ABSTRACT
Low flow is the periodic seasonal phenomenon that occurs in a river during the dry period of the year and the random event of hydrological drought, resulting from less-than-normal precipitation, occurring over several consecutive months or years. Analysis and estimation of low flow characteristics in the past focused primarily on the design of river diversions within the water supply and hydropower systems or on the protection of water quality affected by wastewater discharge. In recent decades there has been a shift toward the assessment of environmental flow management. This chapter deals with the subject of low flow, emphasizing the interaction with instream flow requirement. The first section illustrates the main estimation methods of low flow characteristics (recession curve, flow duration curves, low flow frequency analysis, run analysis, low flow in ungauged catchments). The next section then describes a variety of methods proposed to assess instream flow requirements, including a set of mandatory technical rules adopted in different countries to maintain aquatic life in rivers. Finally, some criteria to help in the choice of estimation methods of instream flow requirements are discussed. Concluding remarks focus on the necessity of overcoming the idea that ecological purpose requires the restoration of the river to an idealized natural state and instead a careful balance should be found between the needs of aquatic life and the water abstraction requirements for essential human uses.
