It has been proposed that conservation laws might not be beneficial for accurate hydrological modeling due to errors in input (precipitation) and target (streamflow) data (particularly at the event time scale), and this might explain why deep learning models (which are not based on enforcing closure) can out-perform catchment-scale conceptual and process-based models at predicting streamflow. We test this hypothesis at the event and multi-year time scale using physics-informed (mass conserving) machine learning and find that: (1) enforcing closure in the rainfall-runoff mass balance does appear to harm the overall skill of hydrological models, (2) deep learning models learn to account for spatiotemporally variable biases in data (3) however this “closure” effect accounts for only a small fraction of the difference in predictive skill between deep learning and conceptual models.

Particle tracers are sometimes used to track sources and sinks of riverine particulate and contaminant transport. A potentially new particle tracer is ~200 nm sized superparamagnetic silica encapsulated DNA (SiDNAFe). The main objective of this research was to understand and quantify the settling and aggregation behaviour of SiDNAFe in river waters. Our results indicated, that in quiescent conditions, more than 60% of SiDNAFe settled within 30 hours, starting with a rapid settling phase followed by an exponential-like slow settling phase in the three river waters we used (Meuse, Merkske, and Strijbeek) plus MilliQ water. From this, we inferred that the rapid SiDNAFe settling was mainly due to homo-aggregation and not due to hetero-aggregation (e.g., with particulate matter present in river water). Incorporating a first-order mass loss term which mimics the exponential phase of the settling in quiescent conditions seems to be an adequate step forward when modelling the transport of SiDNAFe in river injection experiments. Furthermore, we validated the applicability of magnetic separation and up-concentration of SiDNAFe in real river waters, which is an important advantage for carrying out field-scale SiDNAFe tracing experiments.

Stream channel incision and deposition are common after wildfire, and these geomorphic changes may impact runoff mechanisms and the composition of pre-event and event water in runoff. To investigate this, we monitored discharge and electrical conductivity at 6 nested sites within a 15.5 km 2 watershed in the northern Colorado Front Range that had recently burned, experienced large flooding, and well-documented and significant channel erosion and deposition. Over the study period, the watershed experienced seven precipitation events. For each hydrograph, we separate baseflow from runoff using a new method to characterize and account for the strong diurnal signal in the baseflow. Electrical conductivity is used as a tracer in a two-component end-member mixing analysis to separate the event hydrographs into event and pre-event water. Correlation coefficients were computed between key variables of the hydrologic response (such as runoff ratio, volumes of event and pre-event water) to storm and basin characteristics (including stream channel erosion/deposition, fraction of high/moderate burn severity, precipitation intensity, and antecedent precipitation). The strength and significance of correlations was found to vary seasonally. In the early season, event and pre-event volumes did not vary significantly with basin or storm characteristics. In the late season, antecedent precipitation correlated with a decrease in event runoff (R 2 = 0.34) and total runoff (R 2 = 0.40), increased precipitation intensity correlated with an increase in event runoff (R 2 = 0.48), and local erosion correlated with an increase in pre-event runoff (R 2 = 0.60) and total runoff (R 2 = 0.53). These findings indicate that seasonality and post-fire stream channel erosion influence the makeup of runoff response, most likely through their impact on the gradient of the near-stream groundwater table.

