Denis A. Hughes

A decade of Predictions in Ungauged Basins (PUB)—a review

Abstract The Prediction in Ungauged Basins (PUB) initiative of the International Association of Hydrological Sciences (IAHS), launched in 2003 and concluded by the PUB Symposium 2012 held in Delft (23–25 October 2012), set out to shift the scientific culture of hydrology towards improved scientific understanding of hydrological processes, as well as associated uncertainties and the development of models with increasing realism and predictive power. This paper reviews the work that has been done under the six science themes of the PUB Decade and outlines the challenges ahead for the hydrological sciences community. Editor D. Koutsoyiannis Citation Hrachowitz, M., Savenije, H.H.G., Blöschl, G., McDonnell, J.J., Sivapalan, M., Pomeroy, J.W., Arheimer, B., Blume, T., Clark, M.P., Ehret, U., Fenicia, F., Freer, J.E., Gelfan, A., Gupta, H.V., Hughes, D.A., Hut, R.W., Montanari, A., Pande, S., Tetzlaff, D., Troch, P.A., Uhlenbrook, S., Wagener, T., Winsemius, H.C., Woods, R.A., Zehe, E., and Cudennec, C., 2013. A decade of Predictions in Ungauged Basins (PUB)—a review. Hydrological Sciences Journal, 58 (6), 1198–1255.

Panta Rhei-Everything Flows: Change in hydrology and society-The IAHS Scientific Decade 2013-2022

Abstract The new Scientific Decade 2013–2022 of IAHS, entitled “Panta Rhei—Everything Flows”, is dedicated to research activities on change in hydrology and society. The purpose of Panta Rhei is to reach an improved interpretation of the processes governing the water cycle by focusing on their changing dynamics in connection with rapidly changing human systems. The practical aim is to improve our capability to make predictions of water resources dynamics to support sustainable societal development in a changing environment. The concept implies a focus on hydrological systems as a changing interface between environment and society, whose dynamics are essential to determine water security, human safety and development, and to set priorities for environmental management. The Scientific Decade 2013–2022 will devise innovative theoretical blueprints for the representation of processes including change and will focus on advanced monitoring and data analysis techniques. Interdisciplinarity will be sought by increased efforts to connect with the socio-economic sciences and geosciences in general. This paper presents a summary of the Science Plan of Panta Rhei, its targets, research questions and expected outcomes. Editor Z.W. Kundzewicz Citation Montanari, A., Young, G., Savenije, H.H.G., Hughes, D., Wagener, T., Ren, L.L., Koutsoyiannis, D., Cudennec, C., Toth, E., Grimaldi, S., Blöschl, G., Sivapalan, M., Beven, K., Gupta, H., Hipsey, M., Schaefli, B., Arheimer, B., Boegh, E., Schymanski, S.J., Di Baldassarre, G., Yu, B., Hubert, P., Huang, Y., Schumann, A., Post, D., Srinivasan, V., Harman, C., Thompson, S., Rogger, M., Viglione, A., McMillan, H., Characklis, G., Pang, Z., and Belyaev, V., 2013. “Panta Rhei—Everything Flows”: Change in hydrology and society—The IAHS Scientific Decade 2013–2022. Hydrological Sciences Journal. 58 (6) 1256–1275.

A desktop model used to provide an initial estimate of the ecological instream flow requirements of rivers in South Africa

Abstract The South African Water Act of 1998 stipulates that future water resource developments should be environmentally sustainable and that a component of the natural flow of rivers should be reserved to ensure some level of ecological functioning. Detailed methods for quantifying the environmental instream flow requirements of rivers have been available internationally and in South Africa for some time, but the implementation of the new act introduced a degree of urgency and pointed towards the need for rapid, low-confidence assessments that could be used for initial planning. The desktop reserve model was developed to fulfil this requirement, but since its development in 1999 there have been many more detailed IFR determinations. During 2001 the model underwent a complete review based on the results from 97 detailed IFR determinations involving groups of specialists applying the building block methodology (BBM). The paper explains the conceptual links between the BBM and the model, describes the algorithms that constitute the model and discusses some of the problems associated with developing a model of this type from limited data and limited scientific understanding of the underlying principles. Most of the problems are related to the lack of a consistent understanding of the relationships between hydrological variations and ecological functioning of rivers across a range of river types within a large geographic region (South Africa, for example). The paper indicates that despite these problems the model has found a niche in the water resource management industry within South Africa and has the potential to be improved as our scientific understanding of eco-hydrology improves.

