Streamflow Regime Research Articles

A hydrological and water temperature modelling framework to simulate the timing of river freeze-up and ice-cover breakup in large-scale catchments

Ice phenology, defined as the timing of freeze-up and ice-cover breakup, plays a key role in streamflow regimes in cold-region river catchments. River freeze-up and ice-cover breakup events are controlled by meteorological and hydrological variables. In this study, we present a modelling framework consisting of a physically-based semi-distributed hydrological model and the integration of a 1D stream temperature model that can predict the ice duration in cold region rivers. The hydrological model provides streamflow and hydraulic parameters for the stream temperature model to obtain instream water temperature. The model was successfully applied in the Athabasca River basin in western Canada. Calibration was carried out using the water temperature recorded in the stations at the towns of Hinton, Athabasca and Fort McMurray. Model results show consistent correspondence between simulated freeze-up and breakup dates and the hydrometric station data. In the main tributaries of the basin, freeze-up timing spans from the last week of September to the second week of November and ice-cover breakup occurs from the second week of March to the last week of May. The model presents an application of water temperature and ice phenology simulation which can be incorporated in ice-jam flood forecasting and future climate change studies.

Long‐term streamflow trends in Hawai'i and implications for native stream fauna

AbstractClimate change has fundamentally altered the water cycle in tropical islands, which is a critical driver of freshwater ecosystems. To examine how changes in streamflow regime have impacted habitat quality for native migratory aquatic species, we present a 50‐year (1967–2016) analysis of hydrologic records in 23 unregulated streams across the five largest Hawaiian Islands. For each stream, flow was separated into direct run‐off and baseflow and high‐ and low‐flow statistics (i.e., Q10 and Q90) with ecologically important hydrologic indices (e.g., frequency of flooding and low flow duration) derived. Using Mann–Kendall tests with a running trend analysis, we determined the persistence of streamflow trends through time. We analysed native stream fauna from ~400 sites, sampled from 1992 to 2007, to assess species richness among islands and streams. Declines in streamflow metrics indicated a general drying across the islands. In particular, significant declines in low flow conditions (baseflows), were experienced in 57% of streams, compared with a significant decline in storm flow conditions for 22% of streams. The running trend analysis indicated that many of the significant downward trends were not persistent through time but were only significant if recent decades (1987–2016) were included, with an average decline in baseflow and run‐off of 10.90% and 8.28% per decade, respectively. Streams that supported higher native species diversity were associated with moderate discharge and baseflow index, short duration of low flows, and negligible downward trends in flow. A significant decline in dry season flows (May–October) has led to an increase in the number of no‐flow days in drier areas, indicating that more streams may become intermittent, which has important implications for mauka to makai (mountain to ocean) hydrological connectivity and management of Hawai'i's native migratory freshwater fauna.

Fish assemblage–environment relationships suggest differential trophic responses to heavy metal contamination

Abstract While ecotoxicology has long recognised the importance of identifying levels at which contaminants pose threats to biota, most estimates of species responses to toxicants are derived from controlled laboratory studies and may hold limited relevance to natural systems. However, designing appropriate field‐based studies investigating contaminant induced changes in assemblages has been challenging, partially due to the difficulty in identifying comparable uncontaminated reference sites. The aim of this study is to characterise the effects of heavy metal contamination on natural fish assemblages using an ecologically relevant catchment‐scale design. We hypothesise that environmental variables, including discharge, sediment, and landscape variables, can be used to characterise differences in fish species richness and abundances between sites contaminated with heavy metals and uncontaminated reference sites. We apply a geographic information systems approach that uses assemblage–environment relationships developed using hydrologic model outputs, land cover, and topographic data from uncontaminated reference sites to predict expected fish species richness and abundance at sites contaminated with heavy‐metals within the Big River catchment in south‐eastern Missouri, U.S.A. These predicted levels of richness and abundance are then compared to observed assemblages at contaminated sites to estimate the potential impacts of historical lead mining activities on freshwater taxa. We developed models that characterised variation in Centrarchidae (bass and sunfish) richness and abundance, Cyprinidae (minnows) abundance, and Percidae (darters) richness using variables including streamflow regime, suspended sediment concentration, and land cover at uncontaminated sites. Using these relationships, we predicted expected fish species richness and abundance at heavy metal contaminated sites across the Big River catchment and found a significant reduction in centrarchid abundance from field‐collected data compared to predicted estimates. Our results suggest that centrarchids, which tend to occupy a higher trophic level than cyprinids and percids, have lower abundances at sites contaminated with heavy metals than predicted by assemblage–environment relationships. These decreases in abundance are not associated with decreases in centrarchid species richness, cyprinid abundance, or percid richness. This geographic information systems‐based approach provides a useful and ecologically relevant framework for understanding the response of taxa to the presence of contaminants without assuming habitat equivalence across sites. Our findings also suggest the need for further research regarding how heavy metals impact fishes of varying trophic levels in natural settings.