Sustainable water management in semi-arid agriculture practices requires quantitative knowledge of water fluxes within the soil-vegetation-atmosphere system. Therefore, we used stable-isotope approaches to evaluate evaporation (Ea), transpiration (Ta), and groundwater recharge (R) at sites in Senegal's Groundnut basin and Ferlo Valley pasture region during the pre-monsoon, monsoon, and post-monsoon seasons of 2021. The approaches were based upon (i) the isothermal evaporation model (for quantifying Ea); (ii) water and isotope mass balances (to partition Ea and Ta for groundnut and pasture); and (iii) the piston displacement method (for estimating R). Ea losses derived from the isothermal evaporation model corresponded primarily to Stage II evaporation, and ranged from 0.02–0.09 mm d-1 in the Groundnut basin, versus 0.02–0.11 mm d-1 in Ferlo. At the groundnut site, Ea rates ranged from 0.01 to 0.69 mm d-1; Ta was in the range 0.55–2.29 mm d-1; and the Ta/ETa ratio was 74–90%. At the pasture site, the ranges were 0.02–0.39 mm d-1 for Ea; 0.9–1.69 mm d-1 for Ta; and 62–90 % for Ta/ETa.  The ETa value derived for the groundnut site via the isotope approach was similar to those from eddy covariance measurements, and also to the results from a previous validated HYDRUS-1D model. However, the HYDRUS-1D model gave a lower Ta/ETa ratio (23.2%). The computed groundwater recharge for the groundnut site amounted to less than 2% of the local annual precipitation. Recommendations are made regarding protocols for preventing changes to isotopic compositions of water in samples that are collected in remote arid regions, but must be analysed days later. The article ends with suggestions for studies to follow up on evidence that local aquifers are being recharged via preferential pathways

We examined conditions that form or prevent thermal stratification in river pools using field measurements and statistical and three-dimensional (3D) computational fluid dynamics (CFD) modeling. Our motivation is to identify variables that control stratification for exploitation to enhance or prevent thermal gradients as needed to benefit species in rivers. One study pool (UT) is above water storage reservoirs and receives natural flows, and the other pool (PT) is regulated and receives unnaturally high, cold water in summer on the Trinity River, California. Thermal stratification formed in UT pool in spring at a critical flow of 1.01 m 3/s, peaked at 8.1 oC in summer, and exhibited diurnal formation and destruction under sub-critical flows until fall. At PT pool, the 14.2 m 3/s baseflow caused mixing that prevented stratification and formed a spatially homogenous thermal environment. Statistical modeling indicated the daily range in air and inlet water temperature at UT pool best correlated with the occurrence and strength of stratification but were progressively irrelevant as flows increased above the critical value. The 3D CFD model was verified by predicting the observed critical flow and dynamics of stratification at UT pool and isohytes observed at PT pool. The 3D model was then used to explore the thermal stratification process. Results confirmed low flows are the main variable for stratification to form, and the daily range in inlet water temperature drives the strength of the thermal gradient. The model estimated a critical discharge at PT pool of 2.0 m 3/s, twice that for UT pool owing to its 2.6-times larger area, suggesting critical flows scale with pool size. Results show that releasing critical and lower flows in summer on regulated streams may conserve water and provide thermal gradients that benefit poikilothermic species; alternately, higher than critical flows can prevent stratification where needed to improve water quality.

Flooding is a frequent disaster that has a wide-spread footprint globally with significant financial and societal impacts. With availability of Earth observation data from private and public entities at varying spatial, temporal, and spectral resolution as well as data from crowdsourcing, there is no shortage of models. In fact, models and algorithms are abundant and proliferating. However, the question remains where is a global flood model when we need one? Just because models are available does not mean they are usable or accessible and adequate for emergency managers, first responders and other stakeholders who use the model outputs for preparedness, response and resource planning. Often the issue of usability stems from the fact that the models are not always reproducible or replicable. The accuracy and uncertainty associated with the models and how they change based on the scale of analysis and the resolution of input and output datasets are often not communicated properly to stakeholders so they can be part of their decision-making process. The proliferation of machine learning and data driven models that rely on historical data also adds to this problem. This paper discusses several important issues associated with global flood models and provides recommendations that could be used to increase the usability of these models.

