Flow Recession Research Articles

Comparison of streamflow recession between plantations and native forests in small catchments in Central‐Southern Chile

AbstractIn central Chile, many communities rely on water obtained from small catchments in the coastal mountains. Water security for these communities is most vulnerable during the summer dry season and, from 2010 to 2017, rainfall during the dry season was between 20% and 40% below the long‐term average. The rate of decrease in stream flow after a rainfall event is a good measure of the risk of flow decreasing below a critical threshold. This risk of low flow can be quantified using a recession coefficient (α) that is the slope of an exponential decay function relating flow to time since rainfall. A mathematical model was used to estimate the recession coefficient (α) for 142 rainstorm events (64 in summer; 78 in winter) in eight monitored catchments between 2008 and 2017. These catchments all have a similar geology and extend from 35 to 39 degrees of latitude south in the coastal range of south‐central Chile. A hierarchical cluster analysis was used to test for differences between the mean value of α for different regions and forest types in winter and summer. The value of α did not differ (p < 0.05) between catchments in winter. Some differences were observed during summer and these were attributed to morphological differences between catchments and, in the northernmost catchments, the effect of land cover (native forest and plantation). Moreover, α for catchments with native forest was similar to those with pine plantations, although there was no difference (p < 0.05) between these and Eucalyptus plantations. The recession constant is a well‐established method for understanding the effect of climate and disturbance on low flows and baseflows and can enhance local and regional analyses of hydrological processes. Understanding the recession of flow after rainfall in small headwater catchments, especially during summer, is vital for water resources management in areas where the establishment of plantations has occurred in a drying climate.

Development and Verification of the Available Number of Water Intake Days in Ungauged Local Water Source Using the SWAT Model and Flow Recession Curves

Climate change significantly affects water supply availability due to changes in the magnitude and seasonality of runoff and severe drought events. In the case of Korea, despite a high water supply ratio, more populations have continued to suffer from restricted regional water supplies. Though Korea enacted the Long-Term Comprehensive Water Resources Plan, a field survey revealed that the regional government organizations limitedly utilized their drought-related data. These limitations present a need for a system that provides a more intuitive drought review, enabling a more prompt response. Thus, this study presents a rating curve for the available number of water intake days per flow, and reviews and calibrates the Soil and Water Assessment Tool (SWAT) model mediators, and found that the coefficient of determination, Nash–Sutcliffe efficiency (NSE), and percent bias (PBIAS) from 2007 to 2011 were at 0.92%, 0.84%, and 7.2%, respectively, which were “very good” levels. The flow recession curve was proposed after calculating the daily long-term flow and extracted the flow recession trends during days without precipitation. In addition, the SWAT model’s flow data enables the quantitative evaluations of the number of available water intake days without precipitation because of the high hit rate when comparing the available number of water intake days with the limited water supply period near the study watershed. Thus, this study can improve drought response and water resource management plans.

The 2019/2020 floods in the UK: a hydrological appraisal

The 2019/2020 floods in the <scp>UK</scp>: a hydrological appraisal

The Use of Dual and Surge Irrigation to Improve The Efficiency of Surface Irrigation

A field experiment was conducted in a farm(5km northwest of Ramady City) during Autumn Season in a silty clay loam sedimentary soil. The ground water table depth is 120 cm. Field has divided into 10 strips(2.40x80)m with spaces 1m between strips and 2 m between replicates each strips has divided into 3 furrows (20 cm in depth, 80 m length,with 0.2% slop) with each others.Study factors included Contineous Irrigation (conventional), Dual Contineous irrigation, Contineous irrigation with two phases, Surge irrigation, Dual Surge irrigation Cycle ratio was 0.25 with time On-time 4 minutes and off-time 12 minutes for both dual surge irrigation and surge irrigation with discharge of 2 litter/sec. for both advance phase while the discharge after completing advance phase was 0.8 litter/sec. The plots were planted with corn(Zea mays L)and irrigated drainage water with salinity 5.8-6.2 ds/m, and Euphrates water (1.1-1.3 ds/m). Consumptive water use was calculated by Evaporation Pan method. Twenty test has been carried out for advance and recession of flow along furrow by using eight observation stations. Apportinity time and absorbed water depth in each middle furrow was calculated at each station. Practical and Theoretical relative yield of corn were calculated.Results showed that Dual Surge Irrigation system reduced of salty water used to complete advance phase with rate of 12.41% comparing with Dual Contineous irrigation system, The efficiency of Salt- water displacement proceeses downward increased by 11.56%. by the effect of fresh water under Dual surge irrigation system compared with Dual Contineous Irrigation system, The percentage fresh water saving reached to 44.94% and 51.30% in dual surge and dual Contineous irrigation system respectively, Insert Surge Irrigation technique on Dual Contineous Irrigation system leads to reduce salt- water advance phase time, late in recession time in Dual surge Irrigation and Surge Irrigation with the other treatments during advance and post- advance phase, distribution of Dual surge Irrigation system as compared with dual Contineous irrigation system in salt- water distribution efficiency during advance phase and uniformity of total absorption depth along the furrow for all testing months during the season, theoretical relative yield (RY%) has decreased gradually with increasing in depended depth for calculating the average soil salinity (ECe) with assumption of no mixing between fresh and saline water, practical relative yield exceeds the theoretical relative productivity when assuming the condition of mixing between fresh water and salt water, the practical relative yield has distinction on the theoretical yield at depth of 0-90-cm in Maas 1984 equation.