Assessment of Streamflow Regime Alterations in Tang River, China

The biodiversity and integrity of river ecosystems are depending on the natural streamflow regime. Therefore, assessing alteration of hydrologic regimes becomes a fundamental step in river ecosystem protection and restoration. In this paper, the Range of Variability Approach (RVA) was used to analyze the alteration hydrologic regimes from 1959 to 2016 in the Tang River, the upstream of Baiyangdian Lake Basin, China. Several results can be drawn: (i) annual streamflow presented a decreasing trend, and an abrupt change was detected in 1979; (ii) the significant changed indicators of hydrologic variation in upstream of Baiyangdian Lake Basin were the monthly streamflow in March, April, August, November, time of 3-day and 90-day maximum streamflow, date of maximum streamflow occurrence, rise rate and number of reversals; and (iii) monthly water should be restored in flood and delivered in non-flood season, and annual extreme streamflow frequency and duration of high and low streamflow also should be regulated to maintain the streamflow regimes in the Tang River Basin, China. The results will help to provide the suitable ecological streamflow and maintain the integrity of river ecosystem in changing environment.

Impact of global climate change on stream low flows: A case study of the great Miami river watershed, Ohio, USA

Climate change will profoundly affect hydrological processes at various temporal and spatial scales. This study is focused on assessing the alteration of water resources availability and low flows frequencies driven by changing climates in different time periods of the 21st century. This study evaluates the adaptability of prevailing Global Circulation Models (GCMs) on a particular watershed through streamflow regimes. This analysis was conducted in the Great Miami River Watershed, Ohio by analyzing historical and future simulated streamflow using 10 climate model outputs and the Soil and Water Assessment Tool (SWAT). The climate change scenarios, consisting of ten downscaled Coupled Model Intercomparision Project Phase 5 (CMIP5) climate models in combination with two Representative Concentration Pathways (RCP 4.5 and RCP 8.5) were selected based on the correlation between observed records and model outputs. Streamflow for three future periods, 2016-2043, 2044-2071 and 2072-2099, were independently analyzed and compared with the baseline period (1988-2015). Results from the average of ten models projected that 7-day low flows in the watershed would increase by 19% in the 21st century under both RCPs. This trend was also consistent for both hydrological (7Q10, 1Q10) and biological low flow statistics (4B3, 1B3). Similarly, average annual flow and monthly flows would also increase in future periods, especially in the summer. The flows simulated by SWAT in response to the majority of climate model projections showed a consistent increase in low flow patterns. However, the flow estimates using the Max-Planck-Institute Earth System Model (MPI-ESM-LR) climate output resulted in the biological based low flows (4B3, 1B3) decreasing by 22.5% and 33.4% under RCP 4.5 and 56.9% and 63.7% under RCP 8.5, respectively, in the future when compared to the baseline period. Regardless, the low flow ensemble from the 10 climate models for the 21st century seemed to be slightly higher than that of historical low flows. Keywords: climate change, low flows, SWAT, climate models, Great Miami River Watershed DOI: 10.25165/j.ijabe.20191201.4486 Citation: Shrestha S, Sharma S, Gupta R, Bhattarai R. Impact of global climate change on stream low flows: A case study of the great Miami river watershed, Ohio, USA. Int J Agric & Biol Eng, 2019; 12(1): 84–95.

