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Watershed resilience is the ability of a watershed to maintain its characteristic system state while concurrently resisting, adapting to, and reorganizing after hydrological (for example, drought, flooding) or biogeochemical (for example, excessive nutrient) disturbances. Vulnerable waters include non-floodplain wetlands and headwater streams, abundant watershed components representing the most distal extent of the freshwater aquatic network. Vulnerable waters are hydrologically dynamic and biogeochemically reactive aquatic systems, storing, processing, and releasing water and entrained (that is, dissolved and particulate) materials along expanding and contracting aquatic networks. The hydrological and biogeochemical functions emerging from these processes affect the magnitude, frequency, timing, duration, storage, and rate of change of material and energy fluxes among watershed components and to downstream waters, thereby maintaining watershed states and imparting watershed resilience. We present here a conceptual framework for understanding how vulnerable waters confer watershed resilience. We demonstrate how individual and cumulative vulnerable-water modifications (for example, reduced extent, altered connectivity) affect watershed-scale hydrological and biogeochemical disturbance response and recovery, which decreases watershed resilience and can trigger transitions across thresholds to alternative watershed states (for example, states conducive to increased flood frequency or nutrient concentrations). We subsequently describe how resilient watersheds require spatial heterogeneity and temporal variability in hydrological and biogeochemical interactions between terrestrial systems and down-gradient waters, which necessitates attention to the conservation and restoration of vulnerable waters and their downstream connectivity gradients. To conclude, we provide actionable principles for resilient watersheds and articulate research needs to further watershed resilience science and vulnerable-water management.

Intermittent rivers and ephemeral streams are crucial for the water cycle and ecosystem services, yet they are often neglected by managers and researchers, especially in headwater areas. This oversight has caused a lack of comprehensive basemaps for these vital river systems. In headwater regions, water bodies are typically sparse and disconnected, with narrow and less distinct channels. Therefore, we propose a novel hybrid method that integrates topographic data and remote sensing imagery to delineate river networks. Our method reestablishes connectivity among sparsely distributed water bodies through topographic pairs, enhances less distinct channel features using the gamma function, and converts topographic and water indices data into a weighted graph to determine optimal channels with the A* algorithm. The topographic and water indices data are derived from the Multi-Error-Removed Improved-Terrain DEM (MERIT DEM) and an average composite of the Modified Normalized Difference Water Index (MNDWI), respectively. In the upper Lancang-Mekong River basin, our method outperformed five publicly available DEM datasets, achieving over 91% positional accuracy within a 30 m buffer. This hybrid method enhances positional accuracy and effectively connects sparse water bodies in headwater areas, offering promising applications for delineating intermittent rivers and ephemeral streams and providing baseline information for these river systems.

This study investigated the influence of dissolved organic matter (DOM) additions on phosphate sorption kinetics of iron-rich sediments (39–50% hematite and goethite) from an ephemeral stream in the arid Pilbara region of sub-tropical northwest Australia. While phosphate sorption in stream sediments is known to be strongly influenced by sediment mineralogy as well as interactions with DOM, the mechanisms and significance of DOM on P-release from sediments with high sorption capacities, are largely undescribed. We assessed phosphorus (P) sorption behaviours by adding a range of solutions of known inorganic P concentrations that were amended with variable loadings of DOM derived from leachates of leaf litter to sediments from stream pools during the non-flowing phase. We compared the sorption capacity of the sediments and concurrent changes in DOM composition measured using fluorescence spectroscopy. We show that the low-dose DOM addition (~ 4 mg L−1 DOC) had the effect of reducing sediment P adsorption capacity, while for the high-dose DOM addition (~ 45 mg L−1 DOC), it was increased. The high-dose DOM was similar to pore water DOC and likely saturated sediment surface adsorption sites and produced P–OM–Fe complexes. This resulted in increased removal of P from solution. Sediment P sorption characteristics were well fitted to both Freundlich and Langmuir isotherm models regardless of DOC concentration. Langmuir P sorption maxima ranged from 0.106 to 0.152 mg g−1. General P sorption characteristics of these iron-rich sediments did not differ among pools of contrasting hydrological connectivity. Our results show how humic-rich DOM can modulate the sediment P availability in dryland streams. Unravelling the complexities of P availability is of particular significance to further our understanding of biogeochemical processes in aquatic ecosystems where P often acts as a limiting nutrient.

