Low flows

Due to the seasonal variability of precipitation over the HKH region, there is either plenty of water during the monsoon or a deficit during the dry season. In a data-limited region, it is important to have reliable and practical methods for assessing low flows for the planning and design of infrastructure such as hydropower, irrigation and water supply schemes.

The objective of the Low Flow Group is to develop a range of design methods for calculating low flows at gauged and ungauged sites in the HKH region. These should lead to the more accurate design of a wide range of water resource schemes, including estimation of dry season flows, river abstraction, public water supply and irrigation, preliminary estimates of storage yield relationships, reservoir design, and estimation of the energy available for run-of-the river hydropower schemes.

Since its inception, the Low Flow Group has worked on regional capacity-building for estimating and managing low flows through training and research projects. The three-year project (1998-2001) Regional flow estimation for small-scale hydropower assessment (REFRESHA) resulted in a valuable tool (Figure 8.3) for assessing the flow duration curve and, hence, the hydropower potential of the ungauged catchments of Nepal and Himachal Pradesh, India.

Figure 8.3 Modelled distribution of Q95 (as % of mean flow), derived during the REFERSHA project, which is used to estimate the flow duration curve and hydropower potential at ungauged sites in Nepal

The HydrA software that resulted from the project is now used by many developers and planners for better planning and wise utilization of the water resources in the region for hydropower development and small scale irrigation and water supply schemes.

To further improve the understanding of the low flow variation a regional recession model to estimate dry-season flows in ungauged catchments was developed, focusing on areas spanning Nepal and the northern Indian state of Himachal Pradesh with similar topographical and climate conditions. A selection of gauged catchments in Nepal and Himachal Pradesh, India were chosen to develop the regional recession model having good hydrometric quality and natural flow regimes. On the basis of recession constant (k), flow at the beginning of the recession (Q₀) and recession start time (T₀), regionalised equations for predicting each parameter based on catchments characteristics were developed. Performance of the model was found to be reasonably good. The results suggest that such a regional model provides a relatively robust technique for estimating average annual recession flows at ungauged sites and provides water resources managers with the ability to estimate not only the magnitude of flows in ungauged catchments, but also the timing of the flows (Rees et al., 2004). The regional recession model was then incorporated into a modified version of the Low Flows 2000 integrated water resources management (IWRM) tool (Young et al., 2003) and applied to the West Rapti river basin in Nepal (Figure 8.4) and the Uhl in Himachal Pradesh, India.

The aim of the application was to demonstrate the potential for improved water resources management in the HKH. First, the tool enables estimates of the natural dry-season low flows

Figure 8.4 Application of the regional recession model in the Low Flows 2000 software, comparing the estimated natural recession curve (blue) against the estimated artificially influenced recession curve (green) and observed data (red dots) for the West Rapti river at Jalkundi, Nepal

in ungauged catchments to be derived, using the regional model. Second, it enables the impact of water use or "artificial influences" within the catchment to be simulated. Workshops to disseminate the model and the software to local and regional water practitioners were held in Nepal and India in May 2003.

The project Low flow studies in the rivers of Bhutan, was recently initiated by the Low Flow Group and ICIMOD with the support of UNESCO/IHP. The project seeks to improve the database on hydrological information on the rivers of Bhutan, analyse the available low flow data and develop a preliminary methodology for the prediction of low flow in the rivers of Bhutan. The project was due to be completed in March 2006.

Collaborative research was undertaken with the NE-FRIEND Large-scale Variations Group with the aim of (1) identifying large-scale hydroclimatological patterns across Nepal and (2) establishing precipitation-flow regime associations to aid assessment of current and future (under climate change) water availability dynamics for gauged and (extrapolation to) ungauged basins.

The following specific objectives were achieved:

• characterisation of spatial and temporal variability of long-term average and annual river flow and precipitation regimes (shape and magnitude)

• understanding of controls upon large-scale hydro climatological patterns

• assessment of the sensitivity of river flow regimes to precipitation

• application of precipitation-flow associations to predict future river flow and for ungauged basins.

The study produced the following key observations:

• The controls upon spatial patterns in Nepalese precipitation include length and timing of the summer monsoon, orographic and convection effect of the mountains generating rain shadows and localised precipitation 'hot spots' and westerly winter precipitation (Kansakar et al., 2004).

• The factors governing annual flow regimes are the onset and cessation times of the summer monsoon, meltwater and groundwater contributions, and local topography (Hannah et al., 2005b; Kansakar et al., 2002).

• Substantial meltwater (snow and glacier) contributions reduce the inter-annual variability in the flow regime shape, although a similar relationship does not exist for magnitude, which is generally more variable.

• Precipitation-flow links for shape are different for different regions while links are generally more straightforward for magnitude (Hannah et al., 2004, 2005a).

Perhaps most importantly, the results stress that future water resource management and decision-making in the Nepalese Himalaya must (a) be scientifically well-informed and (b) recognise the significant spatial (local and regional scale) and temporal (year-to-year) variation in basin hydro climatological response.

8.2.3 Floods

The main objectives of this research group are to conduct flood studies to help mitigate flood damage and to develop regional design procedures for estimating floods at gauged and ungauged sites in the HKH region.

The project Flood risk and vulnerability mapping using GIS: A case study from Ratu river in central Nepal has been recently completed by the group. The study used GIS and remote sensing data to map flood risk vulnerability and sought to identify community-based disaster mitigation activities to reduce loss of life and property. Findings were disseminated during a national seminar on Flood risk and vulnerability mapping of the Ratu river basin in January 2004 to 45 participants from national, district and local levels. The seminar was also used to discuss project components for the second phase of the project including flood disaster mitigation, development of coping mechanisms, implementation of structural and non-structural measures for flood disaster mitigation and institutional set-ups in the watershed.

The second phase of the study used detailed topographic information and prepared an accurate hydraulic model, using Hydrologic Engineering Center’s River Analysis System (HEC-RAS) for a selected site in Jaleshwor, proposed by the stakeholders. The model predicted accurate water levels and defined inundation areas and identified escape routes during floods of various recurrence intervals. A detailed socio-economic survey was also conducted to check the results obtained from the model with the actual ground information.

A significant development with regard to regional floods was also initiated as a contribution to the floods theme of HKH-FRIEND. The project Regional cooperation for flood forecasting and information exchange in the HKH began in 2001. The project has achieved full cooperation from five participating countries, Bangladesh, Bhutan, China, Nepal and Pakistan, with India as an observing member. Four years of deliberations have resulted in a series of high-level meetings and national consultations which have laid the foundation for regional cooperation, strengthened existing bilateral arrangements and enriched institutional linkages. The project conducted a demonstration and testing phase, during which the five fully participating countries exchanged near-real-time hydrological data from twelve pilot stations.

Following the successful completion of this phase, the partners have recommended that the project should proceed to a second phase and increase the station network to make meaningful interpretations of the real-time data and increase the lead time so that more lives and property can be saved from floods. The project seeks to improve the technical capacities, infrastructure and know-how of the partners.