Effects of small hydropower plants in cascade arrangement on the discharge cyclic patterns

ABSTRACT Because it is assumed that the impacts in the natural streamflow regime promoted by Small Hydropower Plants (SHP) are negligible, environmental licensing for such facilities is usually easier. Nonetheless, recent studies have shown that the operation of SHPs may disturb the natural flow conditions, mainly when the plants are placed in a cascade arrangement. In this context, the main objective of this study is investigating the alterations in flows periodic behavior in a system of six hydropower plants, being five of them SHPs. Daily discharge time series were extracted from eight streamflow gauging stations located in the Jauru River catchment, Brazil, whose period-of-record spans from May/2016 to Aug/2017. By using the wavelet transform, dominant cycles along the time series were identified and their coherence in nearby stations was compared. Among the results, one may observe that, from upstream to downstream, the high frequency cycles became more important whereas the low frequency ones have weakened. Additional analyses indicate that such alterations are not directly related to meteorological factors or to the gradual increasing in the catchment’s drainage area in the downstream direction, which suggests that the operation of SHPs may affect the streamflow natural cycles.

Water-quality trends in US rivers: Exploring effects from streamflow trends and changes in watershed management

We present a conceptual model that explores the relationship of streamflow trends to 15 water-quality parameters at 370 sites across the contiguous United States (US). Our analytical framework uses discrete water-quality data, daily streamflow records, and a statistical model to estimate water-quality trends between 1982 and 2012 and parse these trends into the amount of change attributed to trends in streamflow versus changes in watershed management, such as changes in point or non-point sources related to pollution control efforts. We conceptualize a water-quality trend as an additive function of these two trend components. We found that for most of these records the water-quality trends were more strongly affected by changes in watershed management as opposed to trends in streamflow. However, the importance of these trend components on water quality varied by estimate type (i.e. concentration versus load trends), parameter, and site. Trends in load were more influenced by changes in the streamflow regime than trends in concentration. Trends in major ions, salinity, and sediment were more sensitive to changes in streamflow than nutrients. When results were aggregated by site, 25% of the sites had at least 1 parameter where streamflow trends attributed >7.5% to the water-quality trend for concentrations. For loads, this was the case for 66% of the sites. The findings of this work have important implications for the analysis of water-quality trends. Understanding the relative role of streamflow and management changes can help to isolate the effects of pollution control efforts on water quality and provide clearer understanding of progress, or lack thereof, towards water-quality goals.

A new solution to mitigate hydropeaking? Batteries versus re-regulation reservoirs

Hydropower plants frequently operate at high output during peak hours and at low output (or even shutoff) during off-peak hours. This scheme, called “hydropeaking”, is harmful to downstream ecosystems. Operational constraints (minimum flows, maximum ramps) are frequently used to mitigate the impacts of hydropeaking. However, they reduce the operational flexibility of hydroelectric dams and increase the operational cost of power systems. Another approach to mitigating ecological impacts from hydropeaking is using structural measures, such as re-regulation reservoirs or afterbays. The first contribution of our work is to study the cost-effectiveness of these re-regulation reservoirs in mitigating ecological impacts from subdaily hydropeaking. Our second contribution is assessing energy storage (specifically, batteries) to mitigate the financial impacts of implementing peaking restrictions on dams, which represents the first attempt in the literature. Understanding these mitigation options is relevant for new hydropower dams, as well as for existing ones undergoing relicensing processes. For this, we formulate an hourly mixed-integer linear optimization model to simulate the annual operation of a power system. We then compare the business-as-usual (unconstrained) hydropower operations with ecologically constrained operations. The constrained operation, by limiting hydropower ramping rates, showed to obtain flows close to the natural streamflow regime. As next step, we show how re-regulation reservoirs and batteries can help to achieve these ecological constraints at lower costs. While the former are cost-effective for a very broad range of investment costs, the latter will be cost-effective for hydropeaking mitigation from 2025 onwards, when their capital costs have fallen. If more stringent environmental constraints are imposed, both solutions become significantly more attractive. The same holds for scenarios of more renewable generation (in which the operational flexibility from both alternatives becomes more valuable). After 2030, batteries can match the cost-effectiveness of expensive re-regulation reservoirs. Our findings are valuable for policy and decision makers in energy and ecosystem conservation.