In arid area, the liquid water and water vapor states in soil profiles and fluxes at the upper and bottom interfaces are extremely complex due to heterogeneity of soil textures and the driving forces of heat and matrix potential. In this study, we used Hydrus-1D to simultaneously simulate liquid water, water vapor, and heat transports based on the observed data of atmosphere, soil and groundwater at three soil profiles in an arid area of northwest China. Comparison and contrast of the observed and simulated results at the three soil profiles show that there are diurnal vapor entry and outlet fluxes at the dry surface layer (DSL) of 30 cm in the summer season. The vapor entry and re-evaporation account for about 14% of annual precipitation for the heterogeneity soil profile with a mean groundwater depth of 210 cm. Because of limited soil moisture in this arid area, vapor induced re-evaporation occurs shortly in the early daytime. Moreover, the extent of vapor entry, condensation and re-evaporation strong depends on soil properties and water table depths. The lower water table produces the drier soil surface, allowing more vapor entry, condensation and re-evaporation. Whereas the finer grained soil layers benefits the vapor fixation to produce zero fluxes that substantially inhibit the upward liquid water and vapor fluxes, and thereby reduces soil evaporation. The reduced soil evaporation correspondingly decreases the capillary effect on phreatic evaporation, proven by that soil evaporation decreases slowly with decline of water table and the large extinct depth of phreatic evaporation for the finer grained soil profiles. The estimated extinct depth is 180 cm and 200 cm for the soil profiles consisting of silt loam and loamy sand, respectively, much larger than 100 cm of the sandy soil profile. Additionally, as water table is higher and lower than the extinct depth, the models neglecting the vapor - heat function could respectively overestimate and underestimate soil evaporation.

Floods claim a high toll in fatalities and economic impacts. Despite their societal relevance, there is much more to learn about the projected changes in discharge and flooding. Here we force an operational hydrologic model over the state of Iowa with high-resolution convection-permitting climate-model precipitation to evaluate the response of 140 watersheds to climate change. At the end of the century, under the most aggressive scenario in terms of fossil fuel use, we show that the transition from snow to rainfall and a ~30% increase in extreme precipitation rates lead to a doubling of maximum discharge during the spring and extended the flood season into the fall. Total discharge volumes are also expected to increase. Our results suggest that flood projections based on extreme precipitation increases alone substantially underestimate future risk due to the nonlinearity of the hydrologic response explained by long-term soil moisture memory and its feedbacks with precipitation.

Forested watersheds provide many ecosystem services that have become increasingly threatened by wildfire. Stream nitrate (NO 3 -) concentrations often increase following wildfire and can remain elevated for decades. We investigated the drivers of persistent elevated stream NO 3 - in nine watersheds that were burned to varying degrees 16 years prior by the Hayman fire, Colorado, USA. We evaluated the ability of multiple linear regression and spatial stream network modeling approaches to predict observed concentrations of the biologically active solute NO 3 - and the conservative solute sodium (Na +). Specifically, we quantified the degree to which landscape and stream network characteristics predict stream solute concentrations. No landscape variables were strong predictors of stream Na +. Rather, stream Na + variability was largely attributed to flow-connected spatial autocorrelation, indicating that downstream hydrologic transport was the primary driver of spatially distributed Na + concentrations. In contrast, vegetation cover, measured as mean normalized differenced moisture index (NDMI), was the strongest predictor of spatially distributed stream NO 3 - concentrations. Furthermore, stream NO 3 - concentrations had weak flow-connected spatial autocorrelation and high spatial variability. This pattern is likely the result of spatially heterogeneous wildfire behavior that leaves intact forest patches interspersed with high burn severity patches that are dominated by shrubs and grasses. Post-fire vegetation also interacts with watershed structure to influence stream NO 3 - patterns. For example, severely burned convergent hillslopes in headwaters positions were associated with the highest stream NO 3 ‑ concentrations due to the high proportional influence of hillslope water in these locations. Our findings suggest that reforestation is critical for the recovery of stream NO 3 - concentrations to pre-fire levels and targeted planting in severely burned convergent hillslopes in headwater positions will likely have a large impact on stream NO 3 - concentrations.

