A coupled dynamic leakage loss and flood routing model for ephemeral rivers with complex sections

Ephemeral rivers commonly occur in regions with a shortage of water resources, and their channel configuration tends to change substantially owing to cultivation, tree planting and sand extraction. There is an urgent need to restore degraded river ecosystems. During short-term water conveyance, water storage in sand pits and leakage in dry riverbeds retards the flow of water, which is detrimental for ecological restoration of the riparian zone. A coupled dynamic leakage loss and flood routing model was established to predict the flow processes in the complex river channel of the Yongding River in China. The model mainly included three sub-models of flow dynamics, dynamic leakage loss, and water balance along multiple cross sections of the river channel. The complex section is reflected in the different infiltration properties for each section, and the existence of sand pits. The water head was dominated by flow velocity and the overflow from sand pits. Owing to the difference in landforms and the deposited sediment size of the riverbed bottom, the river channel was divided into 11 cross sections and a sand pit to ascertain the respective infiltration or leakage loss processes. The input parameters of the model came from field surveys of sand pits, river geometry and hydrogeology. The model was also calibrated and validated using monitoring data from ecological water releases into the Yongding River in 2019 and 2020. This coupled model can predict the water leakage loss and flow process of the water head and also provide important guidance for river reconstruction and ecological restoration.

Geomorphological evolution of ephemeral rivers through historical and UAVs images

<p>Ephemeral rivers hydromorphological processes are intermittent and many times of fast response. Therefore they remain still quite unknown. The geomorphological mapping of river forms and geomorphological units is a useful tool to recognize the evolution, changes and the response of river adjustments of hydrological events.</p><p>A diachronic geomorphological mapping has been done in some ephemeral rivers located in Ebro basin, Segura basin and Calabrian ephemeral rivers. We are presenting the specific results of six reaches distributed by the Ebro basin (Tudela, Reajo, Alpartir, Cariñena, Valcodo, Sosa and Seco). The first historical aerial image is that of the American Flight B of 1956-57, another of the mid 80’s, the last official ortophotography available (around 2017), and a specific flight with an unmanned aerial vehicle (UAV) done during the winter of 2019. An altimetry correction has been performed on the first two images.</p><p>Different categories have been identified within the channel (active channel, principal channel and secondary channel), the sediment bars (vegetated, scant vegetated and non-active paleo-bars), the deposits coming from bank failures or tributaries, rocky areas, exhumed old sediment areas, consolidated or unconsolidated granular bed. The categories were mapped at different scales depending on the image quality (for example, from ≤ 1/300 scale of the UAV to ≤ 1/1,000 scale of the American flight).</p><p>This evolutionary cartography allows comparing the geomorphology of each river reach among different dates, considering the different resolution of the images and its limitations (i.e. previously, the results were unified to compare among them), and relating to the fluvial processes and changes on the river and basin.</p><p>This research was funded by ERDF/Spanish Ministry of Science, Innovation and Universities—State Research Agency (AEI) /Project CGL2017-84625-C2-1-R; State Program for Research, Development and Innovation Focused on the Challenges of Society. </p>

River bed dynamic monitoring on ephemeral/intermittent rivers – sand-bed response on topical drylands

