Vladimir U. Smakhtin

Hydrology-based assessment of environmental flows: an example from Nepal

Abstract The paper examines the applicability of several desktop hydrology-based environmental flow assessment methods—Tennant, range of variability approach (RVA) and South African desktop reserve model (DRM)—in the specific context of Nepal. Some of these techniques are modified, following the discussion of their limitations. It is indicated that hydrology-based methods of environmental flow assessment represent a necessary first step in planning for environmental allocations in developing countries. It is shown that use can be made of complementary features of existing environmental flow assessment techniques to arrive at justified estimates of environmental flows, even in the conditions of limited basin-specific eco-hydrological knowledge. The methods described in the paper could also be used in other countries—by relevant departments, agencies and organizations, which are engaged in ecosystem management and preservation of aquatic environment. This research intends to promote the need for environmental water allocation planning in river basin development and to streamline the inclusion of environmental water demand assessments into relevant national policies.

The Indus and the Ganges: river basins under extreme pressure

The basins of the Indus and Ganges rivers cover 2.20 million km2 and are inhabited by more than a billion people. The region is under extreme pressures of population and poverty, unregulated utilization of the resources and low levels of productivity. The needs are: (1) development policies that are regionally differentiated to ensure resource sustainability and high productivity; (2) immediate development and implementation of policies for sound groundwater management and energy use; (3) improvement of the fragile food security and to broaden its base; and (4) policy changes to address land fragmentation and improved infrastructure. Meeting these needs will help to improve productivity, reduce rural poverty and improve overall human development.

Global hydrological models: a review

Abstract Global hydrological models (GHMs) have effectively become a separate research field in the last two decades. The paper reviews and compares 12 known global modelling efforts since 1989, the year the first GHM was published. Structure, strengths and weaknesses of individual models are examined, and the objectives of model development and their initial applications are documented. Issues such as model uncertainty, data scarcity, integration with remote sensing data and spatial resolution are discussed. Editor D. Koutsoyiannis

Basin Closure and Environmental Flow Requirements

A river basin is referred to as ‘closed’ when all its river flow is allocated to different uses. Water requirements of freshwater-dependent ecosystems, often referred to as ‘environmental flow requirements’, only recently started to receive attention. This ‘user’ is still often neglected in river basin management. This paper discusses the place of environmental flow requirements in basin water resources, examines a global pattern of closed/closing river basins and advocates the need to set environmental requirements in advance of major basin developments. It is also suggested that to ensure sustainable water resources development in the future, it is necessary to revise the content of ‘basin closure’ by explicitly introducing environmental flow requirements into basin water management and supporting it with relevant policies.

Hydrological Modeling of Large river Basins: How Much is Enough?

Hydrological modeling is an indispensable component of water resources research and management in large river basins. There is a tendency for each new group working in a basin to develop their own model, resulting in a plethora of such tools for each major basin. The question then becomes: how much modeling is enough? This study reviews hydrological modeling in four large basins (Nile, Mekong, Ganges and Indus). Based on this review, four areas for action to improve effectiveness and reduce duplication in hydrological modeling of large basins are suggested. Model setups and input data, as well as model results, should be published, to allow more coordinated approaches and capitalize on past modeling efforts. More focus is needed on reporting uncertainty, to allow more realistic assessment of the degree of confidence in using results for policy and management. Initiatives are needed to improve the quantity and quality of data for model input, calibration and validation, both traditional hydrological monitoring (improved networks, expansion of automated systems) and new methods for data collection (remote sensing, crowd-sourcing and community based observations). Finally, within each major basin, an appropriate agency should be identified and resourced to take responsibility for data sharing and coordination, to reduce redundancy of effort and promote collaboration.

Multi-year variability or unidirectional trends? Mapping long-term precipitation and temperature changes in continental Southeast Asia using PRECIS regional climate model

The subject of change detection in climate time series has recently received greater interest as the perception of a human-induced change in the climate is now widely accepted. However, changes in regional precipitation and temperature remain uncertain. This study characterizes projected fine-scale changes in precipitation and temperature in continental Southeast Asia over the period 1960–2049. Twenty four annual variables were derived from grid-based daily precipitation and temperature produced by the PRECIS regional climate model under A2 and B2 scenarios. These time series, capturing precipitation intensities (classified as low, medium and high), seasonality and extremes in precipitation and temperature, were subjected to the modified Mann-Kendall trend detection test accounting for long-term persistence. The results indicate that temperature increases over the whole region with steeper trends in higher latitudes. Increases in annual precipitation, mainly restricted to Myanmar and the Gulf of Thailand, result from increases in high precipitation during the wet season. Decreases are observed mainly over the sea and caused by a reduction of low precipitation. Changes in the occurrence of the monsoon affect the low-latitude sea areas only. By showing that significant precipitation change are minor over land areas, these results challenge most of the previous studies that suggested significant precipitation changes over Southeast Asia, often mixing up multi-decadal variability and long-term unidirectional trends. Significant changes in precipitation and temperature may induce higher agricultural yields as steepest temperature and precipitation increases will predominantly affect the coldest and driest land areas of the region.

