With global warming, the hydrological cycle is intensifying with more frequent and severe droughts and floods, placing water resources and their dependent communities under increasing stress. Guidance and insights into the projection of future water conditions are, therefore, increasingly needed to inform climate change adaptation. Hydrological projections can provide such insights when suitably designed for user needs, produced from the best available climate knowledge, and leverage appropriate hydrological models. However, producing such hydrological projections is a complex process that requires skills and knowledge spanning from the often-siloed disciplines of climate, hydrology, communication, and decision-making. Accordingly, this paper bridges these silos, by providing detailed guidance on the important steps and best practices to develop hydrological projections that can effectively support decision-making. Using an extensive literature review as well as our practical experience as climate scientists, hydrologists, numerical modelers, uncertainty experts and decision-makers, here we provide: (i) an overview of climate change hydrological impacts as background knowledge; (ii) a step-by-step guide to produce hydrological projections under climate change that are targeted to water practitioners and decision-making applications, (iii) a summary of important considerations related to hydrological projection uncertainty; and (iv) insights to use hydrological projections and their associated uncertainty for impactful communication and decision-making. By providing this guide for water practitioners, our paper addresses a critical interdisciplinary knowledge gap and supports enhanced decision-making and resilience to climate change threats.

Tropical peatlands play a critical role in regional water cycling, yet most tropical peat swamp forests (PSFs) are anthropogenically disturbed though modification of the water table (e.g. drainage), deforestation, and fire events. These disturbances can alter ecosystem processes including evapotranspiration (ET), thereby creating feedbacks that degrade peatland ecosystem services and result in significant alteration of greenhouse gas budgets. However, our understanding of fine-scale hydrological fluxes in tropical peatland ecosystems is currently lacking. Here, therefore, we aimed to quantify rates of ET from a degraded tropical PSF in Central Kalimantan, in the context of broader peatland hydrology and site meteorology. From March to November 2020, ET ranged from 1.8–7.3 mm d -1, averaged 4.09 ± 0.06 mm d -1 and was consistent between months, despite large fluctuations in precipitation (P) following typical wet/dry seasonality (e.g., 4.1 ± 0.2 mm d -1 in July, compared to 17.5 ± 4.4 mm d -1 in April). Total ET over the nine-month study period was 1127 mm; approximately 37% of total precipitation. Daily ET rates were comparable to previous studies from tropical PSFs, however, the ratio of ET/P was lower than other tropical PSF sites. We suggest that the volume of water lost through canal drainage may be higher at this site than other tropical PSFs, indicating more substantial hydrological alteration through drainage. We expect that with continued hydrological restoration (i.e. canal blocking), ET/P may increase and, if so, could potentially be used as an indicator for changing peatland condition over time.

Cryogenic vacuum distillation (CVD) is a widely used technique for extracting plant water from stems for isotopic analysis, but concerns about potential isotopic biases have emerged. Here, we leverage the Cavitron centrifugation technique to extract xylem water and compare its isotopic signature to that of CVD-extracted stem water as well as source water. Conducted under field conditions in tropical northern Australia, our study spans seven tree species naturally experiencing a range of water stress levels. Our findings reveal a significant deuterium bias in CVD-extracted bulk stem water when compared to xylem water (median bias \($-14.9\textperthousand$\)), whereas xylem water closely aligned with source water (median offset \($-1.9\textperthousand$\)). We find substantial variations in deuterium bias among the seven tree species (bias ranging from -19.3 to \($-9.1\textperthousand$\)), but intriguingly, CVD-induced biases were unrelated to environmental factors such as relative stem water content and pre-dawn leaf water potential. These results imply that inter-specific differences may be driven by anatomical traits rather than tree hydraulic functioning. Additionally, our data highlight the potential to use a site-specific deuterium offset, based on the isotopic signature of local source water, for correcting CVD-induced biases.

Coastal vegetated habitats, including mangroves, saltmarshes and seagrasses, mitigate climate change by storing atmospheric carbon. Previous blue carbon research has mainly focused on organic carbon stocks. However, recent studies suggest that lateral inorganic carbon export might be equally important. Lateral export is a long-term carbon sink if carbon is exported as alkalinity (TAlk) produced via sulfate reduction coupled to pyrite formation. This study evaluates drivers of pyrite formation in coastal vegetated habitats, compares pyrite production to TAlk outwelling rates, and estimates global pyrite stocks in mangroves. We quantified pyrite stocks in mangroves, saltmarshes and seagrasses along a latitudinal gradient on the Australian East Coast, including a mangrove dieback area, and in the Everglades (Florida, USA). Our results indicate that pyrite stocks were driven by a combination of biomass, tidal amplitude, sediment organic carbon, sedimentation rates, rainfall latitude, temperature, and iron availability. Pyrite stocks were three-times higher in mangroves (103 ± 61 Mg/ha) than in saltmarshes (30 ± 30 Mg/ha) and seagrasses (32 ± 1 Mg/ha). Mangrove pyrite stocks were linearly correlated to TAlk export at sites where sulfate reduction was the dominant TAlk producing process, however pyrite generation could not explain all TAlk production. We present the first global model predicting pyrite stocks in mangroves, which average 155 (range 128 – 182) Mg/ha. In mangroves, estimated global TAlk production coupled to pyrite formation (~3 mol/m2/y) is equal to ~24% of their global organic carbon burial rate, thus highlighting the importance of including TAlk export in future blue carbon budgets.