Dry Season Flow Research Articles

The effects of land use changes on streams and rivers in mediterranean climates

We reviewed the literature on the effects of land use changes on mediterranean river ecosystems (med-rivers) to provide a foundation and directions for future research on catchment management during times of rapid human population growth and climate change. Seasonal human demand for water in mediterranean climate regions (med-regions) is high, leading to intense competition for water with riverine communities often containing many endemic species. The responses of river communities to human alterations of land use, vegetation, hydrological, and hydrochemical conditions are similar in mediterranean and other climatic regions. High variation in hydrological regimes in med-regions, however, tends to exacerbate the magnitude of these responses. For example, land use changes promote longer dry season flows, concentrating contaminants, allowing the accumulation of detritus, algae, and plants, and fostering higher temperatures and lower dissolved oxygen levels, all of which may extirpate sensitive native species. Exotic species often thrive in med-rivers altered by human activity, further homogenizing river communities worldwide. We recommend that future research rigorously evaluate the effects of management and restoration practices on river ecosystems, delineate the cause–effect pathways leading from human perturbations to stream biological communities, and incorporate analyses of the effects of scale, land use heterogeneity, and high temporal hydrological variability on stream communities.

Pancheshwar Multipurpose Project: Nepal’s Portion of Water

On September 20, 1996 an overwhelming 96.5% of the members present at the Joint Session of the Nepalese Parliament hurriedly ratified the Mahakali Treaty to fulfill the requirements of Article 126 of the 1990 Constitution of Nepal.The then Prime Minister Sher Bahadur Deuba and Water Resources Minister Pashupati SJB Rana provided CPN-UML General Secretary Madhav Kumar Nepal and CPN-UML Mahakali Study Team Coordinator KP Sharma (Oli) written replies on the “equal sharing” of the Mahakali waters. However, despite the lapse of 15 years, the two critical clauses: i) “...equal entitlement in the use of Mahakali waters without prejudice to their respective consumptive uses...,” and ii) “...precludes the claim, in any form, by either Party on the unutilized portion of the shares of the waters...” still remain obfuscated and clouded in mist. Is “equal entitlement” (50 percent each) considered after deducting the respective consumptive uses as interpreted but later withdrawn by Minister Rana? Or is this “equal entitlement” (50 percent each) considered prior to deducting the consumptive uses as interpreted by Secretary General MK Nepal and then applying the clause “without prejudice” to their respective consumptive uses? No attempts appear to have been made by Nepal to elicit the Government of India’s interpretations on this vital issue.The Ganges basin supports a massive over 523 million people; i.e., 450 million in India, 45 million in Bangladesh, and 28 million in Nepal. With India’s economy growing consistently at a high 8 to 9% annually, the quality of life of her people is definitely on the rise. This will invariably mean drastic increases in her per capita consumption of water. With Nepal contributing over 72% of the dry season flows of the Ganges, India’s eyes and ears are on Nepal either in the form of “...without prejudice to their respective consumptive uses...” or “...precludes the claim, in any form, ...the unutilized portion of the waters...” However, Nepalese governments from the Panchayat era perceived water, whether by design or otherwise, as a mere secondary component of hydropower. Despite the full knowledge that water has become a scarce diminishing resource, the Nepalese Governments continue to be mesmerized by hydropower. This is manifested by the government unveiling in a span of two years her grand dreams of 10,000 MW in ten years and 25,000 MW in twenty years. This article attempts to delve into the intricacies of the Mahakali waters as expounded by Prime Minister SB Deuba’s government during the extremely hurried ratification the Mahakali Treaty in 1996.DOI: http://dx.doi.org/10.3126/hn.v8i0.4925 Hydro Nepal: Journal of Water, Energy and Environment Issue No. 8, 2011 JanuaryPage: 48-56Uploaded date: 23 June, 2011

Rainfall interception by natural and planted forests in the Middle Mountains of Central Nepal

