HS2.1.3

Temporary waters span both terrestrial and aquatic environments, though the terrestrial phase is typically understudied. A key component in the ecology of these water bodies is the length of the hydroperiod. To date, hydroperiod length in temporary waters is determined largely by site visits and camera traps. These methods of determination however, are taxing on resources at fine temporal resolutions (daily). While water level loggers are able to determine hydroperiod length, they are relatively expensive and peak at 50°C, preventing the collection of terrestrial data, particularly within the tropics.

Here we propose an alternative low-cost method for the determination of a temporary pond’s hydroperiod length using anchored HOBO pendant dataloggers of temperature and light intensity. By analysing the environmental data collected at fine temporal resolution across dry and wet seasons - corroborated by daily rainfall collection and frequent site visits - the determination of phase, whether aquatic or terrestrial, using this method was possible. This then extended to the determination of the length of the hydroperiod.

In addition to determining hydroperiod length, this method also provided data on the diurnal temperature dynamics, photoperiod and irradiation intensity of the aquatic and terrestrial phases. Trends in pond drying were also detectable using these data. In the terrestrial phase, the method provided data on soil surface temperatures, which was particularly lacking for the Caribbean. These data are important in understanding environmental stress regimes among aquatic and terrestrial ecosystems, with applications in agriculture, conservation and infrastructure.

How to cite: Campbell, G. and Hyslop, E.: The use of environmental data in detailing the hydroregime of a temporary pond, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-629, https://doi.org/10.5194/egusphere-egu21-629, 2021.

Temporary rivers, characterized by shifts between flowing water, disconnected pools and dry periods, represent over 50% of the world’s river network and future climatic projections suggest their increase. These rivers are understudied, especially when only disconnected pools remain, because gauging stations or hydrological models do not inform of what happens after the cessation of flow. In addition, most of biological indicators for water quality are designed for flowing waters and their adequacy for temporary rivers is uncertain.

The development of biological metrics adequate for the assessment of disconnected pools is difficult, because the high species replacement during and following flow cessation. For this reason, one hydrological variable of paramount importance for the assessment of ecological quality of disconected pools is the time since disconnection from the river flow.

The objective of our work is to present a methodology to estimate the time since disconnection of pools from the river flow. This methodology, following the Gonfiantini (1986) model, is based on the sampling of water stable isotopes in disconnected pools. For pools disconnected from the groundwater, knowing the isotopic modification of the water in time due to evaporation, allows to estimate the relative volume of water evaporated since the pool has been disconnected. However, this approach gets complicated when pools have relevant rainfall inputs or exchanges with groundwater.

Within the Vallcebre research area (42º12’N and 1º49’E), two artificial pools, one covered with a transparent lid to prevent the input of rainfall and another uncovered, were installed to validate this methodology in controlled conditions. From July to November 2020, water volume of these pools were weekly measured and sampled for isotopic analysis. In parallel, meteorological variables were monitored and rainfall was also sampled for water stable isotopes.

To develop and validate an operational methodology for estimating the time since disconnection, we first calculated the relative amount of evaporated water based on the variations of isotopic composition of the covered pool samples, and estimated the time since disconnection (for a given natural pool) using the potential evaporation calculated from the meteorological data. For the uncovered pool, the information of amount and isotopic composition of rainfall was added in a mass balance model. Additionally, the same estimations were calculated with standard information (i.e. the meteorological data obtained from the National Meteorological Service and precipitation isotopes data from the Global Network of Isotopes in Precipitation (GNIP) of the International Atomic Energy Agency). Finally, measured volumes changes in pools, were used to assess the limitations of the operational methodology and the sensitivity of the results to meteorological conditions.

Our approach suggests that changes in isotopic composition can be a reliable method to estimate time since disconnection of pools in temporary rivers to better assess their ecological quality.

How to cite: Llorens, P., González, S., Latron, J., Múrria, C., Bonada, N., and Gallart, F.: Using stable isotopes to estimate the time since disconnection of pools in temporary rivers, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3059, https://doi.org/10.5194/egusphere-egu21-3059, 2021.

