Dry Season Streamflow Research Articles

Modeling climate change projection and its impact on the streamflow in the Yadot watershed, Genale Dawa basin, Ethiopia

ABSTRACT Varied streamflow response to climate between river basins and seasons highlights the importance of further research on different basins and watersheds in different seasons to help plan adaptation options at watershed scale. This study investigated the hydrological impacts of climate change over the Yadot watershed. The multi-model ensemble of three regional climate models (CCLM4.8, RACMO22T, and RCA4) under RCP4.5 and RCP8.5 emission scenarios for 2021–2050 and 2051–2080 periods were used. The SWAT model was used to simulate the streamflow. Climate model projections have indicated that precipitation will slightly increase during both the wet and dry seasons from 0.59 to 2.08% and 0.02–1.59%, respectively. The annual projected precipitation will increase by 0.13–1.66%. The change in the projected maximum and minimum temperatures in both dry and wet seasons increased by a range of 0.61–1.9 °C and 0.65–2.07 °C, respectively. Similarly, the change in the projected minimum temperatures in both dry and wet seasons increased by a range of 1.07–2.01 °C and 0.06–1.66 °C, respectively. The wet and dry season stream flow increased by 6.23–9.36% and 3.16–5.46%, respectively. The findings of this study can help to guide water resources planners and designers in planning and managing water resources effectively for future use.

Assessing the impacts of climate change on streamflow in the upper Omo Gibe basin of Southwest Ethiopia using MIKE NAM

ABSTRACT Climate change affecting the rainfall-runoff processes in the upper Omo Gibe river basin was articulated using bias-corrected ensemble simulations of the CORDEX-Africa under RCP4.5 and RCP8.5 climatic scenarios. CMhyd was used to downscale the ensembles statistically followed by bias-adjustment using distribution mapping. MIKE NAM model was used (1980-2005) to project streamflow for three time periods; the near future (2023-2048), the middle future (2049-2074), and the far future (2075-2100). The average temperature of the basin will rise by 1.59°C and 2.56°C, respectively, under RCP4.5 and RCP8.5, respectively, while, the precipitation for wet and dry seasons decreases by 7.97% and 23.67%, respectively. MIKE NAM has demonstrated a strong correlation between simulated and observed runoff with coefficients of determination (R2) and NSE of more than 0.75. It has been predicted that the mean dry season, wet season, and annual streamflow will decline by 15-27%, 4-13%, and 8-24%, respectively. Intricate study of climate change followed by judicious and timely precautionary measures if not taken today, will decrease in streamflow and put negative consequences on long-term irrigation and hydropower development in the river catchment of upper Omo Gibe basin.

Watershed Hydrological Responses to Land Cover Changes at Muger Watershed, Upper Blue Nile River Basin, Ethiopia

Changes in land cover (LC) are the major factors influencing the hydrological processes within a watershed. Understanding the impacts of LC on watershed hydrology is crucial for planning and predicting land resource utilization, water resources, and sustaining hydrological balance. This study assesses the hydrological responses of LC changes in the Muger watershed located in the Upper Blue Nile River Basin (UBNRB) from 1986 to 2020. We used the Soil and Water Assessment Tool (SWAT) hydrological model to investigate the effects of LC on the hydrological process. The simulations were driven by several datasets, such as watershed elevations, mean climatology, hydrology and soil datasets, and LC satellite maps for three time periods (i.e., satellite imagery taken in 1986, 2003, and 2020). We found that the key LC changes that affected hydrological parameters in the Muger watershed are changes in cultivation land, forest land, and settlement. The expansion of cultivation land and shrinkage of forest and shrub lands triggered surface runoff and a reduction in groundwater between 1986 and 2003. Additionally, settlement was identified as the primary factor contributing to increases in evapotranspiration (ET) and surface runoff. The LC changes that occurred between 1986 and 2020 reduced the average annual, wet season, and dry season streamflow. Between 2003 and 2020, surface runoff decreased by 3.71% due to the effect of land landscape restoration interventions. The outcome of the study can assist decision-makers and planners in preparing adaptable strategies under changing LC conditions within a watershed.