► Rainfall interception loss was measured and modelled for natural and planted forest. ► The difference in rainfall interception loss between the two forests was examined. ► The model simulates interception loss well under monsoonal montane conditions. ► The difference in interception loss is due to differences in LAI and tree density. ► The high annual interception loss from planted forest could affect dry season flows. Measurements of gross rainfall, throughfall and stemflow were made during the 2011 rainy season in an evergreen natural forest and in a planted pine forest located in complex topography in the Middle Mountains of Central Nepal. For the period of observation (20 June–9 September, 2011), measured throughfall, stemflow and derived interception loss in the natural forest were 76.2%, 1.4% and 22.4% of incident rainfall, respectively. Corresponding values for the pine plantation were 83.0%, 0.5% and 16.5%. The revised version of Gash’s analytical model of rainfall interception was calibrated and validated for the two stands, representing a first for montane sub-tropical monsoonal conditions. The results of the modelling corresponded well with observed values, provided an optimised value was used for the average wet-canopy evaporation rate. The model was subsequently used to calculate annual interception losses from the two forests. Modelled estimates of annual interception loss for the planted and natural forest were 291 mm and 319 mm and represented 19.4% and 22.6% of incident rainfall, respectively. The high interception loss from the planted forest is considered a contributory factor towards the observed decline in dry season streamflow following the reforestation of degraded hillsides in the Middle Mountains of Nepal.

Climate change impacts on the water balance of coastal and montane rainforests in northern Queensland, Australia

Summary How the water balance of coastal and montane rainforests in northern Queensland could change in response to climate change was examined using physically based models of interception and transpiration along with long term weather records. Future rainfall and temperature changes were based on the most recent climate modelling for the region and were assumed to fall within the range ±20% for rainfall with a temperature increase of 1–3 K. Climate change will affect the water balance of Australian rainforests primarily via rainfall changes rather than temperature. Any given change in rainfall produces a greater change in downstream runoff, the amplification ranging from 1.1 to 1.5 in the wet season to a factor of 12 in the dry season. Changes in wet season rainfall (80% of the annual total) dominate the total annual amount of water released for downstream flow, but dry season rainfall (20% of the annual total) changes are also very important as they affect onset and the duration of the period when there is no runoff. This period is currently ∼110 days and this would change by ±30 days under the above climate scenarios. There are also potential in situ impacts of climate change that affect how long the rainforest canopy is wet, which may have important implications for the epiphytes and mosses that depend on these wet canopy conditions. Similarly there may be significant impacts on downstream freshwater species whose life cycles are adapted to the current dry season flow regime.

Optimal integrated operation of reservoir-assisted stormwater treatment areas for estuarine habitat restoration

Significant reductions in pollutant loadings can be achieved through coordinated operation of constructed wetlands or stormwater treatment areas (STAs) when connected to retention reservoirs for maintaining ideal water levels and hydraulic residence times in STAs that maximize treatment efficiency. Reservoir-assisted STA systems can provide additional benefits including maintenance of target frequency distributions of stormwater inflows into coastal estuarine systems for ecosystem rehabilitation, dry season flow augmentation in coastal riverine systems for maintaining the lower salinity zone, and supplemental water supply. An implicit stochastic optimization approach is applied to adaptive, multiobjective control of interconnected reservoir-assisted STAs that optimizes fuzzy rule-based operating rules using a genetic algorithm interacting with a simulation model of the stormwater drainage system. The model is applied to the four interconnected reservoir/STAs in the design configuration of the Indian River Lagoon-South project for the St. Lucie River watershed and estuary, South Florida. Results of simulated long-term performance of the optimal operating rules show attainment of targets for remediation of the aquatic ecology of the St. Lucie estuary, riverine environmental protection, and reliable supplemental water supply.