The paradigm of surface water flow regimes is central to the aquatic sciences, where the timing, duration, frequency, magnitude, and rate of change of flow drive physical, chemical, and biological functions in aquatic systems. However, non-perennial streams comprise the majority of the global river network and there is a need to understand not just whether or not a stream periodically dries, but how it dries. Here, we propose to flip the script on flow regimes by presenting a comprehensive 'drying regime' framework to characterize stream drying.  We then identify similar drying characteristics in streams across watersheds with a broad range of climates, physiographic regions, and land uses. Using daily streamflow from 894 U.S. Geological Survey streamflow gages we isolated over 25,000 unique drying events over a period from 1979 - 2018. From these drying events we identified and calculated streamflow metrics that describe timing, duration, magnitude, frequency, and rate of change of stream drying. Using multivariate statistics, k-means clustering, and random forest analyses we grouped drying events into distinct drying regimes and determined the drivers of the clustered regimes. K-means clustering resulted in 4 distinct drying regimes characterized by (1) more frequent drying, (2) longer no-flow duration, (3) drying occurring following low antecedent flows, and (4) flashy high frequency drying, respectively. The majority of gages had more than one drying regime present at different times within each year, suggesting that dominant flow paths or drivers varied through time  Clustered drying regimes show low event-scale spatial coherence, and while drying regimes (1) and (2) show similar frequency throughout the year, (3) and (4) are substantially more frequent during summer months. Based on random forest analysis, land cover characteristics appear to drive drying event assignment to drying regimes more than climate variables. Furthermore, increased importance of individual watershed properties shows that the structural makeup of the watershed is notably more important to how an intermittent system dries than climate or physiographic characteristics. Non-perennial systems have unique functions due to the occurrence of both flowing and dry states, yet most of the past efforts rely on frameworks built around perennial streamflow behavior. Our work presents a novel drying regime framework to allow future studies to more effectively connect river drying to the physical, chemical, and biological functioning in these systems. This framework may also aid in current sustainable river management, including engineered flow regimes that are designed to balance water allocations with ecosystem requirements.

How to cite: Price, A., Jones, C. N., Hammond, J., Zimmer, M., and Zipper, S.: The drying regimes of non-perennial rivers, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6544, https://doi.org/10.5194/egusphere-egu21-6544, 2021.

Temporary streams (i.e. streams that temporarily cease to flow) are becoming a hot research topic in hydrology. These streams provide an invaluable contribution to riverine ecosystems, as they host a variety of habitats (from lotic to lentic and terrestrial) which sustain high biodiversity. Temporary streams can be found in different regions of the world and are characterized by strongly heterogeneous flow patterns, from flashy streams that flow only after rainfall events to rivers that episodically experience droughts. Many recent studies investigated temporary streams, originating interesting observational datasets about event-based or seasonal network dynamics. Empirical or conceptual models are usually employed for assessing the main physical drivers of network dynamics in each specific study site.
In this contribution, we develop and apply novel theoretical tools to understand how the local statistical properties of each reach of the network affect the catchment-scale variability of the active length. In particular, the Stream Length Duration Curve (SLDC) is proposed to efficiently summarize catchment-scale dynamics of the active length, providing an objective way to quantify network dynamics. The concept of SLDC is applied to a number of Italian headwater catchments, where data about temporal changes in the configuration of the flowing stream are available, providing a clue for the characterization of emergent temporal and spatial patterns of network dynamics. The Stream Length Duration Curve can facilitate comparisons across different catchments an time periods, possibly enabling and objective classification of temporary streams. 

How to cite: Durighetto, N., Senatore, A., and Botter, G.: Characterizing temporary stream dynamics: the Stream Length Duration Curve, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7897, https://doi.org/10.5194/egusphere-egu21-7897, 2021.

Ephemeral catchments, where streamflow is frequently zero or negligible, are common across the world yet difficult to model reliably. This paper evaluates probabilistic approaches for modelling streamflow in ephemeral catchments, with a focus on the description of predictive uncertainty using residual error models.

We compare an explicit treatment of zero flows using a censoring approach versus a simpler pragmatic approach where the lower streamflow bound of zero is applied in prediction only. Following a theoretical exposition, empirical comparisons are reported using a daily rainfall-runoff model (GR4J), four residual error schemes (based on log, log-sinh and Box-Cox (BC) transformations with power parameter L = 0.2 and 0.5), 74 Australian catchments with diverse hydroclimatology, and five performance metrics, including reliability, precision, bias and proportion of zero flow days.