Influence of groundwater and topography on stream drying in semi‐arid headwater streams

AbstractNon‐perennial streams comprise over half of the global stream network and impact downstream water quality. Although aridity is a primary driver of stream drying globally, surface flow permanence varies spatially and temporally within many headwater streams, suggesting that these complex drying patterns may be driven by topographic and subsurface factors. Indeed, these factors affect shallow groundwater flows in perennial systems, but there has been only limited characterisation of shallow groundwater residence times and groundwater contributions to intermittent streams. Here, we asked how groundwater residence times, shallow groundwater contributions to streamflow, and topography interact to control stream drying in headwater streams. We evaluated this overarching question in eight semi‐arid headwater catchments based on surface flow observations during the low‐flow period, coupled with tracer‐based groundwater residence times. For one headwater catchment, we analysed stream drying during the seasonal flow recession and rewetting period using a sensor network that was interspersed between groundwater monitoring locations, and linked drying patterns to groundwater inputs and topography. We found a poor relationship between groundwater residence times and flowing network extent (R2 &lt; 0.24). Although groundwater residence times indicated that old groundwater was present in all headwater streams, surface drying also occurred in each of them, suggesting old, deep flowpaths are insufficient to sustain surface flows. Indeed, the timing of stream drying at any given point typically coincided with a decrease in the contribution from near‐surface sources and an increased relative contribution of groundwater to streamflow at that location, whereas the spatial pattern of drying within the stream network typically correlated with locations where groundwater inputs were most seasonally variable. Topographic metrics only explained ~30% of the variability in seasonal flow permanence, and surprisingly, we found no correlation with seasonal drying and down‐valley subsurface storage area. Because we found complex spatial patterns, future studies should pair dense spatial observations of subsurface properties, such as hydraulic conductivity and transmissivity, to observations of seasonal flow permanence.

Estimation of hydrogeological parameters using physically based hydrological modelling and lithology data in ungauged creek catchments of southern Taiwan

ABSTRACT Streamflow recession analysis is widely used for estimating catchment-scale hydrogeological parameters in gauged catchments because of its low cost, ease of use, and efficiency in terms of results. In this work, however, the same methodology was applied to ungauged creek catchments, where low recession flows were simulated using physically based, semi-distributed, continuous hydrological modelling. Moreover, a combination approach was used to reasonably estimate the effective aquifer depth (D) with borehole lithology and digital elevation model data. The estimated hydraulic conductivity (k) values ranged between 10–5 and 10–3 m s–1, and they were comparable with the results of a field test. By contrast, the specific yield (Sy ) values obtained through streamflow recession analysis (10–7 to 10–5) were lower than the results of the field tests at different aquifer depths. However, the Sy values obtained for alluvial catchments (on the order of 10–5) were comparable. Thus, the results of the comparison support the application of the streamflow recession analysis scheme for estimating hydrogeological parameters in ungauged creek catchments.