Assessing the Future Water and Energy Security of a Regulated River Basin with a Coupled Land Surface and Hydrologic Model

To address the water-related issues faced by humans, the planning and construction of dams, water diversion projects, and other water infrastructures have been continuously adopted by decision makers worldwide. This is especially the case for the Yalong River Basin (YRB) in China, which is expected to be one of the most regulated rivers due to reservoir construction and the planned South-to-North Water Diversion project. To understand the potential impact of these water infrastructures on the water resources and hydropower production of the basin and downstream areas, we employ a land surface–hydrologic model with explicit representations of dam operation and water diversions in order to quantify the impact of reservoir operation and water diversion on the future water and energy security of the YRB. In particular, a conceptual reservoir operation scheme and a hydropower-optimized reservoir operation scheme are employed to predict the future release, storage and hydropower generation of the YRB, respectively. Results indicate that reservoirs can have noticeable, cumulative effects in enhancing the water security by reducing the wet season streamflow by 19% and increasing the dry season streamflow by 66%. The water diversion can result in an overall decrease in the streamflow, while the downstream reservoirs are expected to fully mitigate the decline in the dry season streamflow. The hydropower production is likely to decrease by 16% and 10% with conventional and optimized operation schemes, respectively, which suggests that the adaptation of operation rules alone cannot reverse the decline in the electricity production. Our findings can provide implications for sustainable water resource management.

Assessment of GPM IMERG and GSMaP daily precipitation products and their utility in droughts and floods monitoring across Xijiang River Basin

The recent increasing concerns about floods and droughts have brought up a pressing need for high-precision and high-resolution precipitation records. As representative satellite precipitation products (SPPs) in Global Precipitation Measurement (GPM) era, the Integrated Multi-satellitE Retrievals for GPM (IMERG) product and the Global Satellite Mapping of Precipitation (GSMaP) are widely acknowledged as reliable alternative sources. A quantitative evaluation of these new products is of great importance before their extended applications. This study comprehensively assesses the statistical performance and the application potential of IMERG (IMERG-Early, IMERG-Late and IMERG-Final) and GSMaP (GSMaP-NRT, GSMaP-Gauge-NRT, GSMaP-Gauge) products over Xijiang River Basin during 2009–2018 at a daily timescale. Both IMERG and GSMaP products have their own advantages. The results indicate that (1) at the grid scale, the IMERG-Final exhibits the highest correlation with gauge-based precipitation data while GSMaP-Gauge yields the lowest relative mean error. IMERG-Early and IMERG-Late have superiorities in almost all evaluation metrics relative to GSMaP-NRT and GSMaP-Gauge-NRT. (2) The temporal performance in specific months visually demonstrates that GSMaP-Gauge has a lower precipitation detection capacity in wet seasons due to a higher FAR, in opposite to other SPPs. (3) The precipitation detection ability of SPPs varies largely with different precipitation intensities. GSMaP-Gauge can identify nearly 90% rainfall events at the lowest threshold of 1 mm/day, but it tends to overestimate the non- and slight precipitation events up to small or moderate classes and highly underestimate precipitation amount of strong events. At extreme precipitation intensity (≥50 mm/day), >50% of extreme precipitation events were missed by SPPs resulting in unsatisfactory PODs below 0.5. (4) For meteorological drought monitoring, IMERG-Final exhibits highest CCs in all inspected timescales, succeed by GSMaP-Gauge. IMERG-Early and IMERG-Late still perform better than near-real-time GSMaPs in drought research. (5) In the hydrological simulation with the Soil and Water Assessment Tool (SWAT), GSMaP-Gauge-simulation shows the best accuracy regardless of wet or dry seasons with highest NSEs of 0.64 and 0.73, respectively. All near-real-time SPPs present an unacceptable hydrological utility and an underestimation of streamflow in dry seasons. The systematic evaluation of extreme streamflow indices highlights that of the four timelier products, IMERG-Early is most reliable in capturing extreme streamflow. GSMaP-NRT tends to exaggerate the flood magnitude and could be used as reference in estimating severe and continuous floods.