Sustainable groundwater production from karst aquifers is primarily dictated by its recharge rate. Therefore, it is essential to accurately quantify annual groundwater recharge in order to limit overexploitation and to evaluate artificial methods for groundwater enrichment. Infiltration during erratic flood events in karst basins may substantially contribute to aquifer recharge. However, the complicated nature of karst systems, which are characterized in part by multiple springs, sinkholes, and losing/gaining streams, impede accurate quantification of the actual contribution of flood waters to groundwater recharge. In this study, we aim to quantify the proportion of groundwater recharge accrued during runoff events in a karst aquifer. The role of karst conduits on flash flood infiltration was examined during four flood and controlled runoff events in the Soreq creek near Jerusalem, Israel. We distinguished between direct infiltration, percolation through karst conduits, and diffuse infiltration—the latter of which is most affected by evapotranspiration. A water balance was calculated for the 2014/15 hydrological year using the Hydrologic Engineering Center-Hydrologic Modelling System (HEC-HMS). Simulations show that 6.8 to 19.2% of the annual recharge volume was added to the aquifer from infiltration of runoff losses along the creek through the karst system.

This paper presents a methodology for examining the net benefit of site rehabilitation after an ecological disaster. While restoration of the site seems reasonable on the face of it, the cost of proactive restoration can be very high. In this article, we present a tool for decision makers to decide on the optimal route to rehabilitation – proactive or natural rehabilitation (or some combination thereof). We present a case study of an ecological catastrophe that occurred in June 2017 at an ephemeral desert stream in the south of Israel. We estimated the restoration costs and the benefits of restoration over the relevant time frame using a contingent valuation method. Comparing the present costs and benefits revealed a net present value of ILS 355.5 million in favor of proactive restoration of the stream. We also demonstrate that not all sections of the stream pass the benefit cost test, so a higher net benefit could be achieved through partial restoration. Our study demonstrates the importance of cost–benefit analysis when policy makers are contemplating proactive versus natural restoration.

Climate-driven hydrological models rarely incorporate intermittent rivers and ephemeral streams (IRES) due to monitoring difficulties and their perceived minor effect on river networks. Worldwide, IRES represent approximately 50% of river networks and up to 60% of annual flow and are recognized as conduits and processors of organic matter (OM). Climate induced changes in precipitation and discharge (Q) may impact OM fluxes from IRES. We assessed storm-driven source and flux of total suspended solids (TSS) and OM from small IRES in Mississippi, USA. We used linear Pearson correlations to evaluate relationships between water and storm characteristics (e.g., discharge). Stepwise regression was used to predict change in flux. Dissolved OM was derived from saturated flow through soil whereas particulate OM was derived from channel extension during storms. A power log relationship between Q and materials flux indicated that Q was the driver for flux. A 5% increase in Q within IRES may result in flux increase of 2% TSS and 1.7–2.8% OM. Climate change projections of increased storm intensity over a shorter water year will increase channel extension and soil water transfer resulting in higher material flux to downstream reaches. Climate-driven hydrological models of OM flux should incorporate IRES.

Hayami analytical solution for the modelling of a diffusive flood wave is developed and generalized by introducing an additional term, the ‘decay coefficient’ which can influence the shape and the peak of the flood hydrograph while passing in an ephemeral stream and making it robust to handle any shape of the inflow storm hydrograph, respectively. Furthermore, a spreadsheet model of Hayami solution is proposed that takes the inflow flood hydrograph, comprising of unit floods (pulses). It is tested on two Hayami examples, showing identical results. Sensitivity analysis is performed for the selection of a number of pulses that can sway the accuracy of the simulated results. Taking 10 pulses as a reference case, the RMSE converges from 0.7 at one pulse to 0.03 at eight pulses. The significance of this model is proven by its application on the real field data of Yiba catchment, located in Saudi Arabia, where the diffusion coefficient larger than 400 m 2 /s shows the divergence of the RMSE and the values between 50 and 400 m 2 /s are the fair estimates in ephemeral streams. Besides, it can also be used as a benchmark tool for testing other numerical schemes.

