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We have enhanced the ability of a widely used watershed model, Hydrologic Simulation Program — FORTRAN (HSPF), to predict low flows by reconfiguring the algorithm that simulates groundwater discharge. During dry weather periods, flow in most streams consists primarily of base flow, that is, groundwater discharged from underlying aquifers. In this study, HSPF's groundwater storage‐discharge relationship is changed from a linear to a more general nonlinear relationship which takes the form of a power law. The nonlinear algorithm is capable of simulating streamflow recession curves that have been found in some studies to better match observed dry weather hydrographs. The altered version of HSPF is implemented in the Chesapeake Bay Program's Phase 5 Model, an HSPF‐based model that simulates nutrient and sediment loads to the Chesapeake Bay, and is tested in the upper Potomac River basin, a 29,950 km2 drainage area that is part of the Bay watershed. The nonlinear relationship improved median Nash‐Sutcliffe efficiencies for log daily flows at the model's 45 calibration points. Mean absolute percent error on low‐flow days dropped in five major Potomac River tributaries by up to 12 percentage points, and in the Potomac River itself by four percentage points, where low‐flow days were defined as days when observed flows were in the lowest 5th percentile range. Percent bias on low‐flow days improved by eight percentage points in the Potomac River, from −11 to −3%.

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A 2009 study revealed serious flaws in a standard technique used by hydrological researchers to understand how changes in watershed land use affect stream flow behaviors, such as peak flows. The study caused academics and government agencies alike to rethink decades of watershed research and prompted Kuraś et al. to reinvestigate a number of long‐standing assumptions in watershed research using a complex and well‐validated computer model that accounts for a range of internal watershed dynamics and hydrologic processes. For the test site at 241 Creek in British Columbia, Canada, the authors found not only that deforestation increased the severity of floods but also that it had a scaling influence on both the magnitudes and frequencies of the floods. The model showed that the larger the flood, the more its magnitude was amplified by deforestation, with 10‐to 100‐year‐return‐period floods increasing in size by 9%–25%. Following a simulated removal of half of the watershed's trees, the authors found that 10‐year‐return‐period floods occurred twice as often, while 100‐year‐return‐period events became 5–6.7 times more frequent. This proportional relationship between the increase in flood magnitudes and frequencies following deforestation and the size of the flood runs counter to the prevailing wisdom in hydrological science.

Figure 1 During the Archean eon, from 3.8 to 2.5 billion years ago, life on Earth was thriving for the first time, growing in a world with much less land and a faster planetary rotation than today. At the same time, the energy flowing to the early Earth from the Sun was just three quarters of what it is now. Despite the drastically lower levels of solar irradiance, previous research has suggested that the Archean Earth was not a planet encased in ice but instead remained a watery world. To explain this seeming inconsistency, a dilemma known as the “faint young Sun paradox,” researchers have suggested that the planetary greenhouse effect must have been much more potent than today. Previous research suggested that atmospheric carbon dioxide levels would need to have had a partial pressure of approximately 0.06 bar, equivalent to an atmospheric concentration 200 times that of the pre‐Industrial modern era.