Dynamic quantile regression for trend analysis of streamflow time series

AbstractWater availability is an essential factor in maintaining the integrity of ecological processes and is a source of socio‐economic development. However, socio‐economic development increases the pressure on water resources. Thus, understanding the flow regime in a watershed is essential to correct water resource management. In this study, we propose using dynamic quantile regression (DQR) to analyse trends in streamflow time series. DQR is a subset of the general class of semi‐parametric quantile regression models, which is tailored for time series modelling. It allows for autoregressive dynamics and modelling of trending behaviour and seasonal fluctuations. With a single model, it is possible to estimate the impacts of predictor variables on any given quantile of the response distribution instead of simply evaluating such effects on the mean response, as is typically done in other statistical approaches. In other words, it is possible to gain knowledge on how predictors impact the magnitude of streamflow in wet (upper quantiles) and dry seasons (lower quantiles) separately. We used DQR to model a streamflow time series of 27 gauges, distributed in the Araguaia watershed in central Brazil. The results show that, except for a single gauge (Alto Araguaia), there are downward streamflow trends, thus non‐stationary behaviour. The model yielded excellent data fits (pseudo‐R2 above 0.80), and it was possible to obtain a confidence interval for each slope. In our analysis, the usefulness of DQR modelling for assessing trends in streamflow time series is shown and, consequently, for achieving efficient water resource management.

Urban runoff and stream channel incision interact to influence riparian soils and understory vegetation.

Riparian soil processes and vegetation are sensitive to water availability. Urbanization can alter riparian water availability by modifying stream flows and stream channel morphology. In cities, runoff from impervious surfaces tends to increase stormflow magnitudes, causing stream channels to incise, or downcut. This change in channel morphology has been linked to lowered water tables and drier conditions in temperate urban riparian zones, leading to shifts in riparian nitrogen (N) cycling and vegetation communities. In Mediterranean climates with distinct wet and dry periods, there is an additional dynamic to consider: runoff from urban water use can cause streams to flow when they would otherwise be dry. This dry-season stream flow could create increased, rather than decreased, water availability in urban riparian zones. However, channel incision may counteract this effect. We asked whether dry-season stream flow interacted with channel incision to influence riparian soil characteristics and understory vegetation along streams in Sacramento, California, which has a Mediterranean climate with an intense summer dry season. At 40 stream reaches that varied by severity of downcutting and presence of dry-season flow, we sampled soils and vegetation on top of stream banks and at the margin of the low-flow channel, an important location for nutrient cycling in dry climates. We measured soil moisture, organic matter, and ∂15 N, as well as total and perennial understory vegetation cover. We found that channel characteristics associated with incision limited the influence of dry-season stream flow on soil moisture, and this interaction appears to have lasting effects on soil organic matter and perennial vegetation on bank tops. At the stream margin, channel downcutting was associated with reduced soil organic matter and vegetation cover, while dry-season flow was associated with increased vegetation cover. Values of soil ∂15 N pointed to limited hydrologic linkage between stream flows and riparian bank soils along incised streams. Our findings suggest that channel incision could limit the ability of urban riparian ecosystems to mitigate low-flow water quality. However, where streams are not incised in Mediterranean climates, dry-season flows from urban runoff may actually increase riparian productivity and N cycling above historical levels.

Quantifying the impacts of decadal landuse change on the water balance components using soil and water assessment tool in Ghaggar river basin

The landuse and landcover change is considered as one of the most vital components of the global environmental change responsible for influencing the quantity, seasonal patterns and durations of the water flow within basins around the world. The aim of this study is to assess the impacts of changes in landuse landcover on the various hydrological components for Ghaggar river basin for a period of 1985–2015 using Soil and Water Assessment Tool model. The landuse landcover maps have been prepared using various Landsat satellite images for the year 1985, 1995, 2005 and 2015.The fixing changing method has been employed to quantify the impacts of landuse changes on hydrological components by changing the landuse maps of different year and keeping rest of the inputs same. The correlation and Partial Least square regression are used to examine the impacts of change in landuse on the hydrological components. The results unveiled that the increase in agriculture and built-up area and decrease in area of evergreen, deciduous and mixed forest. During the period 1985–2015 there is increase in annual stream flow (0.20%), wet season streamflow (1.55%), surface runoff (12.23%) and water yield (0.22%) on the contrary there is decrease in dry season streamflow (2.1%), Evapotranspiration (0.08%), groundwater flow (8.62%), Percolation (8.02%) and lateral flow (3.92%) owing to changes in landuse landcover. There is an urgent need to adopt soil and water conservation practices in the area to avert these deleterious effects of landuse landcover changes.

