Development of a general river regime index (RRI) for intra-annual flow variation based on the unit river concept and flow variation end-points

• We developed a new index to quantify intra-annual river regimes (RRI).
• RRI was developed in a framework of monthly river regime point (MRRP).
• Data on the river Nile was used to illustrate the benefit and use of the RRI.
• The impacts of lakes, wetlands and dams were clearly demonstrated by using RRI.

SummaryDifferent indices have been developed to classify rivers and assess land use and climate change impacts on rivers. These methods are not universal, but tend to depend on local conditions such as climate. This paper presents a general, dimensionless index for studying the impacts of hydrological changes in river regimes. The concept of the unit river and monthly river regime point (MRRP) is developed first and then the river regime index (RRI) is defined as the sum of MRRPs. RRI is a measure of the intra-annual flow regime of rivers ranging from completely uniform (RRI = 0) to ephemeral or dry (RRI = 1200). Data on the river Nile and its tributaries were used to illustrate the benefit and use of the RRI. The Nile was chosen as it is a long river with tributaries flowing though different climate zones. The RRI proved to be sensitive to climate and the annual distribution of excess rainfall, which decreases from upstream to downstream for the Nile in natural conditions. The impacts of lakes, wetlands and dams were clearly demonstrated. The RRI can be used to classify rivers and to study the impacts of changes in climate, base flow and river regulation. It is also useful as a tool for environmental flow allocation.