Looking Beyond the Old Water Paradox: Does New Water Dominate Quick Hydrographs where Surface Flowpaths Prevail? – A Meta-Analysis of Field Evidence from Small, Forested Catchments

Abstract:

The old water paradox describes the rapid mobilization of previously stored water via subsurface flowpaths during a storm event. Old water is usually stored in the subsurface storages. Thus, old water should dominate storm runoff where subsurface flowpaths prevail if mixing with new water is limited. The argumentum e contrario from this understanding raises the following hypothesis: storm hydrographs of catchments with prevailing near surface flowpaths are dominated by new water. We test this hypothesis using data from the scientific literature. The three runoff characteristics hydrograph response (quick or slow), flowpath (surface or subsurface) and time source (old or new water) serve as basis for a conceptual framework of catchment classification where each possible combination of the three characteristics represents a distinct stormflow generation conceptual model. Small forested research catchments for which conceptual models were developed based on field studies were reviewed and assigned to this classification system. Of the 42 reviewed catchments, 30 provide a complete set of the three characteristics resulting in one of the 8 conceptual models. Four catchments support our hypothesis, however, a larger field support exists for subsurface than for surface flowpath dominated sites. Hence, the resulting theory that hydrographs are dominated by new water where surface flowpaths prevail remains highly uncertain. Two explanations exist for the imbalance of field support between the two flowpath classes: 1) the selection of sites in past field studies was mainly to explain quick hydrograph response in subsurface flowpaths dominated catchments; 2) sites with prevailing subsurface flowpaths are more common in nature. We conclude that field studies at sites covering a broader range of characteristics are necessary to understand stormflow generation. The collection of catchments also allows us to test how the three runoff characteristics relate to climate, soils and other catchment characteristics, which may provide important information to improve future predictions.