Contrasting streamflow regimes induced by melting glaciers across the Tien Shan \u2013 Pamir \u2013 North Karakoram

The glacierized Tien Shan – Pamir – Karakoram mountain complex supplies water to about 42 million people. Yet, the knowledge about future glacial runoff in response to future climate is limited. Here, we address this issue using a hydrological model, that includes the three components of glacial runoff: ice melt, snowmelt and the runoff of rainfall over ice. The model is forced by climate projections of the CMIP5 models. We find that the three components exhibit different long-term trajectories, sometimes opposite in sign to the long-term trend in glacier impacts. For the eastern slope basins, streamflow is projected to increase by 28% (ranging from 9 to 44%, from climate model variation (CMV)) by the late 21st century, under the representative concentration pathway, RCP8.5. Ice melt contributes 39% (25 to 65%, CMV) of the total streamflow increase. However, streamflow from the western slopes is projected to decrease by 5% (−24 to 16%, CMV), due to the smaller contribution of ice melt, less precipitation and higher evapotranspiration. Increasing water supply from the eastern slopes suggests more water availability for currently degraded downstream ecosystems in the Xinjiang province of China, while the likely decreasing streamflow in Central Asian rivers on the western slopes indicates new regulations will be needed.

A high-resolution streamflow and hydrological metrics dataset for ecological modeling using a regression model

Hydrological variables are among the most influential when analyzing or modeling stream ecosystems. However, available hydrological data are often limited in their spatiotemporal scale and resolution for use in ecological applications such as predictive modeling of species distributions. To overcome this limitation, a regression model was applied to a 1 km gridded stream network of Germany to obtain estimated daily stream flow data (m3 s−1) spanning 64 years (1950–2013). The data are used as input to calculate hydrological indices characterizing stream flow regimes. Both temporal and spatial validations were performed. In addition, GLMs using both the calculated and observed hydrological indices were compared, suggesting that the predicted flow data are adequate for use in predictive ecological models. Accordingly, we provide estimated stream flow as well as a set of 53 hydrological metrics at 1 km grid for the stream network of Germany. In addition, we provide an R script where the presented methodology is implemented, that uses globally available data and can be directly applied to any other geographical region.

Nonstationarity driven by multidecadal change in catchment groundwater storage: A test of modifications to a common rainfall–run‐off model

AbstractMany parts of the world are experiencing a drying climatic trend with significant implications for water resources, especially where there is interaction between surface water and groundwater. Where drying leads to a shift in a catchment's hydrological regime, hydrological models may display bias in predictions and therefore degradation in their predictive capacity. We present two modifications to GR4J, a commonly used rainfall–run‐off model, that (a) allow the sensitivity of storage dependant evaporation and streamflow production to differ significantly, effectively mimicking “catchment memory” and (b) account for temporal variation in catchment vegetation cover, for example, due to land use change. We test the original and two modified models on a forested headwater catchment in the south‐west of Australia. In this catchment, a data set of more than 21,000 individual groundwater records collected over a period of more than 40 years chronicles a significant decline in catchment groundwater storage, resulting in a nonstationary streamflow regime. These longer‐term changes were overlaid with a shorter‐term hydrological response to clearing for bauxite mining and subsequent revegetation. The model structural modifications led to substantially improved streamflow prediction when compared with the original model, related to improved ability in the revised models' production store to match changes in the observed groundwater storage within the catchment. The performance of all three models and hence the apparent effectiveness of the modified models was dependent on the hydrological conditions in the calibration period. Best model fits were obtained when the calibration period encompassed both wetter and drier conditions in the hydrological record. The modified models presented here may reduce prediction bias for catchments in other parts of the world where long‐term shifts in hydrologic regime are being experienced and improve prediction for catchments with significant changes in vegetation cover.

Understanding the discharge regime of a glacierized alpine catchment in the Tianshan Mountains using an improved HBV-D hydrological model

Abstract Rivers originating from the Tianshan Mountains, known collectively as the “water tower of Central Asia”, are a key source of fresh water to the densely populated lowlands. Despite of the significance of water resources, our knowledge on the discharge regime in the alpine regions is limited, due to the paucity of in situ measurements and the complexity of contributing sources including rainfall, snowmelt and glacier-melt. In this study, the streamflow regime for the headwater catchment of Manas River basin (MRB) in the Tianshan Mountains is investigated through application of a hydrologic modeling framework, which is based upon an appropriate modification of the hydrological model HBV-D (Hydrologiska Byrans Vattenbalansavdelning-D). The daily precipitation and average temperature are reconstructed based on meteorological station data and remote-sensing observations for the period 1967–2007. Thereafter, the modified hydrological model is evaluated and the temporal distribution of runoff components is quantified via the model simulation. Our primary results include the following: (1) our modified version of the HBV-D model, with stronger physics basis in snow/glacier module and higher spatial resolution, is superior to the original HBV-D model in simulating daily streamflow processes and is capable of reproducing the variations of glacier area and glacier volume during the historical period. (2) Snowmelt is shown to dominate runoff processes in pre-monsoon season, accounting for approximately 61%, 76% and 74% of streamflow in April, May and June, respectively. The monthly contributions from glacier-melt, occurring in the region above 3000 m a.m.s.l (above mean sea level), range from 27% to 44% during the July–September period. (3) The average annual discharge in the MRB is sourced to 48% by snowmelt, 27% by glacier-melt, and the rest by rainfall. Particularly in 1981 when rainfall and snowmelt runoff were substantially reduced, the contribution of the glacier-melt to the river discharge reached 40%, suggesting that the glacier melt contributions to the streamflow in the MRB are especially significant during the dry years in ensuring a sustained water supply. The modified HBV-D model is expected to provide a useful modeling tool in simulating runoff regime in high-altitude mountainous regions and the results constitute a highly significant initial contribution to the formulation of effective adaptation strategies for water resource management under climate change.