<p>Intermittent and ephemeral rivers, prevalent in dryland areas, have less monitoring data than perennial rivers worldwide. It hinders studies about hydro-geomorphology dynamics on these streams, which is especially complex in rain-fed flow regime on tropical rivers. Irregular rain patterns characterise the tropical drylands, which reverberate in the hydro-geomorphological dynamic. Unmanned Aerial Vehicles (UAV) survey is an efficient and cheap technic to monitor these streams since the dry periods expose the riverbed surface. This research aimed to analyse the hydro-geomorphology dynamic on sandy bed reaches of an intermittent tropical river. Five UAV surveys were realised on eight sandy reaches, from headwater to the outlet, between 2020 January 7 and December 9 in the Tigre River – Brazilian Drylands –, a 30Km ephemeral/intermittent. The UAV photos from all the surveys were co-aligned to create matching DEMs. We compared the DEMs to identify channel morphology changes, calculating differences in the riverbed and riverbank. The DEMs comparison enabled to calculate the erosion and sedimentation volume to each reach. Simultaneously, we installed crest stages gauges to monitor the peak water level between the surveys. Lastly, we used five rain gauges to identify the necessary rain volume that generates flow events. The 2020 annual rain volume was close to the historical average, between 530mm and 700mm, on the pediments, up to 1000mm on highland headwaters. The average potential evapotranspiration is around 1400-1800mm/year, due to the tropical climate. There was an average of 3.4 extreme rainfall daily events (over 50mm/day) during the year and the rainest period was between March 15 to 26<sup>th</sup> when rained from 134mm to 376mm around the watershed. The surveys between January 18 and March 8 identified insignificant morphology changes on eight reaches. The peak water levels were between no flow to 0.49m; only the outlet reach showed slight erosion and water level reaching 1.1m. The rain events between March 15/26<sup>th</sup> generate the water level annual peaks at all the reaches, from 1.9m to 5.4m (outlet reach). Seven reaches increased the vertical incision around 20/30cm to 80cm, and localised pools were eroded to up 1.7m deep. The outlet part exhibit around 30 to 40cm of sedimentation even with a water level peak of 5.4m. This unusual response could be caused by backwater effect from the Espinho River flood, which Tigre River is a tributary, that trapped sediment in the Tigre River. These results highlight how dynamic intermittent/ephemeral tropical rivers and showed how low-cost UAV High-Resolution DEMs and stage crests are workable and efficient techniques to monitor ungauged intermittent/ephemeral rivers. Simultaneously, narrow the surveys timespan (Covid-19 pandemic hindered most of the monthly planned surveys) is essential to identify which flow events caused erosion and sedimentation and which rain events trigger flow events.  </p><p>Keywords: Sand-bed rivers; UAV HR-DEMs; Brazilian Drylands; Water Level Stage Crest, riverbed erosion</p>

Natural Pans as an Important Surface Water Resource in the Cuvelai Basin—Metrics for Storage Volume Calculations and Identification of Potential Augmentation Sites

Numerous ephemeral rivers and thousands of natural pans characterize the transboundary Iishana-System of the Cuvelai Basin between Namibia and Angola. After the rainy season, surface water stored in pans is often the only affordable water source for many people in rural areas. High inter- and intra-annual rainfall variations in this semiarid environment provoke years of extreme flood events and long periods of droughts. Thus, the issue of water availability is playing an increasingly important role in one of the most densely populated and fastest growing regions in southwestern Africa. Currently, there is no transnational approach to quantifying the potential storage and supply functions of the Iishana-System. To bridge these knowledge gaps and to increase the resilience of the local people’s livelihood, suitable pans for expansion as intermediate storage were identified and their metrics determined. Therefore, a modified Blue Spot Analysis was performed, based on the high-resolution TanDEM-X digital elevation model. Further, surface area–volume ratio calculations were accomplished for finding suitable augmentation sites in a first step. The potential water storage volume of more than 190,000 pans was calculated at 1.9 km3. Over 2200 pans were identified for potential expansion to facilitate increased water supply and flood protection in the future.