Global water demand projections: past, present and future

A review of global water demand projections (WDPs) show substantial overpredictions or under-predictions. The pre-1990 WDPs, with population as the main driver of change, overpredicted current water use by 20 to 130%. The post-1990 WDPs, with sophisticated modeling frameworks, show substantial underestimation under the ‘business-as-usual’ scenarios and are more downward biased under sustainable scenarios. Overall, the value of long-term country-level projections in global WDPs is inadequate for local water resource planning. To increase the accuracy and value of global WDPs, future WDPs should take into account the spatial variation and influence of rapidly changing key exogenous and endogenous drivers of water demand in different sectors across and within countries, and provide a sensitivity analysis of projections.

The Projected Impact of Climate Change on Water Availability and Development in the Koshi Basin, Nepal

Abstract Water has been identified as a key resource for Nepals economic growth. Although the country has 225 billion cubic meters of water available annually, less than 7% has been utilized. Climate change is a frequent topic in national development discussions in part because of its possible impact on future water availability. This study assessed the likely impact of climate change on water resources development in the Koshi River basin, Nepal, using the Soil and Water Assessment Tool to generate projections for the 2030s and 2050s. Results suggested that the impacts are likely to be scale dependent. Little impact is projected at annual, full-basin scales; but at sub-basin scale, under both the IPCCs A2 and B1 scenarios, precipitation is projected to increase in the upper transmountain subwatersheds in the 2030s and in most of the basin in the 2050s and to decrease in the lower sub-basins in the 2030s. Water yield is projected to increase in most of the basin except for the A2 scenario for the 2030s. Flow volumes are projected to increase during the monsoon and postmonsoon but decrease during the winter and premonsoon seasons. The impacts of climate change are likely to be higher during certain seasons and in some sub-basins. Thus, if infrastructure is in place that makes it possible to store and transfer water as needed, the water deficit due to any changes in rainfall or flow patterns could be managed and would not be a constraint on water resources development. The risks associated with extreme events such as floods and droughts should, however, also be considered during planning.

Simulating the hydrology and mouth conditions of small, temporarily closed/open estuaries

Many small estuaries and coastal lagoons in different parts of the world may be classified as temporarily closed/open ecosystems. They are blocked off from the sea for varying lengths of time by a sand bar, which forms at the estuarine mouth. The lengths of the closed and open phases, which are determined primarily by the interaction of river inflow and the sea in the mouth region, affect the structure and functioning of the estuarine biotic community. Freshwater inflow to such estuaries is normally not measured, and observations on the duration of estuarine mouth openings/closures are very scarce. As a result, relevant management decisions are often made on the basis of general experience and intuitive judgment. This paper describes an innovative approach for linking hydrologic data to mouth state in ungauged estuaries. A key characteristic in the method is the stream/river flow duration curve. It is first established for a daily index, which reflects the upstream catchment wetness and is calculated using rainfall information from the nearest rain gauge(s). This duration curve is then used to convert the current precipitation index time series into a continuous daily inflow time series at the ungauged estuarine mouth location. The conversion is based on the assumption that precipitation index values in a small catchment, and daily inflows to the estuarine mouth correspond to similar probabilities on their respective duration curves. The paper further illustrates how the generated inflow data could be used for the simulation of a continuous time series of estuary mouth openings/ closures. Inflows are routed through a reservoir model, and the estuary mouth is considered open on days when the spillage from an estuarine “reservoir” occurs. The approach is illustrated using limited observed data on estuary mouth conditions from the South African coastline.

Including cultural water requirements in environmental flow assessment: an example from the upper Ganga River, India

The rituals of riparian communities are frequently linked to the flow regimes of their river. These dependencies need to be identified, quantified and communicated to policy makers who manage river flows. This paper describes the first attempt to explicitly evaluate the flows required to maintain the cultural and spiritual activities in the upper Ganga River basin. Riparian dwellers and visitors were interviewed and the responses analyzed to obtain an overview of the needs and motivations for cultural flows. The approach enhances the overall concept of environmental flow assessment, especially in river basins where spiritual values ascribed to rivers are high.