investigation on the necessities of instream flow needs assessment in the rivers using hydro-ecological methods (case study: delichai river in tehran, iran)

in the present research necessities of instream flow needs (ifn) assessment in the rivers with focusing on delichai river as case study is investigated using the hydro-ecological methods. at first, ifn is assessed using two common approaches of the tennant and wetted perimeter. then considering that rainbow trout is the most important and dominant species in this river, ecological condition of the river for this species is investigated and physical habitat evaluation is carried out. according to the results, differences among the evaluated ifn value from the tennant and wetted perimeter methods are very large and results of these two methods will be ambiguous. according to the habitat time series curve, habitat suitability condition is also very different for three life stages of rainbow trout. available suitable habitat is also very variable in different months of the year. thus, assessing a special value for ifn will be very difficult. in case of using wetted perimeter method, stakeholders don't have right to exploit the river, and in case of using methods like minimum of the tennant method for ifn assessment according to the habitat time series irrecoverable tensions for river ecosystem will be created, especially in critical months like spawning of the target species (march & april). generally, at first longitudinal habitat suitability distribution of the river must be extracted and then ifn value must be assessed according to the habitat suitability time series of each reach.