A review of low-flow hydrology

The purpose of this article is to assess the present state of low-flow hydrology, which is the study of the lowest flow in a river during the dry seasons of the year. The topic begins with an examination of low-flow producing processes under natural circumstances, as well as a description of human variables that influence low flows directly or indirectly. Following that, a review of current low-flow estimate techniques from stream flow time-series is presented, including flow duration curves, frequency analysis of severe low-flow occurrences and continuous low-flow periods, baseflow separation, and stream flow recession characterization. The article goes through a number of low-flow features (indices) and their uses. The connections between low-flow features are shown in a separate section. A regional regression approach, graphical depiction of low-flow features, creation of regional curves for low-flow prediction, and application of time-series simulation methods are among the approaches for low-flow estimate in ungauged river catchments discussed in the article. The article provides an overview of current low-flow-related research efforts from across the world. The issue of fluctuating minimum river flows as a consequence of climatic variability, as well as specific uses of low-flow data in river ecological research and environmental flow management, are also addressed. The review is mainly based on research findings from the previous two decades.

Automated analysis of lateral river connectivity and fish stranding risks—Part 1: Review, theory and algorithm

AbstractRiverine fish stranding is of significant concern due to its potentially devastating impacts on fish populations already at risk. Because stranding is dependent on a wide range of biotic and abiotic factors, it is difficult to accurately identify and parameterize fish stranding risks for various river topographies, fish species/lifestages and flow ramping scenarios. This article presents a literature review, new concepts and a novel Python3 algorithm for post‐processing two‐dimensional hydrodynamic numerical model results to identify spatially explicit locations where fish stranding is likely, such as but not limited to downstream of hydropeaking facilities. Compared to previous stranding algorithms, this one is novel in its use of graph theory to find optimal fish escape routes and for its embedding in the free, open‐source river analysis software River Architect. Guided by biological parameter selection and supplied with two‐dimensional hydrodynamic model rasters, River Architect's Stranding Risk module is suitable for characterization of existing pool stranding risks, alternative flow regime and topographic design evaluation and post‐project assessment of rivers during flow recessions.

Runoff recession features in an analytical probabilistic streamflow model

The development of run-of-river hydropower requires the flow duration curve (FDC) to estimate the electricity production. As obtaining an FDC based on gauging station streamflow requires significant cost and time (i.e., more than several years), it is important to predict the FDC in the absence of streamflow data. An analytical probabilistic model of streamflow dynamics (p(Q) model) can calculate the FDC, which requires several rainfall and runoff recession parameters. To estimate the recession parameters in the absence of discharge data, we selected two approaches: the geomorphic recession flow model (GRFM) and geological recession approach (GRA). The GRFM and GRA associate the river network and geology, respectively, with runoff recession features. This study aims to comprehend the characteristics of the GRFM and GRA employed in the p(Q) model by targeting 57 basins throughout Japan. We found different performance trends between the GRFM and GRA that corresponded to specific basin characteristics. While the GRFM performed well in high precipitation basins, the GRA performed well in basins with abundant unconfined groundwater contributions. These results agree with the features in the derivations of the GRFM and GRA. In addition, we found a complementary relationship between the GRFM and GRA based on the baseflow index (BFI), which suggests that the complementary relationship is one method to improve the performance of the p(Q) model in the absence of streamflow data.

Flow‐ecology modelling to inform reservoir releases for riparian restoration and management

AbstractLinked hydrologic, hydraulic, and ecological models can facilitate planning and implementing water releases from reservoirs to achieve ecological objectives along rivers. We applied a flow‐ecology model, the Ecosystem Functions Model (HEC‐EFM), to the Bill Williams River in southwestern USA to estimate areas suitable for recruitment of riparian tree seedlings in the context of managing flow releases from a large dam for riparian restoration. Ecological variables in the model included timing of seed dispersal, tolerable rates of flow recession, and tolerable duration of inundation following germination and early seedling establishment for native Fremont cottonwood and Goodding's willow, and non‐native tamarisk. Hydrological variables included peak flow timing, rate of flow recession following the peak, and duration of inundation. A one‐dimensional hydraulic model was applied to estimate stage‐discharge relationships along ~58 river kilometres. We then used HEC‐EFM to apply relationships between seedling ecology and streamflow to link hydrological dynamics with ecological response. We developed and validated HEC‐EFM based on an examination of seedling recruitment following an experimental flow release from Alamo Dam in spring 2006. The model predicted the largest area of potential recruitment for cottonwood (280–481 ha), with smaller areas predicted for willow (174–188 ha) and tamarisk (59–60 ha). Correlations between observed and predicted patches with successful seedling recruitment for areas within 40 m of the main channel ranged from 0.66 to 0.94. Finally, we examined arrays of hydrographs to identify which are most conducive to seedling recruitment along the river, given different combinations of peak flow, recession rate, and water volume released. Similar application of this model could be useful for informing reservoir management in the context of riparian restoration along other rivers facing similar challenges.

