Observations of the transient characteristics of the hydrological balance

Abstract

The impact of transient input events on the hydrological balance components are studied over a wide range of spatial and temporal scales. First, interception evaporation and energy-balance changes during and following precipitation events are examined at the site spatial and event temporal scale. Second, the hydrological response to spatial and seasonal changes in precipitation and evapotranspiration due to land cover changes are examined on the small-to-large watershed scale, covering the event to seasonal time scales.

Estimates in the literature for interception evaporation using conventional methods vary widely even over the same forest type. We present an alternative method to estimate interception evaporation from eddy covariance measurements combined with a novel use of data-analysis techniques to form base state and event-based ensembles. Application of this method at a tropical rain forest site in the Amazon resulted in mean interception evaporation estimates of 11.6%, comparable to recent conventional studies in that region. Energy balance comparisons between dry and afternoon rain-days show an approximately 15% increase of evaporative fraction on the rain days, with the energy being supplied by a corresponding decrease in the canopy heat storage.

Large differences in the watershed response characteristics of streamflow peak and streamflow peak to watershed precipitation ratio in the Catskill-Hudson Valley Region of New York were observed relative to a watershed’s proximity to a precipitation shadow observed there. There were detectable changes in streamflow and soil moisture recession times, as well as in the diurnal streamflow amplitude over this watershed network during the autumn transition. This provides an independent estimate of autumn or spring onset to complement phenological or satellite-derived measurements, and is important as this shows the timing of the hydrological impacts of abrupt changes in evapotranspiration forcing on the watersheds.

In a network of watersheds in the Amazon, precipitation and subsequent storage and subsurface drainage processes seem to have a greater influence on seasonal changes in soil moisture recession compared with seasonal variations in evapotranspiration due to vegetation state.