Reservoir management under consideration of stratification and hydraulic phenomena

Reservoirs are the most important components in a water resources system. They are used to store water to extend its temporal availability. The physical, chemical and biological characteristics of water change when impounded in reservoirs. This implies the possibility of using reservoirs for the control of the quality of water besides merely satisfying the quantity requirement. This study presents several techniques formulated to manage a reservoir when both quantity and quality of water are of interest. In this study salinity is selected to characterize the water quality status. The approaches are demonstrated using data from the Jarreh Reservoir on the Shapur river in Iran.Water in a reservoir is stratified for most of a year due to difference in density caused by temperature, dissolved and suspended solids. Therefore, in a stratified reservoir the quality of water that is interrelated to density varies with depth. Consequently, this feature could be used in the process of reservoir operational policy determination to improve the quality of water supply. The aim of this research is to analyze different approaches regarding the incorporation of this phenomenon into reservoir operational policies and to propose those which require the least increase in mathematical and computational complexity.Initially, two techniques that rely on the natural process of stratification occurring in a reservoir are presented. The first methodology proceeds stepwise in time alternating optimization and simulation of reservoir operation at each time step. A one-dimensional reservoir dynamics simulation model is employed to simulate the stratification of the reservoir. A constrained nonlinear optimization model is used to identify optimum releases. In the optimization step the reservoir is assumed to be equivalent to the parallel configuration of several smaller hypothetical reservoirs, the number of which being equal to the number of outlets. There is no communication among these hypothetical reservoirs. The applicability of the technique is tested for three hydrologically different years and for a continuous period of five years. Incorporation of inflow stochasticity into the methodology is devised through the integration of an optimization model based on Stochastic Dynamic Programming technique.Next, an iterative technique, in which an optimization model and a reservoir stratification simulation model operate interactively, is presented. One iteration cycle comprises the run of the optimization model and the simulation model: i) Reservoir operation is optimized over the entire time period (year); ii) Simulation of stratification is applied over the entire time period. The optimization model is based on Incremental Dynamic Programming technique. In the optimization model, the hypothetical reservoir concept used in the above model is adopted. However, communication between any two adjoining hypothetical reservoirs is allowed in the model. The one-dimensional reservoir dynamics simulation model simulates the stratification of the reservoir. The applicability of the technique is examined for three hydrologically different years.Reservoirs could also be modelled by assuming that complete mixing of water is occurring throughout its entire volume during a year. It is a simplification as compared with the real behaviour of stratification occurring in reservoirs. Two models are developed based on this assumption to improve the quality of water supply. In one model only the releases are controlled. In the other, both inflows and releases are controlled. Optimization is based on Incremental Dynamic Programming technique. The results from both models show improvements in the quality of water supplied from the reservoir. However, the improvements obtained by manipulating both inflows and releases are more profound.Improving the quality of water supplied from a reservoir by diverting poor quality inflows and satisfying downstream quantity demands are two conflicting objectives. This problem is studied under the multiobjective analysis framework. The reservoir is assumed to be completely mixed throughout its volume during the whole annual cycle. The results show that a cautious balance between the quantity of water supplied for downstream and the volume of inflows diverted would lead to marked reduction in the supply salinity.The study reveals that the quality of reservoir releases could be improved by withdrawals from different elevations in a stratified reservoir. However, the benefits obtained in this way are marginal for the case study reservoir. Similar improvements are observed under the assumption that the reservoir is completely mixed throughout a year. On the other hand, by manipulating the inflows to the Jarreh reservoir these improvements could be enhanced significantly. That is, by-passing of poor quality inflows seems to be a very promising management alternative for improving the quality of water supplied from the reservoir. The assumption of reservoir's complete mixing is warranted for the stratified reservoir by the obtained results. Hence, a relatively simple and straightforward methodology based on the non-stratification assumption proves to be suitable in managing a density stratified reservoir

A system dynamics simulation model for the assessment of water resources in Sri Lanka