Riparian vegetation model to predict seedling recruitment and restoration alternatives

Native riparian vegetation communities have declined downstream of large water infrastructure like dams and diversions, owing to water management operations that prevent successful seedling colonization and recruitment. Altered timing and magnitude of reservoir releases to fulfill competing water demands often lead to reduced peak discharges and flow recession rates that do not support native riparian reproduction processes. To achieve short-term ecosystem function in highly regulated rivers an alternative method might be restoration planting, whereby success depends on identifying appropriate planting location and spatial extents. This study aims to provide a methodology to inform resource managers about the extent of possible natural seedling recruitment under average and wet hydrologic conditions, as well as constrain restoration planting operational uncertainties. Results from field surveys and simulations showed limited favorable areas for successful riparian seedling recruitment under regulated flows, regardless of hydrologic conditions in the basin. However, wet (11.4 ha) hydrologic conditions were more (approximately 11 times) favorable than average (1 ha) conditions for seedling recruitment. Furthermore, model results identified the location and spatial extent (25.6 ha) of favorable restoration planting areas during average flow. This extent is approximately 25 times larger than natural recruitment during an average (hydrological) year and even twice that for natural recruitment for a wet year. This suggests that ground operational activities guided by numerical modeling may effectively constrain planting uncertainties.

Seasonal Variations of the Depletion Factor during Recession Periods in the Senegal, Gambia and Niger Watersheds

The daily depletion factor K describes the discharge decrease of rivers only fed by groundwater in the absence of rainfall. In the Senegal, Gambia and Niger river basins in West Africa, the flow recession can exceed 6 months and the precise knowledge of K thus allows discharge forecasts to be made over several months, and is hence potentially interesting for hydraulic structure managers. Seasonal flow recession observed at 54 gauging stations in these basins from 1950 to 2016 is represented by empirical and usual conceptual models that express K. Compared to conventional conceptual models, an empirical model representing K as a polynomial of the decimal logarithm of discharge Q gives better representations of K and better discharge forecasting at horizons from 1 to 120 days for most stations. The relationship between specific discharge Qs and K, not monotonous, is highly homogeneous in some sub-basins but differs significantly between the Senegal and Gambia basins on the one hand and the Niger basin on the other. The relationship K(Q) evolves slightly between three successive periods, with values of K generally lower (meaning faster discharge decrease) in the intermediate period centered on the years 1970–1980. These climate-related interannual variations are much smaller than the seasonal variations of K.

A comparison of benthic algal and macroinvertebrate communities in a dammed and undammed Mediterranean river (Eel River watershed, California, USA)

AbstractFlow regulation by dams can reduce hydrologic variability, and subsequently, alter river food webs. This study compared the summer algal and macroinvertebrate assemblages from the mainstem Eel River (Mainstem) below Cape Horn Dam to that of the unregulated Middle Fork Eel River (Middle Fork). Despite above average rainfall in 2017, flow regulation produced a steeper spring flow recession limb with only 15.72 cubic meter per second in the dammed Mainstem in late May, which was about half the undammed Middle Fork's discharge of 32.28 cubic meter per second at the same time of year. Peak summer water temperatures in the Mainstem (mid‐August) were later relative to the Middle Fork (late July) based on past years of monitoring. The regulated Mainstem supported less Cladophora with lower mid‐summer algal richness, resulting in a macroinvertebrate community less reliant on Cladophora, with fewer key invertebrates such as Chironominae compared to the unregulated Middle Fork. Dam systems that reduce flow variation in Mediterranean rivers may shift the downstream community away from seasonal proliferation of dense green filamentous algae and invertebrate grazers typical of rivers in this climate, and towards adnate epilithic algae and cyanobacteria and generalist invertebrates, due to the encroachment of riparian vegetation and reduced thermal variation.