Assessment of Flow Changes from Hydropower Development and Operations in Sekong, Sesan, and Srepok Rivers of the Mekong Basin

The Mekong River supports unique biodiversity and provides food security for over 60 million people in the Indo-Burma region, yet potential changes to natural flow patterns from hydropower development are a major risk to the well-being of this system. Of particular concern is the ongoing and future development of 42 dams in the transboundary Srepok, Sesan and Sekong (3S) basin, which contributes up to 20% of the Mekong’s annual flows and provides critical ecosystem services to the downstream Tonle Sap Lake and the Mekong Delta. To assess the magnitude of potential changes, daily flows were simulated over 20 years using the HEC ResSim and SWAT models for a range of dam operations and development scenarios. A 63% increase in dry season flows and a 22% decrease in wet season flows at the outlet of the 3S basin could result from the potential development of new dams in the main 3S rivers under an operation scheme to maximize electricity production. Water-level changes in the Mekong River from this scenario are comparable with changes induced by the current development of Chinese dams in the Upper Mekong Basin and are significantly higher than potential flow changes from the proposed 11 mainstream dams in the Lower Mekong Basin. Dams on the upper sub-tributaries of the 3S basin have very little effect on seasonal flow regimes because most of those projects are run-of-the-river dams and have small reservoir storages. The effects on hourly flow changes resulting from intra-daily reservoir operations, sediment movement, water quality, and ecology require further study. Strategic site selection and coordinated reservoir operations among countries are necessary to achieve an acceptable level of development in the basin and to mitigate negative effects in seasonal flow patterns, which sustain downstream ecosystem productivity and livelihoods.

Hydraulic redistribution by two semi-arid shrub species: Implications for Sahelian agro-ecosystems

Hydraulic redistribution is the process of passive water movement from deeper moist soil to shallower dry soil layers using plant roots as conduits. Results from this study indicate that this phenomenon exists among two shrub species (Guiera senegalensis and Piliostigma reticulatum) that co-exist with annual food crops in Sahelian agro-ecosystems. Real-time measurements were conducted for soil water content, soil water potential and microclimate variables notably; air temperature, relative humidity, wind speed, precipitation and solar irradiance. Additionally, sap flow measurements were conducted in shrub roots using the thermal dissipation technique on intact and coppiced shrubs. Monthly predawn leaf water potential was measured using a portable pressure chamber. Soil water potential (Ψs) at the 20 cm depth declined significantly during the dry season with diel changes in Ψs of −0.6 to −1.1 MPa. These variations were attributed to passive water release from shrub roots resulting in overnight rewetting of drier upper soil layers. Sap flow measurements on tap and lateral shrub roots indicated daily reversals in the direction of flow. During the peak of the dry season, both positive (toward shrub) and negative (toward soil) flows were observed in lateral shrub roots with sap flow in the lateral roots frequently negative at night and rapidly becoming positive soon after sunrise. The negative sap flow at night in superficial lateral roots and the periodic positive flow in the descending tap roots were indicative of hydraulic redistribution. Hydraulic redistribution may be an important mechanism for drought stress avoidance while maintaining plant physiological functions in both shrubs and neighboring annuals in water-limited environments.

Watershed scale application of WEPP and EROSION 3D models for assessment of potential sediment source areas and runoff flux in the Mara River Basin, Kenya

Mara River basin is an internationally acclaimed and ecologically significant basin in the Lake Victoria basin. The recent trend in major land use change in the upstream (forest-to-agriculture conversion) has been the major driver for the current hydrologic alterations. Recent hydrologic data and observations show a reduction in dry season flow, increase in peak flows during wet seasons and an increase in sediment load. This has led to the need for more observation and modeling to understand the impacts of the various land use changes on the sediment and runoff characteristics of the watershed . Two physically based models (Erosion 3D and Water Erosion Prediction Project (WEPP)) were applied to predict the watershed scale sediment and runoff response. Watershed scale modeling results show that sediment yield and runoff vary by slope, land use and soil type. The spatial sediment budget also shows the variability in the erosion and deposition of sediment in the basin. Average simulated erosion in cultivated land was about 120 t/ha/yr and the lowest simulated erosion rate was on bush lands and grasslands, which may indicate change in land use has significant impact on soil erosion in the Mara River basin. It was shown that both WEPP and EROSION 3D models are useful tools for watershed scale application and estimation of runoff and sediment yield. ► We estimated sediment yield and runoff at watershed scale using two soil erosion models. ► We evaluated the two models using field runoff and sediment data. ► We found that sediment and runoff varies by land use, topography and soil type. ► The two models were comparable but WEPP model was better for sediment estimation. ► Agricultural areas contribute about 50% of the sediment budget in the watershed.

