Isotopes are increasingly used in rainfall‐runoff models to constrain conceptualisations of internal catchment functioning and reduce model uncertainty. However, there is little guidance on how much tracer data is required to adequately do this, and different studies use data from different sampling strategies. Here, we used a 7‐year time series of daily stable water isotope samples of precipitation and streamflow to derive a range of typical stream sampling regimes and investigate how this impacts calibration of a semi‐distributed tracer‐aided model in terms of flow, deuterium and flux age simulations. Over the 7 years weekly sampling facilitated an almost identical model performance as daily, and there were only slight deteriorations in performance for fortnightly sampling. Monthly sampling resulted in poorer deuterium simulations and greater uncertainty in the derived parameter sets ability to accurately represent catchment functioning, evidenced by unrealistic reductions in the volumes of water available for mixing in the saturation area causing simulated water age decreases. Reducing sampling effort and restricting data collection to 3 years caused reductions in the accuracy of deuterium simulation, though the deterioration did not occur if sampling continued for 5 years. Analysis was also undertaken to consider the effects of reduced sampling effort over the driest and wettest hydrological years to evaluate effects of more extreme conditions. This showed that the model was particularly sensitive to changes in sampling during dry conditions, when the catchment hydrological response is most non‐linear. Across all dataset durations, sampling in relation to flow conditions, rather than time, revealed that samples collected at flows >Q50 could provide calibration results comparable to daily sampling. Targeting only extreme high flows resulted in poor deuterium and low flow simulations. This study suggests sufficient characterization of catchment functioning can be obtained through reduced sampling effort over longer timescales and the targeting of flows >Q50.

Urban green spaces (UGS) can help mitigate hydrological impacts of urbanisation and climate change through precipitation infiltration, evapotranspiration and groundwater recharge. However, there is a need to understand how precipitation is partitioned by contrasting vegetation types in order to target UGS management for specific ecosystem services. We monitored, over one growing season, hydrometeorology, soil moisture, sapflux and isotopic variability of soil water under contrasting vegetation (evergreen shrub, evergreen conifer, grassland, larger and smaller deciduous trees), focussed around a 150-m transect of UGS in northern Scotland. We further used the data to develop a one-dimensional model, calibrated to soil moisture observations (KGE’s generally > 0.65), to estimate evapotranspiration and groundwater recharge. Our results evidenced clear inter-site differences, with grassland soils experiencing rapid drying at the start of summer, resulting in more fractionated soil water isotopes. Contrastingly, the larger deciduous site saw gradual drying, whilst deeper sandy upslope soils beneath the evergreen shrub drained rapidly. Soils beneath the denser canopied evergreen conifer were overall least responsive to precipitation. Modelled ecohydrological fluxes showed similar diversity, with median evapotranspiration estimates increasing in the order grassland (193 mm) < evergreen shrub (214 mm) < larger deciduous tree (224 mm) < evergreen conifer tree (265 mm). The evergreen shrub had similar estimated median transpiration totals as the larger deciduous tree (155 mm and 128 mm, respectively), though timing of water uptake was different. Median groundwater recharge was greatest beneath grassland (232 mm) and lowest beneath the evergreen conifer (128 mm). The study showed how integrating observational data and simple modelling can quantify heterogeneities in ecohydrological partitioning and help guide UGS management.

No abstract

&lt;p&gt;Stable water isotopes are naturally occurring conservative tracers that act as a fingerprint of water sources and ecohydrological fluxes. Previous studies have shown that some of those fluxes, like evapotranspiration and infiltration, are influenced by vegetation. Thus, land use will play an increasingly important role in water partitioning considering projected climate change-induced shifts of patterns in precipitation and increased atmospheric water demand. The sensitivity of different land use types to drought conditions and their influence on water partitioning varies, and still lacks understanding.&lt;/p&gt; &lt;p&gt;We used stable water isotopes to follow the pathway of precipitation into soil at a lowland headwater catchment with multiple land use types (forest, grassland, arable and agroforestry sites) and integrated our data into a one-dimensional, tracer-aided, plot scale model. The model requires precipitation, potential evapotranspiration and leaf area index as input data and the results were calibrated to real time soil moisture and isotope data. The dataset was collected in the long-term experimental Demnitzer Millcreek Catchment (DMC), Germany, over the growing season of 2021 and includes hydroclimatic conditions as well as isotopes in precipitation, soil water and groundwater. The 2021 conditions, though relatively average in terms of wetness, were affected by a dry spring, an exceptionally large summer storm event (~60 mm) as well as &amp;#8220;memory effect&amp;#8221; of previous intense drought years.&lt;/p&gt; &lt;p&gt;The implementation of the isotope calculations into the model showed that such a simple, low-parameterisation approach with easily accessible input data can be used to estimate the water balance and track isotopic transformations under plot sites with various land use conditions. The most rapid turnover of water was found under arable land use which resulted in short-term crop vulnerability to drought and slow but more rapid recovery and replenishment of moisture deficits. Forest soils showed slower water turnover with lower soil moisture, mainly reflecting higher interception losses and higher transpiration rates. This, together with access to deeper water, means drought stresses build more slowly at forest sites but can last much longer as storage recovery is slow (&gt;1 year) due to high evapotranspiration. Via adapting the model input data, we further simulated drought conditions to assess the &amp;#8220;water footprint&amp;#8221; of alternative land use under drought stress.&lt;/p&gt; &lt;p&gt;Our study illustrated the potential of stable water isotope data for simplified ecohydrological modelling approaches to quantify water partitioning. The different effects of land use types on ecohydrological fluxes were successfully simulated and their drought resilience was estimated. For the DMC and similar lowland catchments with similar soil types (sand at forest, loam at grassland and crops) and land cover in Central Europe with the modelled drought conditions, forest sites will initially be more resilient but more vulnerable to lasting droughts, while grassland and arable sites tend to recover more quickly, but can be rapidly stressed by short-term severe events. The modelling provides an experimental framework for assessing the differential effects of droughts of varying longevity and severity on alternative land use strategies.&lt;/p&gt;

Funding No funding was provided for this study aside from supervisory support through Harper Adams University. Conflicts of interest/Competing interests NAAvailability of data and material Data were sourced from the National River Flow Archive and the EA historical river water quality. These were obtained under licence and are unavailable for distribution without the express permission of NRFA or the EA.