A Climatic Perspective on the Impacts of Global Warming on Water Cycle of Cold Mountainous Catchments in the Tibetan Plateau: A Case Study in Yarlung Zangbo River Basin

Global warming has a profound influence on global and regional water cycles, especially in the cold mountainous area. However, detecting and quantifying such changes are still difficult because noise and variability in observed streamflow are relatively larger than the long-term trends. In this study, the impacts of global warming on the catchment water cycles in the Yarlung Zangbo River Basin (YZRB), one of most important catchments in south of the Tibetan Plateau, are quantified using a climatic approach based on the relationship between basin-scale groundwater storage and low flow at the annual time scale. By using a quantile regression method and flow recession analysis, changes in low flow regimes and basin-scale groundwater storage at the Nuxia hydrological station are quantified at the annual time scale during 1961–2000. Results show annual low flows (10th and 25th annual flows) of the YZRB have decreased significantly, while long-term annual precipitation, total streamflow, and high flows are statistically unchanged. Annual lowest seven-day flow shows a significantly downward trend (2.2 m3/s/a, p &lt; 0.05) and its timing has advanced about 12 days (2.8 day/10a, p &lt; 0.1) during the study period. Estimated annual basin-scale groundwater storage also shows a significant decreasing trend at a rate of 0.079 mm/a (p &lt; 0.05) over the study period. Further analysis suggests that evaporation increase, decreased snow-fraction, and increased annual precipitation intensity induced by the rising temperature possibly are the drivers causing a significant decline in catchment low flow regimes and groundwater storage in the study area. This highlights that an increase in temperature has likely already caused significant changes in regional flow regimes in the high and cold mountainous regions, which has alarming consequences in regional ecological protection and sustainable water resources management.

Investigating meteorological effect on river flow recession rate in an arid environment

ABSTRACT This study explores previously unreported linkage between recession rates of rainfall hyetograph and river flow hydrograph in an arid environment in Sultanate of Oman. Ephemeral streams are hydraulically disconnected from groundwater aquifers and depend on rainfall to produce water that flows only for hours or a few days at most. It therefore prompted to hypothesize that the recession rate of the rainfall event controls the corresponding recession of river flow. To test this assumption, 1-h river flow rates and 20-min rainfall rates in Al-Khoud catchment area for the period 1997–2013 were analysed. The river flow recession rate and antecedent river flow were found to be inversely proportional, while their relation improved with increasing time span of cumulating the antecedent river flow. The results further show that the simulation of river flow recession rate can be improved by incorporating the combined effects of rainfall recession rate and antecedent moisture content.

Uncertainty propagation in coupled hydrological models using winding stairs and null-space Monte Carlo methods

By integrating disciplinary sub-models, coupled hydrological models allow the exchange of output and input fluxes among their sub-models. While such coupling of models can mirror the conceptual representation of the water-cycle, potential uncertainty propagation and aggregation across the sub-models may limit their overall performance. There are limited studies dealing with uncertainty in coupled hydrological models due to the high computational needs, the absence of detailed data, and the lack of efficient uncertainty propagation frameworks. This study presents an effective uncertainty propagation framework using a combination of statistical techniques, multi-variable calibration, and parallel computation. The framework was tested using a synthetic mathematical coupled model and a real‐world, coupled surface water (PRMS) and subsurface (MODFLOW) model. For the synthetic coupled model, the framework has shown its effectiveness to reveal the interplay of input variables, quantify the uncertainties within each sub-model and track their propagation through the coupled modeling system. For the PRMS-MODFLOW model, the framework has demonstrated how uncertainty in input precipitation, surface water, and subsurface water sub-models influences the different segments of a hydrograph. The results also indicate that improved predictions of high flow require a better quantification of input uncertainty. In contrast, baseflow and recession flow uncertainties largely depend on the subsurface and surface water sub-models, respectively. The presented framework, in addition to providing relatively comprehensive uncertainty information on the integrated model outputs, it helps to identify the potential sources of uncertainty that can be used for further model improvement and guide new data collection campaigns.