The Role of Land Cover in the Propagation of Rainfall Into Streamflow Trends

AbstractThe replacement of natural forests with agriculture is generally associated with modifications in the hydrological behavior of a basin. This is particularly notable in the tropics and subtropics. Southern Brazil is a region with extensive agricultural production, forest conservation, and a vast unexplored streamflow data despite substantial rainfall trends observed in recent decades. In this work, we explore trends in the streamflow regime in the majority of the monitored basins in Southern Brazil. Additionally, we evaluate if pristine forested basins and agricultural nonforested basins have significantly different streamflow responses to changes in rainfall. We analyzed annual averages, maxima, minima, and seasonality of a 36‐year data set (1975–2010) of 675 rainfall and 140 streamflow gauges. Results reveal that large trends are widespread in Southern Brazil, especially in basins with areas smaller than 10,000 km2. Changes in the rainfall regime did not directly translate into changes in the streamflow regime. Changes in the annual maximum flow of forested basins were not statistically significant even when the annual average and maximum rainfall increased significantly. Correlations between changes in rainfall and streamflow for two indices, namely, duration of low‐magnitude events and seasonality, were statistically significant (p < 0.05) only for agricultural basins. The results indicate a higher propagation of hydrological changes through anthropogenically modified systems, providing evidence that agricultural basins are more sensitive to changes in the rainfall regime.

Decreased streamflow impacts fish movement and energetics through reductions to invertebrate drift body size and abundance

AbstractStreamflow drives ecological processes across multiple trophic levels making it a “master variable in lotic systems.” In mountain systems, especially those that are regulated, increased frequency of droughts and reductions in snowpack may alter future streamflow regimes and impact ecological processes. We monitored invertebrate drift abundance, size, and diversity as a function of streamflow. We then related these variables to fish movement and energetic efficiencies in the Upper Shasta River in California, above and below a large streamflow diversion. Invertebrate drift biomass was significantly less at impaired flows compared with unimpaired flows, and average body size of invertebrates decreased with decreasing streamflow. Generally, fish movement was greater at the impaired flow site (>50% of the time fish were tracked). Fish movement at the upstream site was negatively related to the size of individual prey items and amount of prey available, and significant drivers were not detected in the flow‐impaired site. Energetic efficiency was reduced by over 70% when search foraging took place, and the net rate of energetic intake was below 0 J/s for low‐flow periods. Our results suggest that fish foraging behaviour may be influenced indirectly by altered streamflow through changes to amount and size of invertebrate drift. A shift to foraging behaviour, coupled with low food availability, results in decreased energetic efficiency. Future prescriptions of flow rates to regulated rivers should account for changes to invertebrate drift, fish behaviour, and fish energetics on seasonal time scales.

Natural and managed watersheds show similar responses to recent climate change

Changes in climate are driving an intensification of the hydrologic cycle and leading to alterations of natural streamflow regimes. Human disturbances such as dams, land-cover change, and water diversions are thought to obscure climate signals in hydrologic systems. As a result, most studies of changing hydroclimatic conditions are limited to areas with natural streamflow. Here, we compare trends in observed streamflow from natural and human-modified watersheds in the United States and Canada for the 1981-2015 water years to evaluate whether comparable responses to climate change are present in both systems. We find that patterns and magnitudes of trends in median daily streamflow, daily streamflow variability, and daily extremes in human-modified watersheds are similar to those from nearby natural watersheds. Streamflow in both systems show negative trends throughout the southern and western United States and positive trends throughout the northeastern United States, the northern Great Plains, and southern prairies of Canada. The trends in both natural and human-modified watersheds are linked to local trends in precipitation and reference evapotranspiration, demonstrating that water management and land-cover change have not substantially altered the effects of climate change on human-modified watersheds compared with nearby natural watersheds.