Ability to accurately predict water availability in a country is vital in planning its water resources development activities. Traditional approach used is based on a water balance and a demand projection. The projections are variant of current trends and subject to considerable uncertainty. Besides, they do not capture dynamic character of climatic, socio-economic and environmental change etc., and their impact on water use. Thus, the traditional approach is subject to a wide margin of error. In contrast, a novel approach, “system dynamics” offers a new way of modeling future dynamics of complex water systems increasing the ability to correctly assess and predict availability and use of water. This paper presents a system dynamics based simulation model to predict water resources in Sri Lanka. Using the model water availability was predicted and Sri Lanka will have sufficient water in the year 2025. However, in several districts water availability will decrease rapidly while others will not get much affected

Non-linear statistical model for the daily stream flow Prediction in the kalu river catchment in Sri Lanka

Having a long record of stream flow is very valuable in planning water resources development projects. However, in many occasions, stream flow records are available for very short periods though very long rainfall records are available. Therefore, possibility to relate rainfall over a catchment to the stream flow at its outlet will enable having a long record of stream flow. Besides prediction of stream flow using already available predicted rainfall will permit taking precautionary measures in water related disaster situations such as floods and droughts. This paper presents a research carried out to find a model to predict daily stream flow of Kalu River at Ratnapura. The model, a non-linear regression model based on Marquardt’s procedure, was developed using measured daily stream flow at Kalu River at Ratnapura and daily rainfall at eight rainfall gauging stations within the catchment above Ratnapura. Data for the period 1987-1994 were used for the calibration of the model while data for the period 1995-2000 were used for verifying it. The model was validated using Nush-Sutcliffe efficiency and pseudo R2 . Nush-Sutcliffe efficiency (78%) and pseudo R2 (85%) show the possibility of the fitted model in predicting daily stream flow of Kalu River at Ratnapura

Rainfall forecasting for flood prediction in the Nilwala basin

Flooding is the major natural disaster in Sri Lanka and reliable forecasts with longer lead time is a way of reducing the damages. In this study a weather model was coupled with a hydrologic model and a hydraulic model for predicting floods in Nilwala river basin in southern Sri Lanka. WRF 3.0 (Weather Research and Forecasting) weather model was configured and used to predict rainfall over the basin 24 h into future. The model was configured by investigating the impacts of its physics options on precipitation forecasting. The impacts of microphysics schemes, cumulus schemes, land surface schemes, long/shortwave schemes and boundary layer schemes on rainfall predictions were investigated. The predictions were compared with observed point rainfall data for three rainfall events to find reasonably good physics combination. It was seen that model physics combination; Ferrier microphysics scheme, Kain-Fritsch cumulus scheme, Rapid Update Curve land surface scheme, Rapid Radiative Transfer Model longwave radiation scheme, Dudhia shortwave scheme and Yonsei boundary layer scheme yields better precipitation predictions over the basin. Output of the weather model was coupled with hydrologic model HEC-HMS 3.3 (Hydrologic Engineering Center-Hydrologic Modeling System) with Clark’s, Snyder’s and SCS transformation methods. In all model runs Green-Ampt loss model was executed with recession base flow method. Before using the model with the WRF output HEC-HMS model was calibrated for historical events and Snyder’s method performed better than other methods in calibration and verification. Snyder’s method produced Nash-Sutcliff efficiencies greater than 70% and 50% in calibration and verification respectively. WRF predicted rainfall for May-2003 was introduced to HEC-HMS and the generated river discharges of sub basin were ingested to the HEC-RAS 4.0 (Hydrologic Engineering Center-River Analysis System) hydraulic model for water profile computations along the Nilwala main river. Output of HEC-RAS was exported to Arc- GIS 9.2 where it was two dimensionally visualized as a flood map. Model was capable of predicting the areas as inundated regions but with underestimation of inundation depth

The Water – Energy – Food Nexus and Climate Change Adaptation

This report explores the exposure and vulnerability of Korea and the Southern African region to climate-driven impacts in the Water-Energy-Food (WEF) nexus. It presents the building of ecological networks as a mean to address climate change - induced alterations of ecosystems and the consequences for humans and nature. Reducing the asymmetry between price and value of water resources is identified as an essential aspect to enable sound resource management use decisions. The report highlights the need for comprehensive tools which assist decision makers in dealing with the complexity of WEF nexus interrelations and facilitate sustainable resource management