Evaluating the Management of a Tropical Reservoir: Implications of Climate Change for Water Availability

Most tropical islands have limited water supply due to the small size of their watersheds and aquifers, unique geology, and distribution of rainfall; each affecting the reliability of water resources for meeting agricultural needs. To improve water supply, reservoirs provide critical storage of surface water. However, climate-driven shifts in the distribution, intensity, and amount of rainfall will alter runoff that feeds reservoirs. Mass-balance modeling was used to evaluate the effects of various management constraints on water availability for the Wahiawā Reservoir (O‘ahu, Hawai‘i) as well as the impact of 20% less dry season rainfall and stream flow to assess the effect of projected climate change on maintaining: (1) a minimum downstream flow; (2) a minimum agricultural supply; (3) a minimum and maximum reservoir level; and (4) a combination of minimum downstream flow and minimum agricultural supply. Under current climate conditions, maintaining a minimal downstream water release could be met with minimal consequences for agricultural supply and reservoir level. However, both agricultural supply and downstream flow had to be reduced during dry periods to keep the reservoir level above a minimum level. Maintaining a minimum downstream flow resulted in a 4.2% reduction in mean agricultural supply and 2.2% reduction in mean reservoir elevation compared to baseline conditions. By contrast, a 20% reduction in dry season water inputs resulted in a 17.8% reduction in mean agricultural supply in the minimum downstream flow scenario compared to current rainfall conditions. Furthermore, the number of days the reservoir level dropped below the minimum needed for a fishery increased (range 1.15–25×) across scenarios compared to baseline. Successful management of surface water within a social-ecological system will be challenged by the increasing unpredictability of rainfall.

Stream Flow Changes and the Sustainability of Cruise Tourism on the Lijiang River, China

Water resources play a critical role in the sustainable development of river-based tourism. Reduced stream flow on the Lijiang River, south China, may negatively impact the development of cruise tourism. We explored the effects of stream flow changes on cruise tourism by determining (1) cruise tourism development indicators, (2) stream flow regime characteristics and their impacts on cruise tourism development indicators, and (3) climate variability and socio-economic factors effecting stream flow. Cruise tourism on the river has experienced rapid growth in recent decades. Stream flow regimes displayed no significant changes between 1960 and 2016, although dry season stream flow was significantly lower than in other seasons. We found that stream flow changes did not have a significant impact on the development of cruise tourism. As precipitation has not changed significantly, policies, including regulated stream flow from hydroelectric reservoirs, are assumed to mitigate reduced stream flow. However, increased irrigation and economic development, combined with future climate change, may increase challenges to cruise tourism. Future reservoir operations should prepare for climate change-related increases in temperature and insignificant changes in precipitation, and adopt adaptive measures, such as rationing water use in various sectors, to mitigate water shortages for supporting sustainable tourism development.

Impacts of climate change and reservoir operation on streamflow and flood characteristics in the Lancang-Mekong River Basin

The Lancang-Mekong River Basin (LMRB) is one of the most important transboundary river basins in Asia. While climate change perturbs the streamflow and affects flood events, reservoir operation may mitigate or aggravate this impact. Therefore, quantitative assessment of the climate change impact and reservoir effect on the LMRB is a vital prerequisite for future hydropower development and environmental protection. This study aimed to estimate the variation of the streamflow and flood characteristics affected by climate change and reservoir operation within the LMRB. A reservoir module was incorporated into the Variable Infiltration Capacity (VIC) model to simulate the streamflow susceptible to the reservoirs. It was found that the reservoirs had a substantial influence on the streamflow during 2008–2016, when many reservoirs were constructed in the LMRB. The reservoirs across the Lancang River (the upper Mekong River located in China) reduced the annual average streamflow by 5% at Chiang Sean station (northern Thailand) in 2008–2016, whereas their influence became undetectable downstream of Vientiane station (northern Laos). The streamflow changes downstream of Mukdahan station at southern Laos (including the stations in Cambodia and southern Vietnam) were mainly attributed to the local reservoirs and climate change. Compared with the baseline period of 1985–2007, the upstream reservoir operation dramatically affected streamflow at the midstream stations with higher dry season streamflow (+15% to +37%), but lower wet season streamflow was less affected (−2% to −24%) in 2008–2016. Climate change increased the magnitude and frequency of the flood by up to 14% and 45%, respectively, whereas the reservoir operation reduced them by 16% and 36%, respectively. Our findings provide insights into the interaction between climate change and reservoir operation and their integrated effects on the streamflow, informing and supporting water management and hydropower development in the LMRB.