A hydrologic connectivity index (HCI) was developed to aid the US Clean Water Act Section 404 jurisdictional evaluation of headwater streams in montane watersheds, using the Cement Creek Watershed in the San Juan Mountains, Colorado as a case study. Jurisdictional determinations are required for many intermittent and ephemeral streams, where significant nexus with downstream traditional navigable waters (TNWs) must be evaluated. Three biophysical metrics (a streamflow metric, distance from the stream to the TNW, and slope between the stream and the TNW) for 239 streams in the watershed were used to develop three indices. These were then combined to develop the HCI, which is a compound, additive index. There was a large range of HCI values for ephemeral streams, which all require a significant nexus evaluation to determine jurisdiction. Statistical analysis of HCI values across stream duration and order classes showed that mean values differed significantly among classes. The flow index component constituted the greatest proportion of the HCI for perennial streams, was sensitive to the flow metrics used, and was greatest for high flows. Although ephemeral streams are only connected to the TNW less than 3 months of the year, the cumulative average annual flow contribution of ephemeral first-order streams can be 5 % of the total to the TNW, and their flow contribution is larger during high flows. Some ephemeral streams with high HCI values are farthest from the TNW but contribute the greatest flow out of all ephemeral reaches, and may have significant nexus with the river. The study shows that the HCI can be useful for initial evaluation of connectivity, significant nexus, and jurisdiction of headwaters at the watershed scale.

In this study, two ephemeral streams are compared in assessing bio-geomorphic change to channels intersected by the Central Arizona Project (CAP) canal. Water and sediment are fully or partially restricted to move downstream due to the CAP canal, which acts as a dam to these streams. Upstream of traversed channels, vegetation cover has increased over time causing the development of a green-up area. Along the canal, several channels remain longitudinally connected via culvert or overchute to the downstream sections of the stream channels. The two streams examined both had longitudinal connectivity, but only one of the streams had a green-up area upstream. Field work and channel surveys reveal that this is due to the culvert’s peak discharge outflow. The stream with a developed green-up area had a culvert size that is approximately three times less than the discharge of this ephemeral stream causing the partial damming upstream during flow events. Green-up zones slowly enlarge over time as bed levels and adjacent desert surface heights increase from continual sediment deposition upstream causing the floodplain to laterally increase due to overbank flooding. It is estimated that the green-up area for the partially dammed stream will increase by approximately 1570 m 2 over the next 36 years. Suggestions for urban and agricultural development are presented in this paper in relation to these dynamic green-up areas. Understanding biogeomorphic processes along dammed ephemeral streams lends valuable insight to riparian conservation efforts and future urban development plans in desert regions.

1. Intermittent rivers and ephemeral streams (IRES) are watercourses that cease flow at some point in time and space. Arguably Earth's most widespread type of flowing water, IRES are expanding where Anthropocenic climates grow drier and human demands for water escalate. 2. However, IRES have attracted far less research than perennial rivers and are undervalued by society, jeopardizing their restoration or protection. Provision of ecosystem services by IRES is especially poorly understood, hindering their integration into management plans in most countries. 3. We conceptualize how flow intermittence governs ecosystem service provision and transfers at local and river-basin scales during flowing, non-flowing and dry phases. Even when dry or not flowing, IRES perform multiple ecosystem services that complement those of nearby perennial rivers. 4. Synthesis and applications. Conceptualizing how flow intermittence in rivers and streams governs ecosystem services has applied a socio-ecological perspective for validating the ecosystem services of intermittent rivers and ephemeral streams. This can be applied at all flow phases and in assessing impacts of altered flow intermittence on rivers and their ecosystem services in the Anthropocene.