Determination of Ecological Instream Flow Requirements at Austrian Rivers via Minimum Flow Tests

摘要

The state of the art of the determination of ecological instream flow requirements is to guaranteethe preservation of a characteristic and river specific composition of species and life stagesof aquatic organism. Based on the Austrian Water Law, the maximum of possible reductionshave to be discussed during the process of permission.The presented method for defining flow requirements as a minimum flow out of minimum-flowtests is a reproducible and low cost way of objectivising the repercussions of water abstractionwithin the stream. By adopting abiotic parameters for different flows and establishing comparisonswith the natural conditions in the stream (guiding view) prior to water abstraction as well asby determining a dynamic pattern of flow to maintain the natural variations in yield, it is possibleto define and minimise the effects on the ecological conditions within the stream.The huge range of consequences in diverted rivers is shown and discussed at 19 case studiesat Austrian rivers and streams. Minimum flow primarily is defined out of statistic comparisons ofFST hemisphere distributions. Depending on type of river, acceptable minimum flow under theaspect of guaranteeing habitat quality is defined between 50 and 60 % MJNQt (mean of annualminimum flows of a period t) for potamal collin and planar rivers and between 30 and 70 %MJNQt for alpine and montan rhithral rivers. At minimum flow, the reduction of depth shows arange between 5 and 20 % for potamal type and 15 to 45 % for rhithral rivers. The wetted areais reduced between 5 to 25 % from situation at guiding view at potamal rivers and between 10and 35 % at rhithral type. Highest changes were measured for parameters aquatic volume andmean velocity. At accepted minimum flow, up to 40 % reductions of volume (rhithral 55 %) and45 % of mean velocity (rhithral 60 %) have been measured.Within a range between minimum and maximum instream flow, a dynamic pattern of flow will becontrolled depending on natural flow up to the point of excess water. Depending on dischargeregime, maximum instream flow was defined between 90 and 145 % MJNQt for potamal riversand between 55 and 160 % MJNQt for rhitral type.One may conclude from that the necessity of single case study judgements to conserve theecological integrity of rivers and the efficiency of the use of water. The discussed method andprocess of defining instream flow requirements ensure the objective presentation and minimizationof the effects. Observed data at 19 case studies in Austria show the outstanding importanceof the parameters volume and flow velocity, which are most effected in comparison to themodel.