Linear Optimal Runoff Aggregate (LORA): a global gridded synthesis runoff product.

No synthesized global gridded runoff product, derived from multiple sources, is available, despite such a product being useful for meeting the needs of many global water initiatives. We apply an optimal weighting approach to merge runoff estimates from hydrological models con- strained with observational streamflow records. The weight- ing method is based on the ability of the models to match observed streamflow data while accounting for error co- variance between the participating products. To address the lack of observed streamflow for many regions, a dissimilar- ity method was applied to transfer the weights of the par- ticipating products to the ungauged basins from the closest gauged basins using dissimilarity between basins in physio- graphic and climatic characteristics as a proxy for distance. We perform out-of-sample tests to examine the success of the dissimilarity approach, and we confirm that the weighted product performs better than its 11 constituent products in a range of metrics. Our resulting synthesized global grid- ded runoff product is available at monthly timescales, and includes time-variant uncertainty, for the period 1980–2012 on a 0.5◦ grid. The synthesized global gridded runoff prod- uct broadly agrees with published runoff estimates at many river basins, and represents the seasonal runoff cycle for most of the globe well. The new product, called Linear Op- timal Runoff Aggregate (LORA), is a valuable synthesis of existing runoff products and will be freely available for download on https://geonetwork.nci.org.au/geonetwork/srv/eng/catalog.search#/metadata/f9617_9854_8096_5291

Figure 10. Mean seasonal runoff calculated for the period 1980–2012.