Possible change in Korean streamflow seasonality based on multi‐model climate projections

AbstractSeasonality in hydrology is closely related to regional water management and planning. There is a strong consensus that global warming will likely increase streamflow seasonality in snow‐dominated regions due to decreasing snowfall and earlier snowmelt, resulting in wetter winters and drier summers. However, impacts to seasonality remain unclear in rain‐dominated regions with extreme seasonality in streamflow, including South Korea. This study investigated potential changes in seasonal streamflow due to climate change and associated uncertainties based on multi‐model projections. Seasonal flow changes were projected using the combination of 13 atmosphere–ocean general circulation model simulations and three semi‐distributed hydrologic models under three different future greenhouse gas emission scenarios for two future periods (2020s and 2080s). Our results show that streamflow seasonality is likely to be aggravated due to increases in wet season flow (July through September) and decreases in dry season flow (October through March). In South Korea, dry season flow supports water supply and ecosystem services, and wet season flow is related to flood risk. Therefore, these potential changes in streamflow seasonality could bring water management challenges to the Korean water resources system, especially decreases in water availability and increases in flood risk. Copyright © 2012 John Wiley & Sons, Ltd.

Vulnerability of the power sector of Bangladesh to climate change and extreme weather events

Rise in temperature and annual precipitation, changes in seasonal rainfall patterns, more frequent and severe extreme weather events, and increased salinity in river water have been observed in Bangladesh in the recent years. Rising temperature will elevate total power consumption and peak power demand especially during the pre-monsoon hot summer season, reduce power plant efficiency and transformer lifetime, and increase the transmission loss. More frequent and severe extreme weather events may cause more disruption in power generation and distribution, and more damage of power infrastructure. Lower river flow in dry season may cause water scarcity in power plants and hamper the production. Increased salinity in river water due to sea level rise may lead to corrosion and leakages in power plants located in the coastal region of Bangladesh. A diversified, decentralized, and climate resilient power system can reduce negative impacts of climate change on power sector of Bangladesh. Adaptation and mitigation strategies must be incorporated in the planning and development of new power systems and the reformation of existing power systems of Bangladesh.

River dolphin distribution in regulated river systems: implications for dry‐season flow regimes in the Gangetic basin

ABSTRACT River flow regulation and fragmentation is a global threat to freshwater biodiversity, ecosystem processes, and associated human activities. Large dams in the Ganges river basin of the Indian subcontinent have severely altered natural flow regimes, particularly in the low‐flow dry season. Altered flows could have negative impacts on endangered species such as the Ganges river dolphin Platanista gangetica. Habitat use by river dolphins was investigated in relation to river channel depth and morphology, over 332 km of the flow‐regulated Gandak River in India. Dolphin distribution patterns were compared across multiple spatial scales in the Gandak, Kosi, Chambal, Sone Rivers and the upper and lower sections of the Ganges main stem. Dolphin presence was recorded in 40% of segments in the Gandak river, with a best count of 257 (range 250–267) and average individual encounter rates at 0.75 dolphins km‐1 (SD 0.89). Bayesian zero-inflated spatial models showed that river dolphin abundance was positively influenced by river depth, presence of meanders and corresponded closely with gillnet fishing. Minimum mid‐channel depth requirements were estimated at 5.2 m for dolphin adults and between 2.2 and 2.4 m for mother–calf pairs. Adult dolphins showed highly similar habitat preferences across regulated or unregulated rivers, for depths >5 m, and meandering channels. Dry‐season habitat availability was reduced as the degree of flow regulation increased across rivers, mainly owing to loss of lateral and longitudinal channel connectivity. Overall encounter rates were reduced from >3 km‐1 in less regulated stretches, to <0.3 km‐1 in regulated rivers. Clustering of dolphins in deep pools increased along the gradient of river flow reduction, with dolphins almost absent from intervening segments because of low flow rates. These results indicate the importance of maintaining adequate dry‐season flows to ensure river habitat availability and connectivity for dolphins. Copyright © 2012 John Wiley & Sons, Ltd.