Rainfall and streamflow trends of Thuchila River, Southern Malawi

This study aimed to assess the long term variability of the rainfall and streamflow in Thuchila river, southern Malawi, from 1985 to 2016. Trends of monthly, seasonal, and annual rainfall and streamflow of Thuchila river were analyzed using Mann-Kendall and Pettit tests at a 5% significance level. The annual, wet, and dry season rainfall in the catchment revealed a statistically non-significant decreasing trend. The monthly rainfall showed statistically non-significant decreasing trends except for April, June, and October. The annual and dry season streamflow exhibited a non-significant decreasing trend, whereas the wet season revealed a non-significant increasing trend. Sen’s slope results were all in agreement with the Mann-Kendall findings. The Pettit test detected different change points for both rainfall and streamflow in the catchment. The dominant change points of decreasing trends appeared between 2005 and 2007. During this period, the country experienced extreme weather events that affected the rainfall pattern. Pearson correlation coefficient was also employed to understand the relationship between rainfall and streamflow. The correlation analysis of the monthly, annual, and seasonal levels showed a linear relationship between rainfall and streamflow in the catchment. The results revealed that the rainfall influenced the streamflow changes of Thuchila river. This vital information about the trends of the streamflow and rainfall in Thuchila catchment plays a crucial role in catchment management.

Hydrological impact of a reservoir network in the upper Gan River Basin, China

AbstractA few relatively large reservoirs, hundreds of small reservoirs, and numerous farm dams were built in the upper Gan River Basin, China. The operation of such a reservoir network can serve as a significant source of variability in the local hydrological regime and should be included in research to better understand the interaction between multiple hydrological processes and watershed management. In this study, a reservoir network module that included reservoirs of multiple sizes was developed and fully integrated into the coupled land surface and distributed hydrologic model system (CLHMS), for a detailed description of the hydrological impact of a reservoir network. A generalized release scheme was employed to determine the outflow of both large and small reservoirs. The integrated model was then evaluated against observations and reanalysis data, which indicate that the model can reasonably reconstruct the reservoir operation, streamflow, and other hydrological variables. Results quantitatively demonstrate that a reservoir network can result in an increased streamflow in dry seasons, a decreased streamflow in wet seasons, a generally larger groundwater discharge, higher groundwater level, a slightly damper soil condition, and a larger amount of evapotranspiration at the basin level. With the integrated model, it is feasible to achieve more sustainable watershed planning and management.

Expanding Rubber Plantations in Southern China: Evidence for Hydrological Impacts

While there is increasing evidence concerning the detrimental effects of expanding rubber plantations on biodiversity and local water balances, their implications on regional hydrology remain uncertain. We studied a mesoscale watershed (100 km2) in the Xishuangbanna prefecture, Yunnan Province, China. The influence of land-cover change on streamflow recorded since 1992 was isolated from that of rainfall variability using cross-simulation matrices produced with the monthly lumped conceptual water balance model GR2M. Our results indicate a statistically significant reduction in wet and dry season streamflow from 1992 to 2002, followed by an insignificant increase until 2006. Analysis of satellite images from 1992, 2002, 2007, and 2010 shows a gradual increase in the areal percentage of rubber tree plantations at the watershed scale. However, there were marked heterogeneities in land conversions (between forest, farmland, grassland, and rubber tree plantations), and in their distribution across elevations and slopes, among the studied periods. Possible effects of this heterogeneity on hydrological processes, controlled mainly by infiltration and evapotranspiration, are discussed in light of the hydrological changes observed over the study period. We suggest pathways to improve the eco-hydrological functionalities of rubber tree plantations, particularly those enhancing dry-season base flow, and recommend how to monitor them.