Eigenmodels to forecast groundwater levels in unconfined river-fed aquifers during flow recession

Low-land alluvial gravel aquifers are formed from, and tend to be recharged, by rivers. These interconnected river - groundwater systems can be highly dynamic with groundwater levels following the seasonality of the hydrological regime of the river. The associated groundwater resources are regularly under stress during summer periods when abstractive demand is high and recharge is low. Predicting lead-times for critical groundwater levels allows for a more flexible and adaptive groundwater management. An eigenmodel approach is proposed here as a way of making such predictions, fast and efficiently. The eigenmodel is a mathematical concept that represents the hydraulic function of a groundwater aquifer as a set of conceptual linear reservoirs, arranged in-series. River recharge, land surface recharge, and groundwater abstraction for irrigation are considered as model forcings. The eigenmodel approach is demonstrated on three wells of the unconfined Wairau Aquifer in the Marlborough District of New Zealand, which are used for water resources management. Individual eigenmodels were calibrated to historic data and predictive uncertainty bounds were determined by Markov chain Monte Carlo sampling. Hindcasting of past recession periods showed a low predictive error of the models and a good coverage of the predictive uncertainty bounds. The main advantage of the approach is a 4-orders of magnitude higher computational efficiency compared to a numerical benchmark model. This allows for probabilistic simulation in operational forecasting of groundwater levels. The framework is implemented as a web application for 30-day operational forecasts that comprises automatic data downloads and model input generation, stochastic simulation, uncertainty estimation, visualization, and daily updates on a website.

Flow Recession Equations for Karst Systems

AbstractThe proper management and exploitation of karst water resources require first and foremost an accurate characterization of the karst aquifer. Analysis of the recession curve of a spring hydrograph is one efficient approach of determining the hydraulic characteristics of a karst system. In the present work, six flow recession equations for various flow conditions are presented that highlight the dichotomy of the dual flow characteristics between the fissured matrix and the conduit system. They are defined in terms of lumped physical parameters that characterize the dominant flow processes occurring in a karst aquifer. The processes consist either of a pressure‐driven flow or a gravity‐driven flow in a conduit that is interacting laterally with the groundwater flow in the surrounding matrix. The recession equations are composed of two distinct functions that represent separately the flood flow component and the base flow component. They allow the separate evaluation of the conduit and matrix drainage processes and capture the strongly varying recession in the early stages as well as the slow decline in the latter stages of the spring recession hydrograph. Approximate equations for large times are also developed that allow the direct estimation of the model parameters of the karst system using simple algebraic formulas. The flow recession equations were applied to three real karst systems and proved to be effective in simulating the observed recession hydrographs and estimating its hydraulic properties.

Three techniques for flow component identification from daily discharge data in Madjez Ressoul catchment, Algeria

Empirical models of recession analysis provide information about surface and ground flow processes during periods of drought. The objective of this research is to evaluate the performance of three different techniques (individual segments, filtering method, genetic algorithms) in assessing the contribution of recession flow component in calculating runoff, using daily records of discharge. These techniques will focus on the values of the recession index covering ten flood events in the period of 1973–2003. The outputs of these models were then compared and their results appeared to validate two particular techniques: the filtering method with one parameter and the genetic algorithm method. The coefficient of determination (R2 = 0.7324–0.935) and the cross probability which equals 0.9 confirm the best separation of all events. Absolute similarities between flow types in the filtering method and genetic algorithms present systematic differences in the calibration form and on the consideration of obstacles and limitations.

How does climate variability affect water quality dynamics in Canada's oil sands region?

In Canada's oil sands region, classic boreal hydrology (i.e., winter low flow followed by peaks during spring freshet and then summer flow recession) combined with erosion of both natural and anthropogenically-exposed bitumen results in seasonal and inter-annual variability in stream water chemistry. Using data collected from all seasons over three years (2012–2015), we investigated the mechanisms driving spatial and temporal change in the concentration of 26 water quality parameters for six rivers draining Canada's oil sands region. Mantel tests showed a strong spatial aggregation of climatic drivers (average daily precipitation, accumulated precipitation, snow water equivalent) associated with west versus east discharge patterns. Wavelet analysis highlighted unique watershed attributes, in particular the importance of developed area in lowering responsiveness to seasonal precipitation. Concentrations of most chemical parameters (20 of 23) showed distinct temporal patterns that were correlated with seasonal changes in hydrology which, in turn, were related to changes in weather. Comparison of concentrations observed in this study with those reported in the scientific literature for the same watersheds showed 81% of comparisons differed significantly. This was likely due to the short duration of previous field campaigns and thus the sampling of a very narrow window of the annual streamflow regime.