Analyses of the magnitude and frequency of a 400-year flood record in the Fish River Basin, Namibia

Ephemeral rivers in dryland regions exhibit a high interannual variability of streamflow regime, mainly dominated by floods. In these environments, floods are a water resource and a potential hazard with important socioeconomic implications. The Fish River (86,600 km2) is the largest ephemeral stream in Namibia and, recently, also the focus of new development plans, including construction of the largest dam in Namibia. The hydrological analysis to support decisions implies large uncertainties owing to spatial-limited and short hydrological records (since 1962). Here we investigate the current and past patterns of extreme floods combining instrumental, historical, and palaeoflood records. Palaeoflood studies were performed at two reaches with preserved sedimentary evidence: at the upper sector (Vogelkranz) upstream of Hardap Dam (~13% catchment area) and in the lower part of the river, in the Fish River Canyon National Park (70% catchment area). In the Hardap reach, the palaeoflood record identified at least eight large floods during the last 350 years, with the largest flood reaching a minimum discharge of 4800 m3 s−1 (150-year return period). In the Fish River Canyon reach, the sedimentary record shows at least 12 large floods over the last 400 years, the largest with an estimated minimum discharge of 8700 m3 s−1. The elevation of alluvial surfaces without flood evidence provided an upper bound for flood stages, associated with discharges of 6400 m3 s−1 and 16,140 m3 s−1 for the upper and mid-lower Fish River respectively. In the upper reach, the flood frequency analysis (FFA) combining systematic and palaeoflood data provided lower discharges (~25%) of the flood quantiles than the ones using only systematic data sets. In the Fish River Canyon reach, incorporation of the palaeoflood data into the FFA results in slightly higher values in the magnitude of the higher flood quantiles (~4–7%). The FFA analysis using an upper limited lognormal distribution function (LN4) fitted with palaeoflood data shows a good performance with a slow behaviour approaching the upper limit. Our flood frequency results suggest that the Hardap Dam should increase the spillway capacity and safety check flood of the original design in order to satisfy the dam safety criteria. However, the projected reevaluation figures calculated from conventional hydrological methods results in an overestimation of the safety floods according to our estimations.

A probabilistic appraisal of rainfall-runoff modeling approaches within SWAT in mixed land use watersheds

A probabilistic approach is presented to assess the performance validity of the empirical Curve Number (CN) and physically-based Green and Ampt (G&A) rainfall-runoff methods in the SWAT model. Specifically, the effects of modeling uncertainties on characterization of the hydrologic budgets and streamflow regimes at various spatial scales and upstream land use conditions are investigated. A Bayesian total uncertainty assessment framework, which explicitly accounts for uncertainties from model parameters, inputs, structure, and measurement data, was employed to explore uncertainties in streamflow simulation using SWAT with different rainfall-runoff methods in a mixed-land use watershed. While the models were trained for streamflow estimation only at the watershed outlet, the performances of the models were compared at different stream locations within the watershed. At the watershed outlet, the CN method had a slightly better, but not significant, performance in terms of streamflow error statistics. Similar results were obtained for the predominantly forested and agricultural tributaries. However, in tributaries with higher percentage of developed land, G&A outperformed the CN method in simulating streamflow based on various performance metrics. In general, the 95% prediction intervals from the models with G&A method covered a higher percentage of observed streamflow especially during the high flow events. However, they were approximately 20–45% wider than the corresponding 95% prediction intervals from the CN methods. Using 95% prediction interval for estimated flow duration curves, results indicated that the models with CN methods underestimated high flow events especially in tributaries with highly developed land use. However, the CN methods generated higher water yields to streams than the G&A method. The results of this study have important implications for the selection and application of appropriate rainfall-runoff methods within complex distributed hydrologic models particularly when simulating hydrologic responses in mixed-land use watersheds. In the present study, while CN and G&A methods in the SWAT model performed similarly at the outlet of a mixed-land use watershed, G&A captured the internal processes more realistically. The subsequent effects on the representation of internal hydrological processes and budgets are discussed.

