Quantifying bias in hydromorphological monitoring: an evaluation of the German LAWA-OS method

Hydromorphological assessment methods as the on-site assessment of the German Working Group on water issues (LAWA-OS method) provide valuable information for a wide range of water management issues like water body assessment, deficit analysis and planning or monitoring of restoration projects. Considering these demands, the question about the assessment variability of such methods arises. Depending on varying aims, scales and approaches different methods may show contradictory assessment results. The objectives of this work are to quantify assessment deviations between different versions of the LAWA-OS method and to identify the causes of these deviations. The hypothesis is that procedural differences between versions act as deviation factors and lead to scoring discrepancies. A pairwise comparison between assessment results of two representative versions show that the LAWA-OS method is very robust against deviation factors on the overall score level. With increasing differentiation of hydromorphological characteristics on the main parameter and single parameter level, the assessment robustness decreases considerably. Particularly, differing numbers of parameters, differing reference scores and differing score aggregation procedures act as factors for substantial assessment deviations between versions. The work in hand provides scientifically based outputs in relation to the reliability, comparability and applicability of the LAWA-OS assessment results for river ecology issues. In this regard, the work in hand contributes to the quality control of the LAWA-OS method and provides valuable insights for practitioners and policy makers.