Comparative study of conceptual versus distributed hydrologic modelling to evaluate the impact of climate change on future runoff in unregulated catchments

Abstract The application of two distinctively different hydrologic models, (conceptual-HBV) and (distributed-BTOPMC), was compared to simulate the future runoff across three unregulated catchments of the Australian Hydrologic Reference Stations (HRSs), namely Harvey catchment in WA, and Beardy and Goulburn catchments in NSW. These catchments have experienced significant runoff reduction during the last decades due to climate change and human activities. The Budyko-elasticity method was employed to assign the influences of human activities and climate change on runoff variations. After estimating the contribution of climate change in runoff reduction from the past runoff regime, the downscaled future climate signals from a multi-model ensemble of eight global climate models (GCMs) of the Coupled Model Inter-comparison Project phase-5 (CMIP5) under the Representative Concentration Pathway (RCP) 4.5 and RCP 8.5 scenarios were used to simulate the future daily runoff at the three HRSs for the mid-(2046–2065) and late-(2080–2099) 21st-century. Results show that the conceptual model performs better than the distributed model in capturing the observed streamflow across the three contributing catchments. The performance of the models was relatively compatible in the overall direction of future streamflow change, regardless of the magnitude, and incompatible regarding the change in the direction of high and low flows for both future climate scenarios. Both models predicted a decline in wet and dry season's streamflow across the three catchments.

A dry season streamflow reconstruction of the critically endangered Formosan landlocked salmon habitat

This study presents a 259-year (1750–2008 CE) reconstruction for October to February (ONDJF) standardized streamflow index of the Chichiawan Stream, the sole habitat of the critically endangered Formosan landlocked salmon (Oncorhynchus formosanus) remaining natural population. The reconstruction was based on the earlywood ring-width variations of Taiwan Douglas-fir (Pseudotsuga wilsoniana), using ensemble empirical mode decomposition for chronology development. Results showed that a higher average ONDJF streamflow in the 1968–2008 period was positively associated with a wider Taiwan Douglas-fir earlywood ring. During that period, the average ONDJF streamflow was related to the Pacific Decadal Oscillation and El Niño-Southern Oscillation development. Spectral analysis indicated that the reconstructed index had significant peaks around 2∼7 years and less significant peaks around 20–35 years. Based on the derived and reconstructed indices, most of the average ONDJF streamflow between 1750 and 2008 was within the normal range. Furthermore, there was no statistical evidence to suggest that the flow regime of the last few decades had changed, which refuted the speculation that the dry season streamflow natural variability in the 1960s and 1970s might have contributed toward the critically endangered status of Formosan landlocked salmon. The results of this study will be useful for developing short- to mid-term strategies to protect the endangered species and manage water resources.

A review of the current state of knowledge of proglacial hydrogeology in the Cordillera Blanca, Peru

The rapidly melting glaciers of Peru are posing new risks to regional dry season water supplies, and this is evident in the Cordillera Blanca, the mountain range with the world's largest concentration of tropical glaciers. Permanent ice loss is causing reductions to dry season streamflow, which is coupled with shifting demands and control over water access and entitlements in the region. A full evaluation of hydrologic inputs is required to inform future water management in the relative absence of glaciers. Over the last decade, new studies have shown groundwater to be a significant component of the regional water budget during the dry season, and it cannot be ignored when accounting for water quality and quantity downstream of the Cordillera Blanca's alpine catchments. Distinctive common features of the Cordillera Blanca's proglacial catchments are sediment‐filled valleys that were once under proglacial lake conditions. The combination of lake sediments with other alpine depositional features results in storage and interseasonal release of groundwater that comprises up to 80% of the valley's streamflow during the driest months of the year. We summarize the emerging understanding of hydrogeologic processes in proglacial headwater systems of the region's principal river, the Rio Santa, and make suggestions for future research that will more clearly characterize the spatial distribution of stored groundwater within the mountain range. As glaciers continue to recede, differences in aquifer thickness and groundwater residence time between alpine catchments in the region will increasingly control dry season water availability at the local and basin scale.This article is categorized under:Science of Water > Hydrological ProcessesScience of Water > Water and Environmental ChangeEngineering Water > Planning Water

Linking fish tolerance to water quality criteria for the assessment of environmental flows: A practical method for streamflow regulation and pollution control