A study of wetland hydrology and ecosystem service provision: GaMampa wetland, South Africa

The GaMampa wetland, a palustrine wetland, comprises less than 1% of the catchment but is widely believed to make a significant contribution to dry-season river flow in the Mohlapitsi River, a tributary of the Olifants River, in South Africa. The contribution of the GaMampa wetland to dry-season flow in the Mohlapitsi River and the impact of increasing agriculture on its hydrological functioning were investigated. Economic analyses showed that the net financial value of the wetland was US$ 83 263 of which agriculture comprises 38%. Hydrological analyses indicated that the Mohlapitsi River contributes, on average, 16% of the dry-season flow in the Olifants River. However, the wetland contributes, at most, 12% to the increase in dry-season flow observed over the reach of the river in which the wetland is located. The remainder of the increase originates from groundwater flowing through the wetland. Furthermore, despite the conversion of 50% of the wetland to agriculture since 2001, there has been no statistically significant reduction in dry-season flow in the Mohlapitsi River. These results highlight the importance of understanding the nature of the full suite of services being provided by a wetland in order to make informed decisions for appropriate management. Editor Z.W. Kundzewicz; Guest editor M.C. Acreman Citation McCartney, M. Morardet, S. Rebelo, L-M. Finlayson, C.M. and Masiyandima, M., 2011. A study of wetland hydrology and ecosystem service provision: GaMampa wetland, South Africa. Hydrological Sciences Journal, 56 (8), 1452–1466.

Phytoplankton composition and constraints to biomass in the middle reaches of an Australian tropical river during base flow

Under high flows, the biomass of riverine phytoplankton can be constrained by short transport times and advective losses. However, under slower flows and longer transport times, secondary factors and sometimes their interaction with flow may constrain phytoplankton biomass. To contribute to a wider understanding of the riverine conditions that constrain phytoplankton biomass, we tested the hypothesis that phytoplankton of the Daly River (tropical Australia) was constrained by transport time during dry-season base flow. The river is virtually undisturbed, with oligotrophic nutrient concentrations during the dry season. The most frequently occurring taxa were planktonic, rather than benthic, and dominated by the dinoflagellate Peridinium inconspicuum which has r-strategist traits that favour rapid growth in a nutrient-deficient environment. Our hypothesis was not supported because increased downstream loads of Chlorophyll a and the domination of P. inconspicuum inferred phytoplankton net growth. Instead, phytoplankton biomass was more likely to be nutrient-limited, although transport time may limit phytoplankton growth over some reaches and for specific taxa. The present study demonstrated that even in the fast-flowing middle reaches of a river (~0.4 m s–1), a population of phytoplankton can be sustained.

Development and application of a hybrid model to analyze spatial distribution of macroinvertebrates under flow regulation in the Lijiang River

Alteration of natural flow regime due to reservoir operation imposes serious impacts on the aquatic ecosystem. To investigate the effects on the spatial distribution of macroinvertebrates, this study developed a hybrid ecohydraulics model which integrated a two-dimensional water quality module with an artificial neural network (ANN) based macroinvertebrate habitat module. The developed model was applied to a compound channel in the middle of the Lijiang River, where the flow in dry season has been largely modified by the Qingshitan Reservoir in the upstream. Semisulcospira amurensis ( S. amurensis) which is the dominant macroinvertebrate species in the whole river basin was studied. The simulation and comparison results indicate that in general the flow regulation in dry season has negative impacts on the distribution of S. amurensis. The distribution area decreases, and in particular it becomes seriously fragmented. The model can be used through scenario analyses to adjust reservoir operation, so as to improve river management for ecological interest. In addition, the hybrid approach proposed in the paper has great potentials in studying this type of problems.