Daily flow time series patching or extension: a spatial interpolation approach based on flow duration curves

Abstract An algorithm is described that was initially developed as a simple method for patching and extending observed time series of daily streamflow. It is based on the use of 1-day flow duration curves for each month of the year and on the assumption that flows occurring simultaneously at sites in reasonably close proximity to each other correspond to similar percentage points on their respective duration curves. The algorithm has been incorporated into a model that allows flows at a destination site to be estimated from flows occurring at several source sites. The model has been applied to six groups of catchments within southern Africa and the resulting streamflow simulations compare favourably with those obtained using a semi-distributed, physically-based, daily time step rainfall-runoff model. The current limitations of the approach and its future potential value are discussed.

Monthly rainfall-runoff models applied to arid and semiarid catchments for water resource estimation purposes

Abstract Problems associated with the application of monthly rainfall-runoff models to arid and semiarid areas for water resource estimation purposes are discussed with respect to the representation of hydrological processes and the model rainfall input. The problems are illustrated using a specific monthly model applied to catchments within the semiarid to arid parts of southern Africa. Some model improvements are suggested and briefly evaluated and the application of a daily model to one of the catchments is compared. The overall conclusion is that while a detailed examination of the available rainfall data can account for many of the inadequate simulation results on an individual month basis, it is difficult to make allowances for the lack of resolution in the normally available information. Other problems related to the structure of individual models can be surmounted by limited model developments, but very little can be done to bypass the rainfall quantification problem.

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.

A semi-distributed, variable time interval model of catchment hydrology—structure and parameter estimation procedures

Abstract The conceptual basis and structure of a semi-distributed, deterministic hydrological model is presented. The modelling time interval used is variable and changes automatically according to a series of user defined rainfall thresholds. The model represents a pragmatic compromise between highly detailed physically based and completely empirical approaches. This is achieved using a semi-distributed approach, but incorporating some effects at scales of less than the sub-area level. Model conceptualisations are based on the authors understanding of physical hydrology, but the complexity of the algorithms has been limited to prevent the models information requirements from being too stringent. The model is designed to be applicable to a range of situations where a simulation of the components of catchment hydrology is required to solve a problem. The paper also briefly discusses the procedures used to estimate parameter values from readily obtainable physical catchment characteristics and presents some example simulations to illustrate the models operation.

Simulating the impacts of land-use and climate change on water resource availability for a large south Indian catchment

Abstract A monthly rainfall-runoff model was calibrated for a large tropical catchment in southern India. Various land-use and climatic change scenarios were tested to assess their effects on mean annual runoff and assured water yield at the Bhavanisagar Reservoir in Tamil Nadu, India. The largest increase in runoff (19%) came from converting forest and savanna (the indigenous control scenario) to agriculture. Mean annual runoff decreased by 35% after conversion to commercial forest and 6% after partial conversion to tea plantations. The predicted climate scenarios of reduced dry season rainfall decreased the annual runoff by 5% while enhanced annual rainfall caused a 17% increase in runoff. Even if land-use and climate changes had relatively large effects on runoff, the changes in reservoir yield which can be assured every year, were often less severe. This was probably due to the buffering effect of the reservoir and variation in the mean annual runoff.

Providing hydrological information and data analysis tools for the determination of ecological instream flow requirements for South African rivers

The new South African Water Act and the inclusion of a component that stipulates that some water has to be reserved to maintain the ecological functioning of rivers has stimulated a renewed interest and need for detailed hydrological data. This has occurred at a time when many of the countries rivers are already affected by impoundments, abstractions, return flows and land-use change impacts. The need to relate the ecological requirements of rivers to the characteristics of the natural flow regime and to convey the details of flow regimes to non-hydrological specialists has presented hydrologists with an exciting challenge. It is rewarding to discover that the status of hydrological research and practice in South Africa, and the methodologies that were already available, were more than capable of responding to this challenge and that the hydrological tools required to assist with the implementation of the new act were either already available or could be developed rapidly.

Incorporating groundwater recharge and discharge functions into an existing monthly rainfall–runoff model/Incorporation de fonctions de recharge et de vidange superficielle de nappes au sein d’un modèle pluie-débit mensuel existant

Abstract Abstract There is an urgent need for an integrated surface water and groundwater modelling tool that is suitable for southern African conditions and can be applied at various basin scales for broad strategic water resource planning purposes. The paper describes two new components (recharge and groundwater discharge) that have been added to an existing monthly time-step rainfall–runoff model that is widely used in the southern African subcontinent. The new components are relatively simple, consistent with the existing model formulation, but based on accepted groundwater flow principles and well understood groundwater parameters. The application of the revised model on two basins in southern Africa with quite different baseflow characteristics has demonstrated that the new components have a great deal of potential, even if the improvement is only to be able to simulate the groundwater baseflow component of total runoff more explicitly. More comprehensive testing and comparison of the results with existing groundwater and geohydrological data is required, while some extensions to the new components need to be considered to ensure that the model can be considered applicable to a wide range of basin and climate types.