The survival of aquatic biota in stream ecosystems depends on both water quantity and quality, and is particularly susceptible to degraded water quality in regulated rivers. Maintenance of environmental flows (e-flows) for aquatic biota with optimum water quantity and quality is essential for sustainable ecosystem services, especially in developing regions with insufficient stream monitoring of hydrology, water quality and aquatic biota. Few e-flow methods are available that closely link aquatic biota tolerances to pollutant concentrations in a simple and practical manner. In this paper a new method was proposed to assess e-flows that aimed to satisfy the requirements of aquatic biota for both the quantity and quality of the streamflow by linking fish tolerances to water quality criteria, or the allowable concentration of pollutants. For better operation of water projects and control of pollutants discharged into streams, this paper presented two coefficients for streamflow adjustment and pollutant control. Assessment of e-flows in the Wei River, the largest tributary of the Yellow River, shows that streamflow in dry seasons failed to meet e-flow requirements. Pollutant influx exerted a large pressure on the aquatic ecosystem, with pollutant concentrations much higher than that of the fish tolerance thresholds. We found that both flow velocity and water temperature exerted great influences on the pollutant degradation rate. Flow velocity had a much greater influence on pollutant degradation than did the standard deviation of flow velocity. This study provides new methods to closely link the tolerance of aquatic biota to water quality criteria for e-flow assessment. The recommended coefficients for streamflow adjustment and pollutant control, to dynamically regulate streamflow and control pollutant discharge, are helpful for river management and ecosystems rehabilitation. The relatively low data requirement also makes the method easy to use efficiently in developing regions, and thus this study has significant implications for managing flows in polluted and regulated rivers worldwide.

Observed changes in flow regimes in the Mekong River basin

Human activities, such as dam construction, significantly altered the flow regimes in the Mekong River, particularly after the completion of two large dams, namely Xiaowan and Nuozhadu in 2010 and 2014, respectively. Streamflow data from 1960 to 2014 obtained from five stations located along the Mekong mainstream are divided into three periods, i.e., the pre-impact period (1960–1991), the transition period (1992–2009), and the post-impact period (2010–2014). The flow regimes were investigated using eco-flow metrics and indicators of hydrologic alteration (IHA). The results show that the construction and initial filling of the upstream dams reduced the annual streamflow in the upstream Chiang Saen gauging station, whereas no clear effect was observed in the downstream Stung Treng station. The operation of dams reduces the streamflow in wet seasons and increases the streamflow in dry seasons, resulting in a unique seasonal variation in the streamflow based on eco-flow metrics in the Chiang Saen gauging station, observed from 2010 to 2014. In addition, the maximum flow values decreased significantly in the Chiang Saen gauging station during the year corresponding to the completion of the upstream dams. The construction and operation of dams clearly have significant impacts on low pulse duration. It is observed that climate change dictated the changes in the annual streamflow during the transition period 1992–2009 (82.28%), whereas human activities contributed more in the post-impact period 2010–2014 (61.88%). The results of this study could provide a reference for reservoir operation in the upstream regions considering both ecological and economic benefits of such operations, as well as maximize the interests of stakeholders in this region.

Urban Streamflow Response to Imported Water and Water Conservation Policies in Los Angeles, California

AbstractLos Angeles has a long history of importing water; however, drought, climate change, and environmental mitigation have forced the City to focus on developing more local water sources (target of 50% local supply by 2035). This study aims to improve understanding of water cycling in Los Angeles, including the impacts of imported water and water conservation policies. We evaluate the influence of local water restrictions on discharge records for 12 years in the Ballona Creek (urban) and Topanga Creek (natural) watersheds. Results show imported water has significantly altered the timing and volume of streamflow in the urban Ballona watershed, resulting in runoff ratios above one (more streamflow than precipitation). Further analysis comparing pre‐ vs. during‐mandatory water conservation periods shows there is a significant decrease in dry season streamflow during‐conservation in Ballona, indicating that prior to conservation efforts, heavy irrigation and other outdoor water use practices were contributing to streamflow. The difference between summer streamflow pre‐ vs. during‐conservation is enough to serve 160,000 customers in Los Angeles. If Los Angeles returns to more watering days, educating the public on proper irrigation rates is critical for ensuring efficient irrigation and conserving water; however, if water restrictions remain in place, the City must take the new flow volumes into account for complying with water quality standards in the region.