Analisis Fluktuasi Hujan dan Morfologi Sungai terhadap Konsentrasi Banjir Daerah Surakarta

This research aims to analyse rain fluctuation and river morphology to flood concentration in Bengawan Solo River, Surakarta. The method of this research is field survey supported by secondary data analysis. The survey was conducted using GPS (Global Positioning System). The result indicates that high intensity rainfall in the research area happened during October till April. River flow in dry season generally decreased in accordance with the distribution of low rainfall (dry month). Whereas, river flow rates seen to increase along with the rainfall that occurs in the rainy season. Other result show that point 11-18 (location in Tanjung village district of Sukoharjo till Semanggi village district of Pasar Kliwon) very superficial which became impact of the river. The point represent floods concentration, therefore if there is rain with high quantity, the water will rise and overflow to surface, especially point 11, 13, and 16 (Tanjung, Kedunggudel, and Telukan village district of Sukoharjo). The processes that occurred at these sites are sedimentation, and lateral erosion (horizontal erosion).

Land use and climate change impacts on the hydrology of the upper Mara River Basin, Kenya: results of a modeling study to support better resource management

Abstract. Some of the most valued natural and cultural landscapes on Earth lie in river basins that are poorly gauged and have incomplete historical climate and runoff records. The Mara River Basin of East Africa is such a basin. It hosts the internationally renowned Mara-Serengeti landscape as well as a rich mixture of indigenous cultures. The Mara River is the sole source of surface water to the landscape during the dry season and periods of drought. During recent years, the flow of the Mara River has become increasingly erratic, especially in the upper reaches, and resource managers are hampered by a lack of understanding of the relative influence of different sources of flow alteration. Uncertainties about the impacts of future climate change compound the challenges. We applied the Soil Water Assessment Tool (SWAT) to investigate the response of the headwater hydrology of the Mara River to scenarios of continued land use change and projected climate change. Under the data-scarce conditions of the basin, model performance was improved using satellite-based estimated rainfall data, which may also improve the usefulness of runoff models in other parts of East Africa. The results of the analysis indicate that any further conversion of forests to agriculture and grassland in the basin headwaters is likely to reduce dry season flows and increase peak flows, leading to greater water scarcity at critical times of the year and exacerbating erosion on hillslopes. Most climate change projections for the region call for modest and seasonally variable increases in precipitation (5–10 %) accompanied by increases in temperature (2.5–3.5 °C). Simulated runoff responses to climate change scenarios were non-linear and suggest the basin is highly vulnerable under low (−3 %) and high (+25 %) extremes of projected precipitation changes, but under median projections (+7 %) there is little impact on annual water yields or mean discharge. Modest increases in precipitation are partitioned largely to increased evapotranspiration. Overall, model results support the existing efforts of Mara water resource managers to protect headwater forests and indicate that additional emphasis should be placed on improving land management practices that enhance infiltration and aquifer recharge as part of a wider program of climate change adaptation.

Spatial and temporal controls on post-fire hydrologic recovery in Southern California watersheds

