Towards the development of an isotope‐enabled rainfall‐runoff model: Improving the ability to capture hydrological and anthropogenic change

Abstract

Global hydrological alterations are being driven by climate change. However, while modelling tools have been instrumental in identifying these changes, their calibration is often dependent solely on streamflow data, which limits the ability to simulate important hydrological processes with the required level of certainty. Constraining hydrological process simulations is required to identify robust model parameters, reducing model uncertainty and providing a baseline model for climate‐related risk assessments. In this study, stable isotope measurements of rainwater, groundwater and stream water (δ2H and δ18O), together with an end member mixing analysis (EMMA) (as a post‐processing step) were integrated with the Jena adaptative modelling system (JAMS)/J2000 rainfall‐runoff model, named J2000iso. The J2000iso models of δ2H and δ18O were able to achieve a streamflow Nash–Sutcliffe efficiency (NSE) of 0.75 and 0.72 and an isotope mixing Kling–Gupta efficiency (KGE) of 0.51 and 0.60, respectively. Compared with the base version, the J2000iso had a much smaller variability with 56% less ensemble variance in the NSE, 13% more simulated interflow which resulted in a better soil‐moisture simulation at an observation point, and lower simulated flow component uncertainty. The J2000iso δ2H model was more robust than the base version during a subsequent validation in terms of KGE and Bias with 0.60 and 0.002 compared with 0.51 and −0.14 for the base version. The EMMA provided a means to improve the hydrograph separation ability of the JAMS/J2000. As many catchments around the world are still ungauged and are affected by anthropogenic activities, the development and enhancement of isotope‐enabled modelling provides a means to improve the simulation of key hydrological processes.

Advancing isotope‐enabled hydrological modelling for ungauged calibration of data‐scarce humid tropical catchments

“…The J2000 model has previously been used in cold climates (Germany, France), tropical climates (Malaysia, Nepal, India, Brazil) and temperate climates (Australia, Turkey, South Africa) and has been proven to work across a range of different climate types and hydrological processes (Bonneau et al, 2023;Donmez et al, 2020;Krause, 2002;Kumar et al, 2017;Künne et al, 2019;Nepal et al, 2021;Watson et al, 2018Watson et al, , 2021. Watson et al (2023) used J2000iso to calibrate/ validate simulated flow components with an end-member-mixing analysis post-processing step. Building on this work, the new isotopeenabled model simulates both water and isotope fluxes for each modelled hydrological process (from precipitation, interception, mixing in the soil, percolation to the aquifer and outflow as different streamflow components).…”

“…Advancements in the availability of global isotope datasets have brought isotope‐enabled models to the forefront of hydrological process simulation, the prediction of future hydrological changes and model calibration for data scarce regions. Isotope‐enabled models use isotopic information to simulate water fluxes and storages (e.g., Stadnyk et al, 2013; Birkel & Soulsby, 2015; Stadnyk & Holmes, 2020; Stevenson et al, 2021; Holmes et al, 2022; Watson et al, 2023). Various distinctions can be made regarding how the isotopes are incorporated into hydrological models.…”

“…In conceptual rainfall‐runoff modelling, there are two main types of isotope‐enabled models. The first use the isotopes to validate or calibrate the simulated hydrograph separation into water source contributions of total streamflow, known as End‐Member‐Mixing integrated models (e.g., Watson et al, 2023). The second use a mass‐balance approach for isotope mixing directly connected to each modelled water flux (e.g., Pesántez et al, 2023).…”