Interpolation of rainfall observations during extreme rainfall events in complex mountainous terrain. Issue 11 (24th November 2022)

• Record Type: Journal Article
• Authors: Page, Trevor; Beven, Keith J.; Hankin, Barry; Chappell, Nick A.
• Is Part Of: Hydrological processes. Volume 36:Issue 11(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-11-24
• Subjects: cokriging -- complex terrain -- extreme events -- orographic enhancement -- rain shadow -- rainfall interpolation -- upland UK; Hydrology -- Periodicals; Hydrology -- Research -- Periodicals; Hydrologic models -- Periodicals; Hydrological forecasting -- Periodicals; 631.432
• DOI: 10.1002/hyp.14758
• Languages: English
• ISSNs: 0885-6087

Abstract

Abstract: The representation of rainfall in space is important for hydrological modelling. Accurate estimation of rainfall is particularly challenging in mountainous regions where observations are often sparse relative to the spatial variability of rainfall. In these regions, orographic processes lead to complex patterns of rainfall enhancement and rain shadow depletion. This study tests Natural Neighbour Interpolation (NNI), ordinary kriging (OK) and ordinary cokriging (CK), to determine if CK improves rainfall interpolation during three extreme rainfall events. Three different elevation indices were considered as secondary variables for CK . Preliminary analysis using long‐term annual average rainfall totals, including additional high elevation rainfall observations, showed that CK with an effective elevation index (a directionally smoothed elevation, corrected for degree of 'orographic processing' and shifted to account for 'wind‐drift' of rainfall) as a secondary variable performed better than NNI and OK with an overall improvement of around 40%. Using rainfall totals for long‐term wind direction and wind speed rainfall classes, CK performance was variable but provided an improvement of approximately 15% for wind direction classes without an easterly wind component. For 15‐min timesteps during extreme rainfall events, there were comparatively small differences (cross‐validation using RMSE ) between interpolation methods, partly attributed to having only relatively low elevation rainfall observations, providing weak constraint. Using cross‐validation and mean bias did, however, show an improvement for both high and low elevation observation classes. Importantly, cross‐variogram estimation provided differing cross‐validation results when estimated for different rainfall accumulation periods: 15‐min, hourly, daily and long‐term. Variograms and cross variograms estimated at a 15‐min timestep frequency were robust for many timesteps, but were difficult to fit automatically for others. Variograms estimated from longer periods were more reliably estimated, but tended to have lower variance and cross‐variance and longer correlation ranges producing a smoother interpolated rainfall field. Given the weak cross‐validation constraint, care must be taken in identifying the most appropriate method and variogram estimation period. Abstract : This study extends previous work using cokriging for interpolating rainfall in complex mountainous terrain. The primary novel aspect of this work is an 'effective elevation' secondary variable for cokriging that includes an index of directional 'orographic processing.' The work is also relatively novel in its investigation of high frequency (down to 15‐min) interpolation of rainfall during extreme events. Importantly, and contrary to many previous studies, relatively high correlations between the effective elevation index and 15‐min, and hourly, rainfall accumulations were found. The work also highlights that where rainfall observation sites are sparse and at relatively low elevations compared to the surrounding terrain, it is not sufficient to rely on simple cross‐validation results to evaluate different methods. This is the direct result of the weak constraint associated with the low elevation rainfall observations.