The methodology we adopt is based on the standard NRFA trend testing approach outlined by Hannaford et al. (2021). In brief, monotonic trends were assessed using the Mann-Kendall (MK) test (Mann 1945; Kendall 1975), a non-parametric rank-based approach that is widely supported for use in streamflow analysis (e.g. Hannaford & Marsh 2008; Murphy et al. 2013). The MK Z statistic (MKZ) follows the standard normal distribution with a mean of zero and a variance of one. A positive (or negative) value of MKZ indicates an increasing (or decreasing) trend. The probability of Type 1 errors set at the 5% significance level allowed the evaluation of statistical significance. A two-tailed MK test was chosen; hence, the null hypothesis of ‘no trend present’ (increasing or decreasing) is rejected when MKZ is outside ±1.96 using traditional statistical testing.
The analysis was undertaken on the full AMAX record available at each station (with the exclusion of rejected and unrepresentative periods). As is widely noted in the literature (e.g. MacDonald & Sangster 2017), decadal-scale variability results in flood-rich and flood-poor periods that can affect the robustness of trends based on the period of data that is selected, users should be aware of this and note that trends have been carried out on the available data and may not reflect longer-term trends (outside of the period of observed data).
The NRFA Peak Flow Dataset Version 12 consists of 917 gauging stations across the UK, of which 893 are classed as ‘Suitable for Pooling’ or ‘Suitable for QMED’ estimation according to the indicative suitability criteria applied in the dataset (see V12 of the NRFA Peak Flow Dataset), and these were selected as the basis for analysis here. ‘Suitable for Pooling’ means either the highest AMAX flow or the 8-year event is likely to be within 30% of its true value, while ‘suitable for QMED’ means that the median AMAX (QMED) is likely to be within 30% of its true value.
The Peak Flow Dataset was then processed based on the following missing data criteria to mitigate the impacts of inevitable gaps in data:
- No more than 10% of missing data.
- 27 or more years of data (≥27 AMAX).
Resultingly, 719 sites were selected for trend analysis from the V12 dataset (see full list of sites).