Review of Yield Comparison Options for Planning Irrigation Reservoirs in Ungauged Watersheds

Abstract

Irrigation reservoir design is often associated with ungauged watersheds. Irrigation system planners have the option of selecting either traditional institutional guidelines or more recently calibrated and verified hydrologic models to compute watershed yield. A design watershed yield is the key for sustainable irrigation reservoir development. Design values not only should lead to reliable estimates but also require the incorporation of safety factors for both rainfall uncertainty and modelling uncertainties. Empirical design yield calculation options in guidelines require explicit rationalization to determine appropriate design levels. Use of popular watershed models , developed and calibrated with other datasets, too requires the guidance to convert the estimations as design watershed yield with the use of safety factors. Therefore, both options need to clearly express the reliability of yield estimations and the associated safety factors for a sustainable practice of water management. Comparison of available alternatives in the light of methods and incorporation of safety is a necessity to select the appropriate option for water resources design in ungauged watersheds. Accordingly, the objective of the present work is to carry out a critical review of available methods to recommend a selection for estimation of streamflow yield from ungauged watersheds. A comprehensive literature review is carried out considering available text books, guidelines and other literature. The study looked at the runoff estimation methods, strengths and difficulties, incorporation of safety factors etc., when deriving design watershed yield from watershed yield. The review of 110 related documents revealed that, although there is literature on models for the estimation of watershed yield, there is a gap in the applicability at various locations and in the conversion to design watershed yield. Irrigation Department Sri Lanka has reported the most detailed direct yield model with threshold considerations. Unit Hydrograph model has many text book references while HEC HMS is the most commonly cited process-based model for runoff estimations. The review shows that, amidst the drawbacks in the present documentation, the best available optionfor a practicing engineer to compute design watershed yield is to use a similar approach that is indicated in the guidelines of Irrigation Department of Sri Lanka. There is an urgent necessity to verify guideline recommended design watershed yield estimation methods by using observed data and take measures to incorporate explicit safety factors.