Defining Dry Rivers as the Most Extreme Type of Non-Perennial Fluvial Ecosystems

We define Dry Rivers as those whose usual habitat in space and time are dry channels where surface water may interrupt dry conditions for hours or a few days, primarily after heavy rainfall events that are variable in time and that usually lead to flash floods, disconnected from groundwater and thereby unable to harbor aquatic life. Conceptually, Dry Rivers would represent the extreme of the hydrological continuum of increased flow interruption that typically characterizes the non-perennial rivers, thus being preceded by intermittent and ephemeral rivers that usually support longer wet phases, respectively. This paper aims to show that Dry Rivers are ecosystems in their own right given their distinct structural and functional characteristics compared to other non-perennial rivers due to prevalence of terrestrial conditions. We firstly reviewed the variety of definitions used to refer to these non-perennial rivers featured by a predominant dry phase with the aim of contextualizing Dry Rivers. Secondly, we analyzed existing knowledge on distribution, geophysical and hydrological features, biota and biogeochemical attributes that characterize Dry Rivers. We explored the capacity of Dry Rivers to provide ecosystem services and described main aspects of anthropogenic threats, management challenges and the conservation of these ecosystems. We applied an integrative approach that incorporates to the limnological perspective the terrestrial view, useful to gain a better understanding of Dry Rivers. Finally, we drew main conclusions where major knowledge gaps and research needs are also outlined. With this paper, we ultimately expect to put value in Dry Rivers as non-perennial rivers with their own ecological identity with significant roles in the landscape, biodiversity and nutrient cycles, and society; thus worthy to be considered, especially in the face of exacerbated hydrological drying in many rivers across the world.

Mapping Paleohydrology of the Ephemeral Kuiseb River, Namibia, from Radar Remote Sensing

The Kuiseb River is one of the major ephemeral rivers of Western Namibia, setting the northern limit of the Namib Sand Sea and outflowing in the Atlantic Ocean at Walvis Bay. Such ephemeral rivers are of the highest importance for the country since they are related both to recent past climatic conditions and to potential water resources. Using high-resolution radar images from the Japanese ALOS-2 satellite, we mapped for the first time the numerous channels hidden under the surface aeolian sediments: while the non-permanent tributaries of the Kuiseb River appear north of its present-day bed, a wide paleochannel system running westward, assumed by previous studies, could be clearly observed in the interdune valleys in the south. Radar-detected channels were studied during fieldwork in May 2019, which produced both subsurface ground-penetrating radar profiles and high-resolution drone-generated digital elevation models. It allowed us to confirm the existence of the “Paleo–Kuiseb” drainage system, a remnant of the Holocene history of the Kuiseb River, moving northward under the progression of the Namib Sand Sea. Our observations also contribute to the explanation of the young age of the linear dunes at the northern edge of the Namib Sand Sea, which are currently active and are pushing the Kuiseb River course toward the north.

Performance of an Unmanned Aerial Vehicle (UAV) in Calculating the Flood Peak Discharge of Ephemeral Rivers Combined with the Incipient Motion of Moving Stones in Arid Ungauged Regions

Ephemeral rivers are vital to ecosystem balance and human activities as essential surface runoff, while convenient and effective ways of calculating the peak discharge of ephemeral rivers are scarce, especially in ungauged areas. In this study, a new method was proposed using an unmanned aerial vehicle (UAV) combined with the incipient motion of stones to calculate the peak discharge of ephemeral rivers in northwestern China, a typical arid ungauged region. Two field surveys were conducted in dry seasons of 2017 and 2018. Both the logarithmic and the exponential velocity distribution methods were examined when estimating critical initial velocities of moving stones. Results reveal that centimeter-level orthoimages derived from UAV data can demonstrate the movement of stones in the ephemeral river channel throughout one year. Validations with peak discharge through downstream culverts confirmed the effectiveness of the method. The exponential velocity distribution method performs better than the logarithmic method regardless of the amount of water through the two channels. The proposed method performs best in the combination of the exponential method and the river channel with evident flooding (>20 m3/s), with the relative accuracy within 10%. In contrast, in the river channel with a little flow (around 1 m3/s), the accuracies are weak because of the limited number of small moving stones found due to the current resolution of UAV data. The poor performance in the river channel with a little flow could further be improved by identifying smaller moving stones, especially using UAV data with better spatial resolution. The presented method is easy and flexible to apply with appropriate accuracy. It also has great potential for extensive applications in obtaining runoff information of ephemeral rivers in ungauged regions, especially with the quick advance of UAV technology.