This study is concerned with the effect that mean annual precipitation (P) has on the relationship between bankfull channel width (Wbf) and drainage area (Ada). Several other studies have been conducted in which relationships were developed for predicting Wbf as a function of Ada and P. In most cases, however, the relationships were developed for specific regions, e.g., physiographic regions. This study is unusual in that it evaluates the relationship between Wbf, Ada, and P over a broad area (i.e., across a range of geologic, terrestrial, and climatic environments). In one study, where a broad area was considered, the relationship between Wbf, Ada, and P was found to be linear. The dataset for this study was compiled from data in U.S. Geological Survey flood-flow-frequency reports, regional curve studies (i.e., studies in which Wbf vs. Ada relationships are developed) and other sources. A total of 435 sites that span across 12 states of the continental U.S. are represented in the dataset. Streams represented in the dataset are alluvial and have widths from 1 to 110 m, drainage areas from 0.50 to 22,000 km2, and mean annual precipitation depths ranging from 22 to 277 cm/yr. Data from the U.S. Environmental Protection Agency's Wadeable Streams Assessment study were employed in validating the results of this study. An analysis of covariance (ANCOVA) model was developed and it was determined that the intercept coefficient for the relationship between Wbf and Ada varies as follows: for P < 50 cm/yr the intercept coefficient (α) is constant; for 50 cm/yr ¡Ü P ¡Ü 100 cm/yr, α increases with P, and for P ¡Ý100 cm/yr, α is again constant. Across all values of P, the slope coefficient is constant (90% Confidence level). Changes in the relationship between Wbfand Ada are attributed to vegetation by noting that biome types changes from shrubland to forest as P increases from 50 to 100 cm/yr. These findings can be incorporated in regional curve studies and landscape evolution models (i.e., models which aim to integrate hydrology, land use history, geomorphology and climate change with models of vegetation succession).

Water availability in the ecosystem is one of the most crucial environmental factors that determines global terrestrial biome distribution. However, aridity/humidity conditions in the geologic past are difficult to quantify, mainly owing to the lack of a proper parameter. By using modern global climatic data, we here examine five selected previously proposed aridity indices (AIs), in which the climatic variables involved, including both precipitation and temperature, are simple and likely available in studies of paleoclimatology and paleoecology, although with different degrees of uncertainty. They were first evaluated along the modern climatic zones of eastern China, with the main metric of Thornthwaite humidity index (HI) and with the supplementary reference of soil moisture index (SMI) and near-ground atmospheric relative humidity (RH). Then AIs and the mean annual precipitation (MAP) were further statistically compared with HI, SMI, and RH, respectively, based on 1189 monitored data sets from meteorological stations over the world. The results show that the Köppen aridity index (AIKöppen), expressed as mean annual precipitation divided by mean annual temperature plus a constant of 33, is the most accurate and precise index among all selected indices, supported by the highest correlation coefficient respectively to HI, SMI, and RH, three widely-employed major indicators sensitive to hydrological dynamics in climatology and meteorology. Specifically, AIKöppen does well mirror corresponding HI along four representative transects from North America, South America, Africa, and Australia, which cover the typical arid and humid climates and span the main terrestrial biome types. Moreover, our results also distinctly reveal that, as also shown by many studies on modern climate, precipitation alone is inadequate to measure the hydrological condition, because both temperature and evapotranspiration are two other critical factors that strongly influence water balance in the ecosystem, meanwhile evapotranspiration is mainly affected by temperature. Based on the validated AIKöppen, we briefly discuss the aridity/humidity condition in China during the middle Miocene. The results demonstrate that moisture did decrease westward, but it is also clear that western China in the middle Miocene appears not to have been as dry as previously thought, indicated by the AIKöppen values representing a sub-humid to humid climate.

Water availability in the ecosystem is one of the most crucial environmental factors that determines global terrestrial biome distribution. However, aridity/humidity conditions in the geologic past are difficult to quantify, mainly owing to the lack of a proper parameter. By using modern global climatic data, we here examine five selected previously proposed aridity indices (AIs), in which the climatic variables involved, including both precipitation and temperature, are simple and likely available in studies of paleoclimatology and paleoecology, although with different degrees of uncertainty. They were first evaluated along the modern climatic zones of eastern China, with the main metric of Thornthwaite humidity index (HI) and with the supplementary reference of soil moisture index (SMI) and near-ground atmospheric relative humidity (RH). Then AIs and the mean annual precipitation (MAP) were further statistically compared with HI, SMI, and RH, respectively, based on 1189 monitored data sets from meteorological stations over the world. The results show that the Köppen aridity index (AIKöppen), expressed as mean annual precipitation divided by mean annual temperature plus a constant of 33, is the most accurate and precise index among all selected indices, supported by the highest correlation coefficient respectively to HI, SMI, and RH, three widely-employed major indicators sensitive to hydrological dynamics in climatology and meteorology. Specifically, AIKöppen does well mirror corresponding HI along four representative transects from North America, South America, Africa, and Australia, which cover the typical arid and humid climates and span the main terrestrial biome types. Moreover, our results also distinctly reveal that, as also shown by many studies on modern climate, precipitation alone is inadequate to measure the hydrological condition, because both temperature and evapotranspiration are two other critical factors that strongly influence water balance in the ecosystem, meanwhile evapotranspiration is mainly affected by temperature. Based on the validated AIKöppen, we briefly discuss the aridity/humidity condition in China during the middle Miocene. The results demonstrate that moisture did decrease westward, but it is also clear that western China in the middle Miocene appears not to have been as dry as previously thought, indicated by the AIKöppen values representing a sub-humid to humid climate.

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