The explicit approach is most beneficial in "mid-ephemeral" catchments (5-50% zero flows) where it offers substantial improvements over the pragmatic approach. The BC0.2 and BC0.5 transformations are Pareto optimal: BC0.2 achieves better characterisation of predictive uncertainty, whereas BC0.5 attains lower volumetric bias. In "low-ephemeral" catchments (<5% zero flows) the pragmatic approach is sufficient, whereas in "high-ephemeral" catchments (>50% zero flows) both approaches incur limitations and further method development is warranted. The findings provide guidance on improving probabilistic streamflow predictions in ephemeral catchments.

How to cite: Kavetski, D., McInerney, D., Thyer, M., Lerat, J., and Kuczera, G.: Improving representation of zero flows in probabilistic hydrological modelling of ephemeral catchments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3640, https://doi.org/10.5194/egusphere-egu21-3640, 2021.

The non-perennial streams and rivers are predominant in the Mediterranean region and play an important ecological role in the ecosystem diversity in this region. This class of streams is particularly vulnerable to climate change effects that are expected to amplify further under most climatic projections. Understanding the potential response of the hydrologic regime attributes to climatic stress helps in planning better conservation and management strategies. Bouregreg watershed (BW) in Morocco, is a strategic watershed for the region with a developed non-perennial stream network, and with typical assets and challenges of most Mediterranean watersheds. In this study, a hybrid modeling approach, based on the Soil and Water Assessment Tool (SWAT) model and Indicator of Hydrologic Alteration (IHA) program, was used to simulate the response of BW's stream network to climate change during the period: 2035-2050. Downscaled daily climate data from the global circulation model CNRM-CM5 were used to force the hybrid modeling framework over the study area. Results showed that, under the changing climate, the magnitude of the alteration will be different across the stream network; however, almost the entire flow regime attributes will be affected. Under the RCP8.5 scenario, the average number of zero-flow days will rise up from 3 to 17.5 days per year in some streams, the timing of the maximum flow was calculated to occur earlier by 17 days than in baseline, and the timing of the minimal flow should occur later by 170 days in some streams. The used modeling approach in this study contributed in identifying the most vulnerable streams in the BW to climate change for potential prioritization in conservation plans.

How to cite: De Girolamo, A. M., Brouziyne, Y., Benaabidate, L., Aboubdillah, A., El Bilali, A., Bouchaou, L., and Chehbouni, A.: Modeling the response of non-perennial streams to climate change impact: The Bouregreg watershed in Morocco, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9414, https://doi.org/10.5194/egusphere-egu21-9414, 2021.

Most of the basins in the Mediterranean Region are characterized by a large spatial gradient in rainfall and temperature and heterogeneity in lithology, soil, and land use. Such environmental factors determine a specific hydrological regime of the river systems that generally includes periods of absence of flow and flash flood events.

In the past decades, several countries in South Europe did not invest resources for the monitoring of the intermittent river systems. Currently, several basins have limited time series of streamflow and water quality data. In addition, it is not rare the case of climate stations not well distributed in the basin as well as the presence of several gaps in the time series.

The lithology and geological features are among the main factors affecting the flow regime, playing a crucial role in groundwater and surface-water interaction and water exchange for which the flow may appear and disappear along with the river network. In such a complex environment, the hydrological and water quality model set up and run may be challenging.

Through a case study, this work aims to face some challenges and to define problem-solving in simulating hydrology in Mediterranean basins. The area is characterized by (i) heterogeneity in lithology with karst areas, (ii) limited flow data availability for calibrating the model, (iii) flow intermittence in the river network. The Soil and Water Assessment Tool (SWAT) was applied to the Canale D’aiedda  (Puglia, Italy), a temporary karst river basin under the Mediterranean climate and with limited data availability. Different solutions were tested to simulate the hydrological processes in the karstic areas including both GIS elaborations and model parameters settings and modifications. Among the main parameters, infiltration and transmission losses and soil hydraulic parameters resulted in the most relevant in simulating hydrology in the karst areas. To calibrate the model, a split-in-space procedure was adopted to overcome the limited streamflow measurement availability. Finally, a zero-flow threshold was introduced to predict the number of zero-flow days in the intermittent river reaches, simulating accurately the flow intermittence and the extreme low flow.

The results show that by using specific strategies in setting-up and calibrating the model, the SWAT model is able to simulate daily streamflow with acceptable performances in complex river basins.

How to cite: Ricci, G. F., Centanni, M., Gentile, F., and De Girolamo, A. M.: Simulating streamflow in a temporary karst river system, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9555, https://doi.org/10.5194/egusphere-egu21-9555, 2021.