The current study investigates the spatial and temporal dynamics of post-fire vegetation and the subsequent influence on seasonal and annual hydrologic responses in chaparral-dominated watersheds. Post-fire climatology, burn severity, slope aspect, and vegetation behavior are evaluated for two basins burned during the 2003 Old Fire in the San Bernardino Mountains in Southern California. Climate and discharge data are used to evaluate seasonal and annual variability of post-fire hydrologic fluxes. Data obtained from the Moderate Resolution Imaging Spectroradiometer (MODIS) are used to estimate Enhanced Vegetation Index (EVI) and differenced Normalized Burn Ratio (dNBR). A Savitzky–Golay filtering technique and an integrated EVI annual fraction are utilized to assess vegetation recovery under a range of potential controls. Vegetation recovery is highly variable in both watersheds and is related to slope aspect (solar and water availability), initial biomass levels, and burn severity. South and west facing slopes show higher pre-fire EVI (biomass) and significant loss of vegetation cover after fire. Vegetation in both watersheds responds to an extreme wet season during the second post-fire year, however recovery rates are not sustained. North and east aspects show the quickest biomass gain relative to pre-fire conditions by the end of the study period (WY 2010), while the west and south slopes show lower biomass recovery. High burn severity areas show the slowest recovery across all slope aspects, with these regions just approaching 90% of pre-fire biomass by the end of the seven-year post-fire period. The variable rate of vegetation recovery across the watersheds results in significant changes in annual and seasonal discharge throughout the post-fire period. Runoff ratios remain elevated in both systems and there is increased dry season flow for much of the study period, indicating that plant water consumption and flowpaths are not back to pre-fire behavior by the end of WY 2010.

Hydrological impacts of inflow and land-use changes in the Gorai River catchment, Bangladesh

This study looks at the changes in water balance in the Gorai River Catchment in the Bangladesh delta before and after operationalization of Farakka Barrage. Results show that inflow into the catchment has decreased, but major changes in land use within the catchment have also impacted runoff. Model scenarios demonstrate that although increasing inflow from upstream is the most effective method to increase dry season flows into the delta, reduction in cultivation of water-intensive crops would provide an alternative solution.

OPTIMIZING ENVIRONMENTAL FLOWS BELOW DAMS

ABSTRACTThe protection of riverine ecosystems downstream from dams requires reservoir operations to satisfy environmental flow (e‐flow) requirements. In practice, this often involves re‐regulation of downstream river flows within the constraints of water resource demands for human supplies and irrigation. In this paper, we develop a method to assess how reservoir storage may be used more effectively to meet both water supply and downstream ecosystem needs. The latter are met by providing seasonally variable baseflows, including dry season flow recession at natural rates, and high‐flow pulses and bankfull flows synchronized with natural events. A series of reservoir operating rules are derived based on both reservoir storage and reservoir inflows to trigger reservoir releases. For a given water supply reliability, the method seeks to minimize the degree of hydrological alteration below a dam. We present a case study of the Tanghe Reservoir in China's Tang river basin to show that the proposed method could help reservoir managers and planners evaluate e‐flow scenarios to reduce the negative impacts of dams and reservoirs on riverine ecosystems. Copyright © 2010 John Wiley & Sons, Ltd.

Trends in rainfall and runoff in the Blue Nile Basin: 1964–2003

AbstractMost of the water from the Nile originates in Ethiopia but there is no agreement on how land degradation or climate change affects the future flow in downstream countries. The objective of this paper is to improve the understanding of future conditions by analysing historical trends. During the period 1964–2003, the average monthly basin‐wide precipitation and monthly discharge data were collected and analysed statistically for two stations in the upper 30% of the Blue Nile Basin and monthly and 10‐day discharge data of one station at the Sudan–Ethiopia border. A rainfall–runoff model examined the causes for observed trends. The results show that, while there was no significant trend in the seasonal and annual basin‐wide average rainfall, significant increases in discharge during the long rainy season (June to September) were observed at all three stations. In the upper Blue Nile, the short rainy season flow (March to May) increased, while the dry season flow (October to February) stayed the same. At the Sudan border, the dry season flow decreased significantly with no change in the short rainy season flow. The difference in response was likely due to the construction of weir in the 1990s at the Lake Tana outlet that affected the upper Blue Nile discharge significantly but affected less than 10% of the discharge at the Sudan border. The rainfall–runoff model reproduced the observed trends, assuming that an additional 10% of the hillsides were eroded in the 40‐year time span and generated overland flow instead of interflow and base flow. Models concerning future trends in the Nile cannot assume that the landscape runoff processes will remain static. Copyright © 2010 John Wiley & Sons, Ltd.