Ecohydrological investigations commonly use the stable isotopes of water (hydrogen and oxygen) as a conservative ecosystem tracer. This approach requires accessing and analyzing water constrained within plant and soil matrices. Generally, there are six steps that researchers must pass through to retrieve hydrogen and oxygen isotope values from these plant and soil matrices: (i) sampling, (ii) sample storage and transport, (iii) extraction, (iv) pre-analysis processing, (v) isotopic analysis, and (vi) post-processing and correction. At each of these steps cumulative errors can be introduced which sum to non-trivial magnitudes. These errors can impact subsequent interpretations about water cycling through the soil-plant-atmosphere continuum. But these steps in the research ‘process chain’ are just the tip of the iceberg when it comes to uncertainly in published findings. At each of these discreet steps, there are multiple possible options to select from resulting in, as we will show, tens of thousands of possible combinations used by researchers to go from plant and soil samples to isotopic data. In a newly emerging science, so many options can create interpretive confusion and major issues with data comparability. This points to the need for the development of shared standardized approaches. Here we critically examine the state of the process chain, reflecting on the issues associated with each step, and end with suggestions to move our community towards standardization. We hope that critically assessing this common approach will help us see the current problem in its entirety and facilitate community action toward agreed upon standardized approaches.

Using annual water balance analyses may mask intra-annual variability in runoff generation, which could limit our understanding of the similarities and differences between water- and energy-limited catchments. This may be especially limiting in comparisons between catchments close to the threshold between water- and energy-limitation. For this study, we examined runoff generation as a function of catchment storage in four watersheds, with focus on two that exist close to these thresholds to identify how year-to-year variability in storage resulted in intra-annual variations of runoff generation efficiency. Specifically, we focused on one energy-limited catchment in the humid subtropics and one water-limited in a Mediterranean climate. We used measured and calculated daily water balance components to calculate variations in the relative magnitude of daily storage. We isolated precipitation events to draw connections between storage and runoff generation at intra-annual scales and compared our findings to the same metrics in two intensely energy-limited landscapes. We observed distinct stages in daily storage across water years in watersheds at the threshold, where systems experienced wet-up, plateau, and dry-down stages. During the wet-up, precipitation was partitioned to storage, and runoff ratios ( RR) were low. In the plateau, storage was filled, precipitation was partitioned to runoff, causing high RRs. During the dry-down, storage decreased as precipitation was partitioned to evapotranspiration and runoff, causing low RRs. The critical role of evapotranspiration during the growing season resulted in relatively higher RRs during the wet-up than during the dry-down for a given storage value. Thus the same storage amount was partitioned to evapotranspiration or runoff differently throughout the year, depending on the storage stage. Despite their different positions on opposite sides of the threshold, the similarity between the two focus catchments suggests a potential characteristic behavior of systems at the threshold common to both humid and semi-arid landscapes.

Land use affects evapotranspiration rates and is a primary driver of the catchment water balance. The water balance of two catchments in southeastern Australia dominated by either grazed pasture or blue gum ( Eucalyptus globulus) plantation was studied, focusing on the patterns of evapotranspiration (ET) throughout the year. Rainfall, streamflow, and groundwater levels measured between 2015-2019 were combined to estimate annual ET using a water balance equation. In the pasture, eddy covariance was used to measure ET from the catchment. Sap flow measurements were used to estimate tree transpiration in May 2017 – May 2018 and Feb 2019 – Feb 2021 in two different plots within the plantation. The tree transpiration rates were added to direct evaporation, estimated as a percentage of annual rainfall, to calculate ET from the plantation catchment. ET in the pasture showed strong seasonal cycles with very low ET rates in summer and ET rates in spring that were larger than the transpiration rates in the plantation, where trees transpired consistently throughout the year. The estimated annual ET from the water balance equation were comparable to ET estimated from other measurements. In the pasture, ET on average accounted for 88% of annual rainfall, while ET in the plantation was on average 93% of rainfall, exceeding it in the years with annual rainfall lower than 500 mm. The difference between the ET rates in the plantation and the pasture were approximately 30 to 50 mm y­ -1. The larger ET rates in the plantation are reflected in a gradual decrease in the groundwater storage. The differences in ET rates were thus enough to cause a decrease in groundwater storage in the plantation, while the groundwater levels in the pasture remained stable.