LAND USE, MICROCLIMATE, AND SURFACE RUNOFF LINKAGES: SPACE-TIME MODELING FROM ROKEL-SELI RIVER BASIN, SIERRA LEONE

Abstract. This study mainly utilized the Soil and Water Assessment Tool (SWAT) together with SWAT-CUP, both free and open source software (FOSS), to construct a distributed hydrologic flow model for the Rokel-Seli River basin, Sierra Leone, in a bid to spatiotemporally evaluate the role of changes in land use/land cover (LULC) and microclimate on streamflow regimes. The model was informed by LULC data derived from three Landsat satellite images collected in 2002, 2010, and 2016. The LULC data was generated with the aid of several python libraries accessed through the FOSS Anaconda Navigator. LULC change analysis demonstrated that between 2002 and 2016, urban, agricultural, water, and mining lands expanded significantly but forest cover reduced the most (−5.7 %). While average annual surface runoff dramatically increased from 2002 to 2010 (31.7 %), the period after the reservoir construction (2010–2016) recorded lower increase in surface runoff (0.9 %). Result of the study suggested that the construction of a major reservoir to support hydroelectricity in concert with significant loss of forest cover and shrub played a greater role in increasing surface runoff compared with the influence of microclimate.

Sensitivity of streamflow to climate change in California

Climate change is rapidly altering the global water cycle, exposing vulnerabilities in both social and environmental systems. However, uncertainty in future climate predictions makes it difficult to design and evaluate strategies for building climate resilience. In regions such as California, characterized by stressed water-supply systems, high natural climate variability, and substantial uncertainty in future precipitation projections, alternative approaches to assessing climate risks may be useful. Here, we develop a hydrologic sensitivity approach to estimate regional streamflow responses to climate change in California. We use statistical models to predict monthly streamflow from physical catchment features and evaluate how flow changes with incremental changes in precipitation and temperature. The results indicate unique regional and monthly flow responses to climate change, with early summer flows (May–July) in interior mountain region having the greatest sensitivity to temperature and winter flows (December–March) in the xeric region having the greatest sensitivity to precipitation. When evaluated over the range of global climate model projections for mid-century (2040–2069), models generally suggest shifts in streamflow regimes towards higher wet season flows and lower dry season flows relative to historical conditions. The sensitivity analysis provides insight into catchment- and regional-scale hydrologic responses in California and complements other approaches for understanding the consequences of climatic change for water and risk management.

Putah Creek hydrology affecting riparian cottonwood and willow tree survival.

Creating or recruiting new riparian forests to improve Lower Putah Creek (LPC) ecosystem functions is challenging under the modified stream flow regime developed after historic gravel mining and installation of Monticello Dam upstream. Hydrologic connectivity between riparian trees, shallow groundwater, and the low flow channel is essential towards maintaining these forests and related habitats through the annual summer and multi-year drought periods typical in this Sacramento Valley region of California. Despite increased average summer flows, significant mature cottonwood and willow tree mortality along the LPC riparian areas below the Putah Creek diversion dam in 2014 raised concerns over the soil and hydrologic factors affecting riparian vegetation survival. A forensic analysis was conducted combining annual canopy coverage fractions and tree ring studies with daily soil-water balances, low flow records, and available groundwater level Information from the past few decades to determine the key hydrologic factors affecting riparian tree survival along the LPC. The 2011-2016 drought was linked with greater than prior average soil-water deficits in 2012-2015 and lower initial soil-water storage on March 1 of 2012 and 2014 that would be expected to stress the trees. However, such stress was not apparent in decreased tree ring spacing during this period from mature (40-50years old), deceased, and living trees. Tree canopy coverage declined dramatically (by as much as 50% as compared to the previous decade average) only in the summer of the 2014 despite a ~ 35% increase in average summer flows from 2011 to 2014. However, the regional water table aquifer levels declined at an average rate of ~ 35mm/day in 2014 (as compared to ~ 17mm/day in previous decade) and by several meters overall between 2011 and 2016 suggesting that deceased trees lacked access to the water table aquifer or lateral stream seepage. The increased rates of water table decline and overall depth may be associated with a large increase in adjacent irrigated almond orchard areas in 2014-2016. Knowledge of the dynamic hydrologic factors controlling sustainability of riparian trees should better inform and guide future tree restoration efforts along the LPC.