Investigating the roles of climate seasonality and landscape characteristics on mean annual and monthly water balances

This paper explores the effects of seasonal variability of climatic inputs on mean annual and monthly water balances and the roles of climate, soil properties and topography in modulating these impacts. The analyses are carried out with the use of a theoretically derived and physically-based water balance model, which includes simple sub-models for evapotranspiration, infiltration, surface runoff by both infiltration excess and saturation excess, and subsurface runoff. Numerical experiments for different watershed conditions showed (1) the effects of seasonality are most important when the seasonal variabilities of precipitation and potential evapotranspiration are out of phase, and in arid climates, (2) the seasonality effects are found to be high in basins with fine grained soils and flat topographies, in which case surface runoff dominates, and also high in basins with coarse grained soils and steep topographies, where subsurface runoff dominates. Indeed, there appears to be a critical combination of soil type and gradient at which a switch from subsurface to surface runoff dominance takes place. As one moves from well drained to poorly drained catchments, the amplitude of the resulting seasonal fluctuations initially decreases and beyond some threshold value of drainability it might increase again due to the switch from subsurface to surface runoff dominance. The study also highlighted the important role of the unsaturated zone storage, the effects of soil permeability and their interactions with topographic slope, in determining both annual and seasonal water balances. The results of the study are summarized in terms of soil type and three dimensionless numbers: climatic dryness index, storage capacity index and drainability index.