Insight into the rainfall-soil moisture (SM) response to land cover is critical for soil hydrological process modeling and management. In this study, five typical land-cover types (forest, shrub, grass, crop, and bare land) and four rainfall patterns (heavy, intermediate, light, and continuous rains) were selected to assess the effects of SM response characteristics on the Loess Plateau of China. We monitored SM at five depths on each land-cover type at 1-h intervals over the growing season of 2019. The results showed that rainfall patterns and land-cover typestogether determined the SM response process and infiltration efficiency. A minimum accumulated rainfall amount of 5 mm was the threshold to trigger a 10-cm SM response. Rain events with higher intensity and smaller sum triggered a quick surface SM response, while larger amounts could percolate deeper and faster. Land-cover change significantly altered the rainfall-SM response dynamics and rainwater utilization efficiency after 20 years of ecological construction. Revegetation sites (mean values of forest, shrub, and grass) increased the soil wetting depth by 14.7%, shortened the SM response time by 27.3%, and accelerated the SM wetting front velocity by 67.2%, which promoted a 35.2% rainfall transformation rate (RTR) across the 1-m profile over all rainfall events (R 1-13). Moreover, planted forest showed the highest RTR of R 1-13 and the maximal increase in soil water storage, which did not aggravate the soil water deficit across the 1-m profile over the growing season. Therefore, we present evidence that planted forests, instead of shrubs, may be beneficial for water conservation if precipitation is greater than 550 mm. The findings of this study prove the role of revegetation on rainwater infiltration capacity and efficiency and can help improve the management of afforestation in arid and semiarid regions.

Canopies shape net precipitation patterns, which are spatially heterogeneous and control soil moisture response to rainfall. The vast majority of studies on canopy water fluxes were conducted in forests, whereas grassland canopies are often assumed to be spatially homogeneous, therefore likely not inducing patches of heterogeneity at and below the soil surface. Yet, some studies on short-structured vegetation, such as grasslands, proposed the importance of canopy-induced heterogeneity for net precipitation, but systematic investigations on the effects on soil wetting patterns are missing. Therefore, in this study, we investigated soil moisture response to rainfall in a managed temperate grassland by exploring the individual impacts of spatially varying throughfall, vegetation height and antecedent soil moisture status on the soil wetting patterns. We applied linear mixed effects models to disentangle the role of grassland canopy versus abiotic drivers. The spatial average soil water response showed that less and less water was stored in the upper parts of the soil as the growing season progressed and the soils dried, indicating bypass flow. Spatial variation of grass height was a significant driver of soil wetting patterns along with precipitation and antecedent soil moisture status. Soil wetting was suppressed in locations with tall canopy, although surprisingly this was not directly related to throughfall patterns. Instead, our results suggest that seasonally drier conditions and grassland stemflow kick off preferential flow. Ultimately, our results confirm that spatial variation of the canopy affects soil moisture wetting patterns not only in forests and indicate a strong influence of preferential flow on soil water patterns.

The projected climate change for Norway through the 21st century predicts that the temperature will increase significantly. Events with heavy rainfall will be more intense and occur more frequently. Rain floods will increase in magnitude and also occur more frequently. Extreme flooding and heavy rain will significantly impact the sediment dynamics in rivers. In the mountain areas, floods are often associated with erosion, transport and deposition of coarse sediment along the streams. These processes are related to bed load transport and pose a hazard in addition to the elevated water discharge and have to be included in management plans for river basins. This paper studies the bed load delivery from sources that contribute the most to the sediment budget in the Gudbrandsdalslågen river basin during the large magnitude floods in 2011 and 2013. More than 100 debris slides and debris flow were triggered in the tributary river Veikleåi by the heavy rain and snowmelt during these floods.  The volume of the contribution from debris flows and erosion and deposition of the river bed was determined by subtracting digital elevation models acquired during repeated airborne LIDAR surveys. In the river Dørja the supply of sediment from a number of debris flows caused extensive aggradation and channel changes. In their new position, lateral erosion by these channels triggered slides on the adjacent slopes. The contributing volumes of debris flows, lateral erosion, and river-bed erosion and deposition were determined from the LIDAR surveys. Relations obtained from studies of sediment transport in modern glacier rivers were used to obtain estimates of the ratio of bed load versus suspended load derived from the Pleistocene moraine deposits. Several monitoring stations using conventional methods for measuring bed load and suspended load recorded very large volumes of sediment delivery during both of the extreme floods. The results of the study may be used to identify necessary locations for check-dams and erosion protection works in order to adapt to climate change.

