge fluxes were sourced from separate storage volumes and sampled at markedly different average sampling flux. Because determining the ages and sources of water that supply transpiration and groundwater recharge was a major challenge in ecohydrology, these findings are ground-breaking. Indeed, I was the first to measure and quantify what was referred heretofore as the “missing exit age” of transpiration. The mechanisms underlying the phenomenological manifestations of ecohydrological separation, as explored and uncovered in my dissertation, have direct implications for how we measure and model the transport of water, nutrients, and pollutants at various scales in space and time

Soil pore water evaporation and temperature influences on clay mineral paleothermometry

Clay mineral isotope paleothermometry is fundamental to understanding Earth's climate system and landscape evolution. Status quo methods, however, assume constant factors, such as temperature and water isotopic compositions, and ignore seasonality, soil water evaporation and depth dependent temperature changes. We propose first-order modifications to address these factors and test them in a modeling framework using published data from various settings. Our forward model reveals that neglecting evaporation and temperature variability may lead to significant underestimations of clay formation temperatures, especially in Mediterranean settings. Our inverse model indicates that high-latitude Eocene clay formation temperatures were ~8°C warmer than modern, while Eocene river sediments in the Sierra Nevada show evaporation-influenced trends, suggesting that previous paleoelevation estimates were underestimated. Our framework demonstrates that explicit consideration of soil pore water evaporation and temperature variability is necessary when interpreting clay mineral isotope data in the context of temperature, hydroclimate and elevation reconstructions

Prevalence and magnitude of groundwater use by vegetation:A global stable isotope meta-analysis

The role of groundwater as a resource in sustaining terrestrial vegetation is widely recognized. But the global prevalence and magnitude of groundwater use by vegetation is unknown. Here we perform a meta-analysis of plant xylem water stable isotope (δ(2)H and δ(18)O, n = 7367) information from 138 published papers – representing 251 genera, and 414 species of angiosperms (n = 376) and gymnosperms (n = 38). We show that the prevalence of groundwater use by vegetation (defined as the number of samples out of a universe of plant samples reported to have groundwater contribution to xylem water) is 37% (95% confidence interval, 28–46%). This is across 162 sites and 12 terrestrial biomes (89% of heterogeneity explained; Q-value = 1235; P < 0.0001). However, the magnitude of groundwater source contribution to the xylem water mixture (defined as the proportion of groundwater contribution in xylem water) is limited to 23% (95% CI, 20–26%; 95% prediction interval, 3–77%). Spatial analysis shows that the magnitude of groundwater source contribution increases with aridity. Our results suggest that while groundwater influence is globally prevalent, its proportional contribution to the total terrestrial transpiration is limited

Tritium and trees: A bomb peak perspective on soil water dynamics in semi-arid apple orchards

Understanding the relationship between agroforest age and soil water dynamics is crucial for effective land and water resources management. However, the complexities of these dynamics, such as soil water recharge and depletion, hamper in-depth understanding, particularly in water-scarce regions. In this study, we examined soil water recharge and depletion in relation to the stand age of apple trees, a widely planted and representative deep-rooted agroforest, over four years in a semi-arid region on China’s Loess Plateau (CLP). We collected soil cores to >20 m depth from four apple orchards (referred to as ‘agroforests’) with variable stand ages (established in 2008, 2005, 1998, and 1994). For comparison, we selected adjacent cropland as land use prior to agroforestry practices (‘control’). We measured soil water content and tritium distributions to model soil water dynamics and estimate water ages across different soil profiles. Our results show that recharge amounts (and depths) in shallow soils were 298.4 mm (7 m), 303.4 mm (6.6 m), 300.6 mm (5.4 m), and 483.1 mm (7.6 m), whereas deep soils had net depletions of 111.1 mm, 391.9 mm, 192.8 mm, and 108.9 mm for AP2008, AP2005, AP1998, and AP1994, respectively. The tritium peak depths, which indicate the 1963 bomb peak depth, significantly differed between agroforested and non-agroforested plots. In particular, agroforestation reduced the seepage velocity of soil water over 20 years. Furthermore, our tritium tracer water age model suggests that the age of transpired deep soil water exceeded 200 years in the oldest orchard. These findings highlight a complex interaction between newly infiltrated water and existing water, possibly due to variations in soil pore size distributions. The results of this study offer valuable insights into the ecohydrological impacts of agroforestation on the CLP and in similar climatic regions

Water woes: the institutional challenges in achieving SDG 6

Background: Sustainable Development Goal (SDG) 6 envisions a future where everyone has access to clean water and sanitation. Yet, as 2030 looms closer, the complexity of achieving this target becomes apparent, with issues far surpassing basic water infrastructure and utility challenges. The underlying problems lie in broader spheres such as governance, policymaking, and financing. Main body: The global landscape of water management is marked by complexities that transcend the operational troubles of water utilities. Financial sustainability is a monumental task. And while it is true that water utilities struggle with revenue generation, the broader picture reveals systemic challenges. The true cost of water provision often extends to ecosystem services such as watershed protection. Often, these services are not internalized in the revenue models of utilities but are typically subsidized by governments or simply not considered. Balancing affordability for users with cost recovery for service providers, however, is not just an arithmetic exercise. It is also a question of equitable policies. Non-revenue water (NRW), resulting from physical losses such as leaks, theft, and inaccurate [or lack of] metering, exacerbates existing financial strain. Annual NRW losses are estimated at an astonishing 126 billion cubic meters, costing roughly USD 39 billion. But at the most fundamental level of achieving SDG 6 is misgovernance. Effective water governance demands consistent policies, coherent collaboration among diverse stakeholders, and comprehensive strategies that cater to specific regional contexts. Current models often suffer from fragmented policies, inadequate public-private partnerships, and weak engagement mechanisms. A glaring gap exists between academic advancements in water management and their practical implementation in policymaking. Moreover, international cooperation, while vital, reveals an unequal landscape in knowledge exchange. Knowledge transfer is often skewed, favoring dominant nations while sidelining voices from the Global South. This emphasizes the need for an inclusive, equitable, and context-specific global cooperation model. Conclusion: The road to realizing SDG 6 is multifaceted, and while on-the-ground solutions are essential, the real success lies in addressing the foundational challenges. This requires innovative financial solutions, reimagining water governance structures, and ensuring all voices, especially from the Global South, are heard and integrated into global policies. As 2030 nears, it is the synergy of governance, finance, and technology that will ultimately make clean water and sanitation a reality for all

Agriculture and Aquaculture in a Changing World

<p>A research brief on agriculture and aquaculture </p

Hydroelectric Dams as an Energy Source

<p>Op-ed piece on hydroelectric dams as an energy source</p

A Combined Reaction-Diffusion and Random Rate Model for the Temporal Evolution of Silicate Mineral Weathering

<p>A general mathematical model of static disorder in describing the apparent time-dependence of silicate mineral weathering</p