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There is no general theory to explain differences in baseflow between catchments, despite evidence that it is mainly controlled by climate and landscape. One hypothesis is that baseflow fraction (the ratio between baseflow and precipitation) can be primarily attributed to the aridity index (the ratio between potential evapotranspiration and precipitation), that is, that there is a "baseflow Budyko curve." Comparing catchment data from the United States and the United Kingdom shows, however, that aridity is not always a good predictor of baseflow fraction. We use the revised Ponce-Shetty annual water balance model to show that there is no single "baseflow Budyko curve" but rather a continuum of curves emerging from a more universal model that incorporates both climate and landscape factors. In humid catchments, baseflow fraction is highly variable due to variations in a catchment's wetting potential, a parameter that describes catchment storage capacity. In arid catchments, vaporization limits baseflow generation, which leads to lower variability in baseflow fraction. Generally, when the magnitude of precipitation is important, the aridity index only partly explains baseflow response. Adapting the model to explain variability of the baseflow index (the ratio between baseflow and total streamflow) shows that the aridity index is generally a poor predictor of baseflow index. While the wetting potentials and other parameters are obtained by fitting the Ponce-Shetty model to annual catchment data, their links to physical properties remain to be explored. This currently limits the model's applicability to gauged catchments with sufficiently long records.

Plain Language Summary Baseflow originates from stored water (e.g., groundwater) and sustains river flow in dry periods, which makes it an important water resource. Baseflow is known to vary with climate and landscape properties such as geology or vegetation, but there is no universal theory to explain this variability. To explore baseflow variability, we use data from several hundred catchments in the United States and the United Kingdom. We investigate whether a catchment's baseflow fraction, that is, the fraction of rainfall that becomes baseflow, can be attributed primarily to the aridity index, a commonly used climate index. The aridity index is defined as the ratio between potential evapotranspiration (available energy) and precipitation (available water). We find that in humid catchments (low aridity index), baseflow cannot be attributed primarily to the aridity index. Rather, a catchment's capacity to store water determines how much precipitation becomes baseflow. In arid catchments (high aridity index), the aridity index can be seen as the primary determinant of baseflow fraction. It strongly influences how much of the precipitation can be evaporated back to the atmosphere and thus cannot become baseflow. These results might help to assess how water availability (in the form of baseflow) changes with changing climate and land use.