Recent revisions to the definition of the “waters of the United States” (WOTUS) have considerably altered how wetlands are federally regulated under the Clean Water Act. The two most recent modifications to WOTUS, the Clean Water Rule (CWR) and the Navigable Waters Protection Rule (NWPR), represent two opposing approaches to the federal wetland policy. Despite their implementation, the impacts of these rules on the regulation of wetlands have as of yet been poorly characterized at broad spatial scales. Using New York State (NYS) as a case study, we evaluated the jurisdictional statuses of more than 373,000 wetlands under the CWR and the NWPR to assess the landscape-scale effects of WOTUS re-definitions. We found that statewide and within each of NYS’s hydrologic regions, the NWPR protects fewer wetlands and less total wetland area than the CWR. The efficacy of the two regulations varied considerably in space across NYS, highlighting the need for comprehensive, nationwide assessments of wetland policy outcomes. We also observed that both rules produced non-uniform patterns in jurisdiction across a range of landscape positions and wetland sizes, preferentially protecting large wetlands close to the stream network. This effect was particularly pronounced under the NWPR, which excludes all geographically isolated wetlands from protection. Our findings in NYS emphasize the existence of unique patterns in protected wetlands across spatial scales, highlighting the value in applying geospatial analyses to evaluate environmental policy.

Buffer strips continue to feature in the management of agricultural runoff and water pollution in many countries. Existing research has explored their efficacy for reducing environmental problems in different geoclimatic settings but, the evidence on the efficacy of different vegetation treatments is less abundant than that for other buffer strip characteristics, including width, and is more contradictory in nature. With policy targets for various environmental outcomes including water or air quality and net zero pointing to the need for conversion of agricultural land, the need for robust experimental evidence on the relative benefits of different vegetation types in buffer strips is now renewed. Our experiment used a replicated plot scale facility to compare the efficacy of 12 m wide buffer strips for controlling runoff and suspended sediment loss during 30 sampled storms spanning 2017-2020. The buffer strips comprised three vegetation treatments; a deep rooting grass ( Festulolium cv. Prior), a short rotation coppice willow and native broadleaved woodland trees. Over the duration of the monitoring period, reductions in total runoff, compared with the experimental control, were in the order: willow buffer strips (49%); deciduous woodland buffer strips (46%); grass buffer strips (33%). The corresponding reductions in suspended sediment loss, relative to the experimental control, were ordered: willow buffer strips (44%) > deciduous woodland buffer strips (30%) > grass buffer strips (29%). Given the three-year duration of our new dataset, our results should be seen as providing evidence on the impacts during the establishment phase of the of the treatments.

In many parts of South Africa, soil erosion rates are high, and likely to be exacerbated by the longer droughts and more intense rainfall that are predicted in long-term regional climate change scenarios. Suspended sediment loads (SSL) and yields (SSY) are accepted means of expressing and comparing sediment transport and soil erosion rates. Land care and water security initiatives in South Africa require these data to provide benchmarking, and trajectories of change. International researchers began in the 1970s to investigate SSL estimation approaches. These investigations typically used near-continuous turbidity data from installed probes as a surrogate for sampled SS, and auto-samplers to monitor SS concentration and develop sediment rating curves. Biophysical and socio-economic conditions in South Africa differ markedly from the northern hemisphere environments where foundational studies were conducted. SSL estimations in South Africa are associated with extreme hydrological regimes, remote study areas and lack the resources required to collect and analyse representative SS data. There is a dearth of measured SS data, and of observed SSL and SSY for South African catchments. Using measured SS data from the Tsitsa River catchment (Eastern Cape, South Africa) we found that a discharge-weighted interpolation estimator was more appropriate than regression estimators, and that SSY responses to biophysical factors were in some ways more similar to northern hemisphere norms than expected. Lack of technical, infrastructural, human and financial resources were our main constraints to monitoring and estimating SSY. Our findings highlight the challenges of, and provide some guidance for, estimating directly measured SSL in the southern Africa region and inform future research in resource scarce areas.